M.Tech thesis

“NANOCRYSTALLINE POROUS SILICON BASED

INTEGRATED MEMS PRESSURE AND

TEMPERATURE SENSOR”

By

Mr. Ishita Sinha

Roll No –

Reg. No -

Under the guidance of

Prof. H. Saha

Thesis submitted in partial fulfillment

Of the requirements for the degree of

Master of Technology in Nanoscience and Technology

May 2007

IC Design and Fabrication Centre

Department of Electronics & Telecommunication Engineering

Jadavpur University

ACKNOWLEDGEMENT

I would like to take this opportunity to express my gratitude and heartiest thanks to Prof. H. Saha, IC Design and Fabrication Center, Department of Electronics & Telecommunication Engineering, Jadavpur University, for his inspiration, encouragement and guidance in the fulfillment of my project work.

I convey my sincere thanks to the Head of the department of Nano Science & Technology Prof.S.Mukherjee for kindly assigning my M.Tech project work in the I.C Centre. I am also thankful to the faculty members, technical and non technical staffs of the same department of their help when ever I needed it.

I am also grateful to Dr. U.Ganguly and Mr. Ashok Mondal for their immense help during the experiment. My sincere thanks also goes to Mrs.Chirasree Pramanick for her constant help throughout the progress of my work.

I am grateful to Prof.S.Basu, Ex Professor, I.I.T Kharagpur and at present Research Advisor I.C Design & Fabrication Centre, Jadavpur University, Kolkata, for necessary suggestions and helpful scientific discussions to improve the quality of my project work.

I would also like to pay my gratitude to Mrs. Joyita Das, Mr.Palash Basu,

Mr. Biplob Mondal, and my all my friends of IC Center, Department of electronics and Telecommunication Engg, Jadavpur University, Kolkata.

Last but certainly not the least, I would also pay my heartiest thanks to my parents, my husband , my respected teachers and my friends and colleagues for their encouragement and support at different level of my work.

CONTENTS

Chapter : 1 Introduction

1.1 General Introduction 1.2 Objective of this project 1.3 Literature Review 1.4 Scope of this work References

Chapter : 2 Overview of micromachining and micromachined pressure and temperature sensor

2.1 Introduction

2.2 Overview of Micromechanical Structure 2.2.1 Micromachining

2.2.2 Anisotropic Etching of Silicon

2.2.3 Top view of the micromachined silicon membrane References

Chapter : 3 Fabrication of the sensor

3.1 Introduction 3.2 Fabrication steps 3.2.1 Cleaning 3.2.2 Oxidation 3.2.3 Photolithography 3.2.4 Micromachining

3.3 Flowchart For Fabrication of Silicon MEMS Piezoresistive Pressure Sensor

3.4 Flowchart For Fabrication of porous Silicon Pressure Sensor 3.5 Flowchart for fabrication of nanocrystalline porous silicon based pressure and temperature sensor

References

Chapter: 4 Design and simulation of the sensor

4.1 Design of nanocrystalline porous silicon based integrated pressure and temperature sensor

4.2 Sensing Principles

4.2.1 Pressure sensing principles 4.2.2 Temperature sensing principles

4.2.3 Integrated temperature and pressure sensor 4.3 Equivalent model of the sensor

4.4 Design of Conventional Pressure sensor using lateral contacts 4.5 Results and Discussions

References

Chapter : 5 Summary and Conclusion

5.1 Summary 5.2 Conclusion

Chapte1: Introduction

1.1

General Introduction:

Micro Electro Mechanical systems (MEMS) are integrated micro devices or systems combining electrical and mechanical components fabricated using Integrated Circuit (IC) compatible batch-processing techniques and range in size from micrometers to millimeters. These systems can sense, control and actuate on the micro scale and function individually or in arrays to generate effects on the macro scale.

Using similar fabrication techniques as building microprocessors, we are now able to build sensors and actuators on the same microscopic level with the processor chip. Measured in microns, thermal sensors, pressure sensors, inertial sensors, flow and viscosity sensors, resonators, levers, gears, transmission systems, micro-mirrors, valves, pumps, motors, can be batch produced together on the same chip with the processing unit. They indeed compose a “system on a chip”[1].

Monocrystalline silicon MEMS pressure sensor suffers from high temperature sensitivity while polysilicon MEMS suffer from lower piezosensitivity .To overcome both these limitations, the idea of porous silicon based MEMS pressure sensor has been conceived[2]. The extremely large surface-to-volume ratio (>=500m2_/cm3_{) of a}

porous silicon nanostructure, the ease of its formation and control of its surface morphology through variation of the formation parameters, its compatibility of silicon IC technology and hence amenability to the development of smart systems-on-chip sensors—all these properties have made it a very attractive sensing material[3,4].

Chapter 1: Introduction

1.2

Objective of the Work :

There are two objectives of the present work :

• Design and simulation of nanocrystalline porous silicon based MEMS integrated pressure and temperature sensor by ANSYS 10.0

• Fabrication of nanocrystalline porous silicon based MEMS integrated pressure and temperature sensor.

1.3 Literature Review :

Following the invention of bipolar transistor in 1947, a great deal of effort was put into characterizing the properties of single crystal semiconductors .In 1954, smith reported the piezoresistive effect of silicon ands germanium, which is a change of resistance with applied stress[5].This discovery enabled production of Semiconductor - based sensors.

Silicon strain gauge, metal diaphragm sensors were first introduced commercially in 1958[2]. Piezoresistive pressure sensors have piezoresistors mounted on or in a diaphragm. For thin diaphragms and small deflections, the resistance change is linear with applied pressure[7]. In these early sensors high-cost, low-volume biomedical and aerospace applications were targeted[11]. The fabrication of early pressure sensors started with metal diaphragm sensors with bonded silicon strain gauges. The strain gauges were bonded by epoxies, phenolics, or eutectics [8]. These first designs had

Chapter 1: Introduction

low yield and poor stability due to such factors as thermal mismatch with the metal– epoxy–silicon interface[9].

Metal diaphragms were quickly superseded by single crystal diaphragms with diffused piezoresistors. These new types of sensor had many advantages related to the properties of silicon and the availability of high-quality silicon substrates. Hysteresis and creep associated with metal diaphragms were eliminated. At low temperatures (<500° C), silicon is perfectly elastic and will not plastically deform [9], but instead will fracture in a brittle manner. Silicon obeys Hooke’s law up to 1% strain, a tenfold increase over common metal alloys . Also, the ultimate tensile strength of silicon can be three times higher than that of stainless steel wire [10]. As a piezoresistive material, silicon has gauge factors that are over an order of magnitude higher than those of metal alloys [5].

Some of the first silicon diaphragms were created by mechanical milling spark machining followed by wet chemical isotropic etching, to create a cup shape. These diaphragms were bonded to silicon supports by a gold–silicon eutectic (Teutectic = 370 °

C). While this technique of fabrication had the advantages of increased sensitivity and reduced size, cost was still high, and diaphragms were created one at a time, rather than in batch mode[2].

Chapter 1: Introduction

In 1960 the invention of the planar batch-fabrication process tremendously improved the reliability and cost of semiconductor devices. In addition, the planar process allowed for the integration of multiple semiconductor devices onto a single piece of silicon (i.e., monolithic integration)[2]. This invention heralded the beginning of the IC industry. Although the early planar process produced relatively large devices (>mm), it was a tremendously scaleable process that could micromachine an increasing number of devices. Micromachined pressure sensors were available in 1963 [7] and advances in fabrication technology have led to the bulk-micromachined sensors available today.

By the late 1960s and early 1970s, three key technologies were being developed: anisotropic chemical etching of silicon , ion implantation, and anodic bonding [13,2]. Ion implantation was used to place strain gauges in single-crystal silicon diaphragms. Ion implantation is generally better than diffusion for doping because both the doping concentration and doping uniformity are more tightly controlled. Anisotropic etching improved the diaphragm fabrication process in a number of ways[6]:

(i) diaphragm sizes and locations were now well controlled by IC photolithography techniques;

(ii) strain gauge placements were improved;

(iii) anisotropic etching was well suited to batch fabrication, allowing hundreds of diaphragms to be created simultaneously; and

(iv) overall size was decreased further.

Anodic bonding, which uses voltages (500–1500 V) and heat (400–600 C), was used to bond finished silicon diaphragm wafers to Pyrex glass supports[19]. Several types

Chapter 1: Introduction

of glass formulations designed to reduce thermal mismatch to silicon were used. Anisotropic etching and anodic bonding are batch techniques, and hence hundreds (or more) of pressure sensors could be manufactured simultaneously

on a single wafer. This amounted to a significant cost reduction.

By 1970s and 1980s, MEMS commercialization was started by several companies (e.g., IC Transducers, Foxboro ICT, Transensory Devices, IC Sensors and Novasensor) that produced parts for the automotive industry. The microsensor companies began to move toward higher-volume, lower cost applications , specifically, the automotive industry [16].

The mechanical properties of single-crystal silicon are excellent, as reported in a landmark article . It has high strength, high stiffness, high mechanical repeatability, high Q, and no mechanical hysteresis. Furthermore, single-crystal silicon is available in large quantities with high purity and low defect densities[10].Piezoresistive gauge factors in silicon are higher than in metal, but temperature coefficients of resistance (TCRs) are high. Because of high TCRs, silicon microsensors often require temperature compensation techniques. He discussed the development of many micromechanical devices and has been instrumental in increasing the awareness of the possibilities that MEMS has to offer[20] .

In 1984 Howe and Muller at the University of California, Berkeley (UCB) developed the polysilicon surface micromachining process and used it to produce MEMS with integrated circuits [15]. This technology has served as the basis for many MEMS

Chapter 1: Introduction

The direct bonding method was first reported in 1985[23]. This method was first used for making silicon-on-insulator (SOI) material, but was quickly applied to micromachined devices [22]. Also, surface-micromachined devices have been reported, which have silicon nitride [24] or polysilicon [25]

diaphragms. These sensors decrease required die size and may simplify integration with electronics, but at the cost of reduced sensitivity and reproducibility of mechanical properties[16,21].

In 1989 Researchers at UCB and MIT independently developed the first electrostatically controlled micromotors that used rotating bearing surfaces [14]. Although no commercial product presently uses this micromotor technology, it served as a valuable technology driver for the field of MEMS[1].

By 1990s, A tremendous increase in the number of devices, technologies, and applications (too many to mention individually) has greatly expanded the sphere of influence of MEMS—and it continues today[1].

So this period is often called the micromachining period [2], since diaphragm dimensions are shrinking to hundreds of micrometres and minimum feature sizes are shrinking to micrometres. Also, anisotropic etching and bonding technologies are being improved[2].

In 1991 Microhinges developed at UCB by Pister extended the surface micromachined polysilicon process so that large structures could be assembled out of the plane of the substrate, finally giving MEMS significant access to the third dimension[1].

Chapter 1: Introduction

During the last decade the interest of researchers to silicon, which was considered before as quite well-known material, has grown enormously. The triggering point was the paper by Dr Leigh Canham (Defence Research and Evaluation Agency, UK) who published the observation of bright red photoluminescence from the surface of electrochemically etched Si wafer [12]. Porous silicon a substance which is produced by a treatment of Silicon wafers in hydrofluoric acid solutions was known since the fifties due to the works by Uhrlir, Turner , Memming and Schwandt [17]. The material was considered as suitable for electronic applications (local insulation, gettering of impurities, sacrificial layers, etc.) but never in relation with optical applications. Energy gap of silicon (1.1 eV) corresponds to the infrared region and is indirect that makes radiative recombination processes quite ineffective[3].

The observation of red bright photoluminescence from PS has produced a sort of sensation (although the first publication on the visible light emission from porous silicon was made in 1984 by Pickering et al. pose of building Si-based Light-Emitting Devices (LEDs). In 1990, Canham [18] reported that if porous silicon is further etched in HF for hours after preparation, it emits bright red light when illuminated with blue or UV light. Efforts of scientific community undertaken during the years 1991 to 1996 brought many useful results about the aspects of PS formation and its physical and chemical properties(19).

Chapter 1: Introduction

1.3

Scope of the Project Work:

The latest piezoresistive pressure sensors are up to ten times more sensitive than old transducers and with response times as rapid as a millisecond. These sensors are used for applications as automotive, hi-fi, aerospace , robotics , space and medical equipment industries. More than a dozen applications for pressure sensors have been identified and silicon thin-diaphragm piezoresistive sensors are responsible for many of these system needs. These sensors are used in vehicles to control the ignition and the composition of the petrol mixture, in audio systems to compensate for loudspeaker resonance and in medical for dialysis, middle ear diagnosis, and disposable blood pressure meters[25]. But these sensors also show temperature sensitivity. To compensate for the temperature effect, a p–n-junction-based temperature sensor is often integrated with the silicon pressure sensor for commercial applications[24].

Chapter 1: Introduction

References

[1] Microelectromechanical systems (MEMS): fabrication, design and applications Jack W Judy

[2] Micromachined pressure sensors: review and recent developments W P Eatony and J H Smith, Smart Mater. Struct. 6 (1997) 530–539.

[3] Porous Silicon-Based Sensors: Prospects and Challenges H. Saha and C Pramanik Materials and Manufacturing Processes, 21: 239–246, 2006 [4] Porous Silicon as Pressure Sensing Material U. Gangopadhyay

Journal of the Korean Physical Society, Vol. 47, November 2005, pp. S450_S453

[5] Piezoresistance effect in germanium and silicon, C.S. Smith, Physical Review,94(1), pp. 42-49 (1954).

[6] Anisotropic etching of silicon IEEE Trans. Bean K E 1978 Electron Devices ED-25 1185–93

[7] Pressure sensitivity in anisotropically etched Thin diaphragm pressure sensors Clark S K and Wise K D 1979 IEEE Trans. Electron Devices ED-26

1887–96 [23]

[8] Piezoresistivity of silicon quantum well wire C Pramanik, S Banerjee, H Saha and C K Sarkar

[9] Micro-diaphragm pressure sensor S. Sugiyama, T. Suzuki, K. Kawahata, Shimaoka M. Takigawa and I. Igarashi Toyota Central Research And

Development Labs., Inc.

[10] Silicon as a Mechanical Material Kurt e. Petersen, member, IEEE proceedings of the IEEE. vol. 70, no. 5, may 1982

[11] Development of a Miniature Pressure Transducer for Biomedical Applications IEEE transactions on electron devices, vol. ed-26, no. 12, december 1979

Chapter 1: Introduction

[12] Porous silicon : mechanisms of growth and applications V. Parkhutik Solid-State Electronics 43 (1999) 1121±1141

[13] A Novel Convex Corner Compensation for Wet Anisotropic Etching on (100) Silicon Wafer Huai-Yuan Chu and Weileun Fang

[14] Materials issues in microelectromechanical systems (MEMS) S.M. Spearing Acta mater. 48 (2000) 179±196

[15] Surface Micromachining for Microelectromechanical Systems James m. Bustillo, Roger T. Howe, fellow, IEEE, and Richard s. Muller, life fellow, IEEE [16] High-temperature Pressure and Temperature Multi-function Sensors

Liu Xiaoviei, Wang Wei, Wang XiIian Liu Yuqiang. Liu Zhenmao

[17] Electrode design and planar uniformity of anodically etched large area porous silicon S M Hossain, J Das, S Chakraborty, S K Dutta and H Saha [18] Light-Emitting Porous Silicon: Material science, Properties, and Device Applications P. M. Fauchet, Senior Member, IEEE , L. Tsybeskov, C. Peng, S. P. Duttagupta, J. von Behren, Y. Kostoulas, J. M. V. Vandyshev, and K. D. Hirschman

[19] A Microsystem Based on Porous Silicon-Glass Anodic Bonding for Gas and Liquid optical Sensing Luca De Stefano , Krzysztof Malecki , Francesco G. Della Corte , Luigi Moretti , Ilaria Rea , Lucia Rotiroti and Ivo Rendina Sensors 2006, 6, 680-687

[20] Micro-diaphragm pressure sensor Sugiyama S, Suzuki T, Kawahata K, Shimaoka K, Takigawa M and Igarashi I 1986 Tech. Digest 1986 Int.

Electron Devices Meeting (IEDM ’86) pp 184–7

[21] Fabrication techniques for integrated sensor Guckel H and Burns D W 1986 microstructures Tech. Digest IEEE Int. pp 122–5

[22] Surface micromachined micro-diaphragm pressure sensors Sugiyama S, Shimaoka K and Tabata O 1991 Digest Tech. Papers 1991 Int. Conf. on Solid-State Sensors and Actuators (Transducers ’91) pp 188–91

Chapter 1: Introduction

[23] Silicon –on- Insulator(SOI) by Bonding and Etch-Back, J.B.Lasky, S.R.Stiffler, F.R.White and J.R.Abernathy, Technical Digest, IEEE International Electron Device meeting, IEDM 85, pp. 684-687 (1985).

[24] Silicon fusion Bonding for Pressure sensors, K.E.Peterson, P.W.Barth, J. Poydock, J.Mallon and J.Bryzek, Technical Digest: IEEE Solid state Sensor and Actuator Workshop, Hilton Head 88, p. 144 (1988).

[25] An integrated pressure and temperature sensor based on nanocrystalline Porous silicon C Pramanik, H Saha and U Gangopadhyay, J. Micromech. Microeng.16 (2006) 1340–1348

[26] A Silicon Piezoresistive Pressure Sensor Ranjit Singh, Low Lee Ngo,Ho Soon Seng, Frederick Neo Chwee Mok

Chapter 2: Overview of Silicon Micromachining

2.1 Introduction

Anisotropic etching of silicon is used to realize the micromechanical structures. The pressure sensors can be broadly classified as piezoresistive and capacitive. In piezoresistive sensors, the resistivity and the geometrical dimensions of the sensor changes on application of stress which in turn affects the resistance. In case of capacitive sensors, on application of stress the deflection of the diaphragm affects the distance between the metal plates of a parallel plate capacitor, which in turn changes the capacitance of the sensor.

2.2 Overview of Micromechanical Structures

2.2.1

Development of silicon microsensors often required the fabrication of micromechanical parts. These micromechanical parts were fabricated by etching areas of the silicon substrate away selectively to leave behind the desired geometries. This is micromachining, used to designate the mechanical purpose of the fabrication process, which were used to form these micromechanical parts. Silicon micromachining is of great importance for the development of inexpensive, batch fabricated, high performance sensors, which can easily be interfaced with microprocessors.

The following properties of silicon have made micro machining feasible [1]:

(a) Silicon can be readily oxidized by exposing it to steam or dry oxygen. It allows silicon wafer to be masked selectively during chemical etching.

Chapter 2: Overview of Silicon Micromachining

(b) Single crystal silicon is brittle and can be cleaved like diamond but it is harder than most metals. It is resistant to mechanical stresses and the elastic limit of silicon is greater than that of steel.

(c) A single crystal silicon can withstand repeated cycles of compressive and tensile stresses.

(d) The crystal orientation of single crystal silicon wafer decides the rate of chemical etching in certain etching solutions, which is important in creating various structures. Bulk micromachining and surface micromachining are the two distinctly different approaches of micromachining silicon for realizing microsensors and actuators. Isotropic and anisotropic etching of silicon has been used for realizing micro mechanical parts from bulk silicon wafer and forms the basis of “bulk micromachining”. In another approach for micro machining called “surface micro machining”, the silicon substrate is primarily used as a mechanical support upon which the micro mechanical elements are fabricated. The bulk of the silicon wafer itself is not etched in surface micro machining. There are no holes through the wafer and no cavities on the backside. When bulk micro machining silicon, the backside of the wafer is conventionally protected against an etchant with an oxide or nitride layer in which windows are opened where the micro mechanical structures are to emerge. An accurate alignment of the etch windows is essential to obtain the structures at a proper position with respect to the photolithographic patterns at the front. In surface

micro machining a sacrificial layer is deposited on the silicon substrate, which may be coated first with an isolation layer. Windows are opened in the sacrificial layer and the

Chapter 2: Overview of Silicon Micromachining

micro structural thin film is deposited and etched. Selective etching of the sacrificial layer leaves a freestanding micro mechanical structure[2].

There are two main methods of etching – wet etching and dry etching. Wet etching is done with the use of chemicals. A batch of wafers is dipped into a highly concentrated pool of acid and the exposed areas of the wafer are etched away. Dry etching refers to any of the methods of etching that use gas instead of chemical etchants. Bulk micromaching of silicon uses wet and dry etching techniques in conjunction with etch masks and etch stops to sculpt micromechanical devices from silicon substrate. The selective etching of silicon can be carried by using isotropic and anisotropic etchant. The isotropic etchant under-etch large area in lateral direction than the area defined by mask opening. On the other hand, anisotropic etchant, which are also known as orientation dependent or crystallographic etchant, etch the silicon surface at different rates in different directions in the crystal lattice. They can form well-defined shapes with sharp edges and corners[3].

2.2.2 Anisotropic etching of silicon

One common MEMS (Micro-Electro Mechanical Systems) fabrication technique is the anisotropic etching of crystalline silicon, where etch rate is a function of orientation. The anisotropic etching of silicon is ubiquitous process in micromachining. Complex microsystems can be generated using the anisotropic properties of single crystal silicon in an orientation dependent dissolution reaction. V-groove structures for example, useful for the passive alignment of optoelectronic

devices are easily fabricated using an anisotropic etchant like KOH or tetramethylammonium hydroxide[4].

Chapter 2: Overview of Silicon Micromachining

Modern exacting demands in this rapidly growing industry require fundamental understanding of these processes in order to achieve a well defined anisotropic ratio nada good surface finish. Mostly used technology for bulk structuring for microsensors and actuators is the anisotropic etching with KOH. Specifications for the etched structures (such as high etch rate ratios of <110> and <100> to <111> planes, short etch times and minimum roughness) can be obtained by optimization of the etch parameters. For sensor applications <100> oriented silicon is mostly used[4].

Anisotropic etchants of silicon such as EDP, KOH and hydrazine are orientation dependent. This means that they etch the different crystal orientations with different etch rates. Anisotropic etchants of silicon etch the <100> and <110> crystal planes significantly faster than the <111> crystal planes. The etch rate for <110> surfaces lies between those for <100> and <111> surfaces. Figure 1 demonstrates the basic concepts of bulk micromachining by anisotropic etching of a <100> silicon substrate. For example for a <100>-silicon substrate etching proceeds along the <100> planes while it is practically stopped along <11> planes. Since the <111> planes make a 54.75Ο _{angle with the <100> planes, the slanted walls (Fig.1.1) result. Due to the}

slanted <111> planes, the size of the etch mask opening determines the final etch result. If the etch masks openings are rectangular or square and the sides are aligned with the <110> direction, no undercutting of the etch mask feature takes place[3]. The width of the bottom surface W is given by [4],

Wb = Wo – 2 t cot(54.75Ο ) (1)

Chapter 2: Overview of Silicon Micromachining

where Wo is the width of the etch mask window on the wafer back surface, and t is the etched depth. If <110> oriented silicon is etched in KOH water etchant, essentially straight walled grooves with sides of <111> planes can be formed. The anisotropic etch rate in the <100> direction of monocrystalline silicon of <100> oriented wafers was investigated focusing on the dependence of temperature and concentration of potassium hydroxide. Advantages and disadvantages of the different etching conditions are anisotropic direction selectivity, speed and surface roughness of the pattern. In isotropic etching all orientations or planes etch at the same rate, hence a square hole would get rounded corners. In anisotropic etching, because of the differences in rates [5], some planes grow while others disappear. There are two main classifications that describe how the initial mask shape will evolve into the final etched shape. Firstly, etched shapes may be classified as either pegs or holes. Holes are lower than the surface of the wafer and pegs are higher than the wafer, Holes enlarge with time while pegs shrink.

After long times, holes are dominated by slow planes, while pegs become dominated by fast planes. Secondly, within a shape (be it a peg or a hole) there can be two corners: convex (peg like) and concave (hole like). While etching convex corners, fast planes dominate: fast planes increase in length while slow planes decrease in length. For concave corners, slow planes dominate[5].

Chapter 2: Overview of Silicon Micromachining

Figure 1(a) : Anisotrophic Etching of Silicon

Chapter 2: Overview of Silicon Micromachining

Etchants can be characterized by using the following characteristics: a) Direction dependency (isotropic or anisotropic)

b) Etch rate

c) Anisotropic etch rate ratio (only for anisotropic etchants, 1:1 to 400:1 for <100>/<111> planes

d) Dopant dependence/selectivity

e) Temperature of etching (20Ο_{C to 100}Ο_C)

Direction dependency

The most important feature in classifying silicon etchants is their ability to have different etch rates in different directions of the crystal lattice that is exposed to them. Isotropic etchants etch in all directions with the same etch rate, resulting in rather rounded shaped pits and also previous sharp edges and corners. The result of anisotropic etchants, on the other hand, is different, looking perpendicular on each of the crystal planes. This makes it possible to fabricate sharply formed structures or narrow gaps, whose borders have to coincide with the crystal planes. Depending on what kind of structure is desired, the proper etchant has to be chosen.

Etch rate

Basically the etch rate can vary with temperature, mix of ingredients, sometimes optical circumstances (light intensity), or it can be stable over a wide range [9].

If an anisotropic etchant is chosen, the ratio of etch rates concerning the different crystal planes, can vary in a wide range. Again, the desired result influences the

Chapter 2: Overview of Silicon Micromachining

choice of etchant, since ratios from 1:1 to 400:1 are possible, if one compare the <100> and the <111> planes.

Dopant dependence (selectivity)

Another very important attribute is the dopant dependency of etchants. Some etchants are very selective on the material that they are exposed to, so that a doped layer or a layer of different material can be used as etch stop or a direction of a much higher etch rate. If this is not desired, it is advantageous to choose a non-selective etchant[10].

Etching temperature

In general lower temperatures are better than higher ones, since temperature induced stress concentrations are minimized when the processing temperature is low. In addition, the occurrence of hazardous gases is lower at low temperatures.

The etching process carried out in this project is by using EDP which has following properties

:-EDP has three properties, which make it indispensable for micromachining: a) It is anisotropic.

b) It is highly selective.

Chapter 2: Overview of Silicon Micromachining

Other than EDP , the following etchants can be used for micro machining :

KOH and water

The main advantage of KOH is [4-7], that it is orientation dependent with much higher <110>: <111> etch rate ratio than EDP, therefore useful for groove etching on <110> wafer.

HNA

The HNA system is highly variable in its etching rates and characteristics depending on

a) Silicon dopant concentration b) Mix ratios

c) Degree of etchant agitation

A major disadvantage is that SiO2 is etched somewhat for all mixtures, so that it can

only be used for short etching times, otherwise, Si3N4 can be used instead.

TMAH

Tetra-Methyl ammonium hydroxide (TMAH) is an anisotropic silicon etchant that is gaining considerable use in silicon micromachining due to its excellent silicon etch rate, etch selectivity to masking layers, degree of anisotropy, and relatively low toxicity.

Chapter 2: Overview of Silicon Micromachining

2.2.3 TOP VIEW OF MICROMACHINED SILICON MEMBRANE

Chapter 2: Overview of Silicon Micromachining

Chapter 2: Overview of Silicon Micromachining

References

[1] Silicon as a mechanical material ”, K.E.Peterson, IEEE Proc. Vol. 70,1982. [2] Comparison of Bulk- and Surface- Micromachined Pressure Sensors William P. Eaton, James H. Smith, David J. Monk, Gary O’Brien, and Todd F. Miller Micromachined Devices and Components, Proc SPIE, Vol 3514, p. 431 [3] Foundation of MEMS Chang Liu Prentice Hall Chapter-10,11

[4] Anisotropic etching of silicon, K.E.Bean, IEEE Trans. ED-25,10,p.1185,1978. [5] A Novel Convex Corner Compensation for Wet Anisotropic Etching on (100) Silicon Wafer Huai-Yuan Chu and Weileun

[6] Anisotropic etching of silicon, D.B.Lee, J. Appl.Phys. Vol. 40,No.1,p.4569, 1969.

[7] Surface Micromachining for Microelectromechanical Systems James M. Bustillo, Roger T. Howe, fellow, IEEE, and Richard S. Muller,

life fellow, IEEE

[8] A new theory for the anisotropic etching of silicon and some underdeveloped chemical micromachining concepts”, D.L.Kendall, Journal of Vacuum

Science Technology A, 8(4), pp. 3598-3604 (1990).

[9] Silicon Carbide as a new MEMS technology Pasqualina M. Sarro

[10] Methods for the Fabrication of Convex Corners in Anisotropic Etching of (100) Silicon in Aqueous KOH”, H.L.Offerins, K.Kuhl nad H.Sandmaier, Sensors and Actuators A,25-27, pp. 9-13 (1991).

Chapter 3 :

Fabrication

Of The Sensor

3.1 Introduction :

3.1.1 Resistivity of the silicon wafer

We measured the resistivity with the 4-probe resistivity meter and measured the average resistivity of the wafer. We placed the wafer at 5 different positions (a) Lower part (b) Upper part (c) middle part (d) Right part & (e) Left part of the wafer and measured the resistivity by passing a known value of current (µA) & noting down the voltages at the different positions of the wafer.

Using the formula: Rs = f * (v)/(I).

where "f' is the Scaling factor=4.53, Rs is the Sheet resistance.

3.1.2 Type test of the silicon wafer

We measured the type of the silicon wafer by the method of Hot Probe test. In this method, we warmed one terminal of the probe and placed the terminals on the wafer. If the wafer is of P- type, then a negative voltage will be displayed in the meter and if the wafer is of N -type then a positive voltage will be displayed in the meter. In our case the reading in the meter was negative. Hence we came to conclusion that the wafer is of P type.

Chapter 3 :

Fabrication

Of The Sensor

3.2 Fabrication Steps:

3.2.1 Cleaning

Step1: Sample + Acetone. Then boil for 3min. And then treat in Ultrasonic cleaner for 3min. to remove dust particles,oil and greese.

Step2: Dip in Methanol to avoid Oxidation of the sample.

Step3: (1:1) conc. H2SO4 + H202. Wait till the completion of reaction. Then clean in

DI-water to remove the oxide of the sample.

Step4: Dip in 10% HF solution. Then clean in DI-water to remove the oxide of the sample.

Step5: Treat the sample with Std. Cleaning-1 [H20 (5):H202(1):NH40H(1)] at a temp- of

70°C for 10min. And then pass through cold water to remove the residual organic material.

Step6: Treat the sample with Std. Cleaning-2 [H2O (6): H2O2(1):HCI(1)} at 700C for

10min. Then pass through DI-water to remove the residual organic material and dry with N2.

Chapter 3 :

Fabrication Of The Sensor

3.2.2 Oxidation

Oxidation is a fundamental process in all silicon device fabrication. Oxidation of silicon wafers is used for: (i) passivation of the silicon surface (i.e., the formation of a chemically and electronically stable surface), (ii) masking of diffusion and ion implantation, (iii) dielectric films, and (iv) an interface layer between the substrate and other materials, such as in chemical and biosensors. Silicon, when exposed to the air at room temperature, will grow a native oxide (about 20 A thick). Thicker oxides can be grown at elevated temperatures in dry or wet oxygen environments. At a given temperature, the relationship between the thickness of oxide and time is parabolic. The rate of growth is also affected by the partial pressure of the oxygen atmosphere and the crystal orientation.

The ability to grow a chemically stable protective layer of silicon dioxide SiO2 on a

silicon wafer makes silicon the most widely used semiconductor substrate. The silicon dioxide layer is both an insulating layer on the silicon surface and a preferential masking layer during the fabrication sequence. Semiconductors can be oxidized by various methods. These include thermal oxidation, electrochemical oxidation and plasma A silicon dioxide layer is grown in an atmosphere containing either oxygen (O2)

or water vapour(H20) at temperature in the range of 900°C to 1300°C .

The following chemical reactions describe the thermal oxidation of silicon in oxygen or water vapour:

Si(solid) + 2H20(gas) →SiO2(solid) + 2H2(gas)

Chapter 3 :

Fabrication Of The Sensor

The silicon - silicon dioxide interface interface moves into the silicon during the oxidation process. This creates a fresh interface region, with surface contamination on the original silicon ending up on the oxide surface. The system used for Oxidation comprises of:

<a> Gas flow system consists of a nitrogen and oxygen cylinder at the back with the controls at the front to control the flow (Iit/min) of the gases. Then we have the bubbler with temperature controller.

<b> The Furnace consists of a quartz furnace separated from the body by quartz wool. The furnace also consists of a PID controller with 3-zone digital temperature controller along with other facilities like alarm setting.

Chapter 3 :

Fabrication Of The Sensor

The Working:

Step1: The clean silicon wafer is mounted on a quartz stack with two butters. Step2: The temperature of the furnace is set at 1000°C at all the 3 zones.

Step3: The Nitrogen cylinder is opened at 2Kg/cm3 _{and the flow is adjusted at 2lit/min}

for 5min.to purge the furnace.

For Dry Oxidation:

Step4: Then oxygen cylinder is opened at 2Kg/cm3 along with Nitrogen for 2min.

Step5: Now Nitrogen flow is stopped and oxygen at 1litlmin. is made to flow through the furnace for 5min.ln this way dry oxidation is done.

Step6: Reduce the oxygen flow and switch on the bubbler.

For Wet Oxidation:

Step7: Now Oxygen is made to flow through the bubbler with temperature at 93°C and the flow is increased to 11itlmin slowly and kept it for 15min.

Step8: Again dry oxidation is done for 5min.

Step9: The oxidized wafer is brought out from the furnace and the color is observed to find the thickness. Also the wafer is observed under the microscope.

Chapter 3 :

Fabrication Of The Sensor

3.2.3 Photolithography

Chapter 3 :

Fabrication Of The Sensor

Microcircuit fabrication requires the precise positioning of a number of appropriately doped regions in a slide of semiconductor, followed by one or more interconnection patterns. These regions include a variety of implants and windows in protective cover layers through which connections can be made to the bonding pads. A sequence of steps is required, together with a specific layout pattern, for each of these regions lithographic processes are used to perform these operations and are carried out in succession during circuit fabrication. The major steps in lithography are:

<1>Fabrication of masks (or pattern generation); and Lithography is the technique of transferring the pattern on the mask to a layer of radiation sensitive material (resist) which, in turn, is used to transfer the pattern to the films or substrates through etching I processes. The radiation used may be optical, X-ray, electron beam (e-beam), or ion beam. Each technique involves a specialized technology.

<2> Transfer of the pattern to the wafer. In Photolithography, a film of the photo resist is first applied to the substrate. Radiation is shone through a transparent musk plate, on which has been imprinted a copy of desired pattern in an opaque material. The resulting image is focused on to the resist-coated substrate, producing areas of light and shadow corresponding to the image on the mask plate. In those regions where light was transmitted through the plate, the resist solubility is altered by a photochemical reaction. Shadowed areas remain unaffected in solubility. This step is

Chapter 3 :

Fabrication Of The Sensor

preferentially removes the resist areas of higher solubility. This step is called development. Depending on the type of the resist, the washed-away may be either the illuminated or shadowed regions of the coating. A resist that loses solubility when illuminated form a negative image of the plate and is called a negative resist. If exposure increases resist solubility resist is washed away in the areas corresponding to the transparent zones of the mask plate. The resist image is identical to the opaque image on the plate, and the pattern is a photographic positive. Therefore, the resist is called positive resist. After development, the substrate bearing the patterned resist, is exposed to an echant. The enchant removes those portions of the substrate unprotected by the resist while the covered areas remain unetched. Finally, the resist coating is removed and discarded, leaving a duplicate of the mask plate pattern etched into a substrate film.

Chapter 3 :

Fabrication Of The Sensor

The Working:

Step1: The cleaned wafer is put on the top of the spinner hold by vacuum.

Step2: The negative photo resist is spread on the center of the wafer and the spinner is switched at a speed of 400r.p.m. for 25 sec. for uniform distribution.

Step3: The wafer is put inside the furnace for pre-bake at a temperature of 90°C for 20min.

Step4: The wafer is then put inside the furnace at 120°C for 20min.We used special photo resist, which did need hard bake.

Step5: The wafer is then put under the microscope and the vacuum pump is put in the holder of the mask-aligner.

Step6: The wafer is then loaded and the lid is closed and properly aligned by watching under the microscope thereafter.

Step7: The U-V source is then put in proper place and the wafer is then exposed for 5 sec. to U-V ray.

Step8: The film over the wafer is then developed and rinsed in Acetone to remove photo resist.

Chapter 3 :

Fabrication Of The Sensor

3.2.4 Micromachining

Take a solution of 18gm by weight pyrocatachol, 102cc by volume ethylene diamine, and 48 cc by volume DI water in a beaker and heat it to a temperature of 110 °C.Then insert the sample in the solution. The etching rate is observed as 80-90 micron/hour at a constant temperature of 110.The required thickness of the mask is nearly 20 micron for both pressure and temperature sensors.

Chapter 3 :

Fabrication

Of The Sensor

Figure 4(b) : Top View of the micromachined silicon membrane

Chapter 3 :

Fabrication Of The Sensor

3.2.5 Porous Silicon Formation

Porous silicon is a derivative of bulk crystalline silicon . Porous silicon consists of numerous pores and inter-connected silicon crystallites resembling sponge-like / Fur-tree-like structure , the pores ranging from < 2nm to a few microns.

Porous silicon is currently gaining interest in silicon microsystem technology for its multifarious application in sensing and photonic devices .The extremely large surface to volume ratio ( 500 m2 _cm-3_{) of the porous silicon nanostructures, the ease of its}

formation and control of the surface morphology through variation of the formation parameters, its compatibility to silicon IC technology and hence amenability to the development of smart systems-on chip sensors have made it a very attractive sensing material. Porous silicon is a three phase mixture of silicon, oxide and voids. The elastic properties of porous silicon which are drastically different from those of bulk silicon are responsible for the peculiar characteristics of the porous silicon layers: its fragility or mechanical instability[1].

There are different techniques for preparation of porous silicon. Two methods have mainly been applied for the preparation of PS: (i) anodic etching in HF and (ii) chemical (stain) etching. Our studies on PS have primarily been focused on PS

to PS formation by anodic etching in order to increase the porosity of the PS layers. Many of the physical properties e.g. electrical and optical properties of porous silicon differ significantly from those of crystalline silicon and these are closely related to the

Chapter 3 :

Fabrication Of The Sensor

Porous silicon nanostructure which in turn depends on various parameters like formation current density, HF concentration, c-Si surface morphology, doping type and level of c-Si etc[2].

It is commonly prepared by anodic dissolution of Silicon in a solution containing HF and ethanol as shown in figure - 5

Anodic reaction during pore formation:

Figure – 5

Chapter 3 :

Fabrication Of The Sensor

Porous Silicon is Characterized by a term called POROSITY

Porosity = Volumetric fraction of pores with respect to total volume of porous silicon. Porosity ranges generally from 20% to 90%[3].

Figure 6 : FESEM of the nanoporous silicon surface

Figure 7 : Porous Silicon based pressure sensor

Chapter 3 :

Fabrication Of The Sensor

Conventional process

CONVENTIONAL PROCESS

Chapter 3 :

Fabrication Of The Sensor

LATERAL CONTACT CONFIGURATION

Integrated Pressure and Temperature Sensor

References :

[1]

[2] Porous silicon -mechanisms of growth and applications V. Parkhutik Solid-State Electronics 43 (1999) 1121±1141

[3] An integrated pressure and temperature sensor based on nanocrystalline Porous silicon C Pramanik, H Saha and U Gangopadhyay

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor 4.1 Design of nanocrystalline porous silicon based integrated pressure and temperature sensor

Silicon-based piezoresistive pressure sensors have been widely used for various applications ranging from Automobiles, robotics, space, process industries to biomedical applications. But these sensors also show Temperature sensitivity. To compensate for the temperature effect, a p–n-junction-based temperature sensor is often integrated with the silicon pressure sensor for commercial applications [1]. There are also recent reports on the development of an integrated pressure and temperature sensor on silicon in a suspended microchannel for high-pressure flow studies [2]. Integrated pressure and temperature sensors based on silicon and polysilicon have also been reported [3]. However, the existing limitation of such a sensor is low-pressure sensitivity, which makes them incompetent for ultra-low-low-pressure applications such as vacuum measurements in the range of a few millitorrs and in biomedical applications such as respirators. Various methods to improve the sensitivity paved the way to increased nonlinearity along with the requirement of complex fabrication steps [4]. The increasing urge towards improving the performance of pressure sensors without considerably increasing the cost and complexity of fabrication has stimulated the use of nanocrsytalline silicon, which is also IC compatible as an active material for pressure sensors due to its increased piezoresistive coefficient [5].This fact has motivated the use of porous silicon, which is a natural nanocrystalline material fabricated nonlithographically by electrochemical etching of silicon, as the active pressure sensing material. Various nonlithographic techniques have been reported to fabricate well-defined nanostructured porous silicon

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor [4]. The pressure sensing characteristics of the porous silicon–silicon structure have been recently reported [6].In such reports, the structure of the porous silicon–silicon pressure sensor is in the sandwich configuration. An added advantage of such a structure is that it can be used for temperature sensing also since the reverse saturation current of the porous silicon–silicon heterojunction is reported to vary with temperature [7].

4.2 Sensing principles:

4.2.1 Pressure sensing principle: The functional principle of a piezoresistive

nanocrystalline porous silicon based pressure sensor is basically similar to that of a bulk silicon piezoresistive pressure sensor.However, an enhancement in piezoresistive coefficient is expected in nanocrystalline porous silicon due to the change in conductivity of the porous silicon layer caused by the deformation in the quantized valence subbands in the nanocrystallites on application of pressure [4,7]. This deformation leads to a redistribution of the carriers within the different energy bands, which is measured as a change in the resistance of the nanocrystalline membrane as a function of applied pressure. The piezoresistive coefficient in porous silicon can be tailored by changing the dimensions of the silicon nanocrystallites.

4.2.2 Temperature sensing principle: A porous silicon–silicon heterojunction has

been found to possess temperature sensitivity similar to that of a p–n junction and can be used as a temperature sensor. It has been observed that in reverse biased conditions, the porous silicon–silicon heterojunction is more sensitive to temperature change than in the forward biased mode. This is because under the reverse bias,

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor silicon takes place. The injection increases considerably with increasing temperature and subsequently the resistance of the porous silicon layer decreases significantly. In the forward biased condition, the current through the heterojunction results primarily from the electron injection from the aluminum electrode rather than hole flow from the substrate to porous silicon. This electron injection is apparently tunneling current through the potential barrier, which has weak temperature dependence[13].

4.2.3 Integrated pressure and temperature sensor: To fabricate an integrated

pressure and temperature sensor, it is important to maintain an optimum distance between the two sensors such that both the sensors attain a temperature within a small tolerance of say±0.3% and also avoid the cross-coupling effect. For our structure of the pressure and temperature sensor, when the ambient temperature rises to a particular value, the surface of the sensor chip gets heated and a temperature gradient is developed along the lateral direction. This temperature gradient is primarily due to the conduction heat losses laterally through silicon and the convection losses from the edges. The boundary conditions in our case are that there is almost no temperature gradient just at the center and the conducting heat flux towards the edges is equal to the convective heat loss from the edges[13].

4.3 Equivalent Model of the Sensor: To analyze the cross-coupling effect, an

equivalent electrical model diagram of the integrated sensor system as shown in figure 8 has been proposed. It is observed from that the resistance between the two sensors should be large compared to the sensor resistance to avoid any cross coupling.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

.

Figure 8 : Equivalent Model of the Sensor

In the above figure, Rps and Rpst represent the resistance of the porous silicon layer of the pressure and temperature sensors respectively, Rsi and Rsit represent the resistance of the underlying silicon layer in the pressure and temperature sensors, respectively, Rs represents the isolation resistance between the two[13].

From figure 8, it is observed that to avoid cross coupling and maintain a 0.3% variation in temperature, the following relationship (equation (2a)) should be maintained:

Rs >> Rps + Rsi or Rs >> Rpst + Rsit, (2a) and separation between the two sensors d should be such that

d < 0.7L, (2b) where 2L is the length of the sensor chip.

Now, Rs=ñps(Lps-Wps)/2Wpstps+ñsi0.7L/Wpstps+ñpst(Lpst-Wpst)/2Wpsttpst (2c) Rps = ñpstps/WpsWps, Rpst = ñpsttpst/WpstWpst,

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

where Lps, Lpst, Wps and Wpst are the length of the porous silicon layer and contact width of the pressure and temperature sensors, respectively, tps, tpst, tsi and tsit represent the thickness of the porous silicon layer and silicon layer in the pressure and temperature sensors, respectively, ñps, ñps, ñsi and ñsit represent the resistivities of the porous silicon layer and silicon layer in the pressure and temperature sensors, respectively.

Porosity for the temperature sensor is also 55% to minimize the fabrication steps. The temperature sensor is also micromachined to the similar thickness as pressure sensor for a faster thermal response. The area of the temperature sensor is chosen to be 500 µm × 500 µm and the area of the temperature sensor is 600 µm × 600 µm. The total thickness of the pressure and temperature sensor is 20 µm of which the thickness of the porous silicon layer is 15 µm[13].

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

3.4 Design of Conventional Pressure sensor using lateral contacts :

A piezoresistive pressure sensor consists of a thin monocrystaline silicon membrane supported by a thick silicon rim as shown in Figure 5. The diaphragm is fabricated by etching away the bulk silicon on a defined region until a required thickness is reached. Piezoresistors are made by diffusing or implanting into the membrane typically close to the edges. The diaphragm acts like a mechanical stress amplifier. The silicon is not only used as a substrate for the diffused resistors but also as an elastic material [3].

Figure 10 : A Piezoresistive pressure sensor

When a pressure difference is applied across the device, the thin diaphragm will bend downward or upward, indicating traction or compression on the piezoresistors. The resistance change caused by this stress can be easily measured.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor Figure 6 shows the four piezoresistors connected in the Wheatstone bridge configuration. Two resistors are oriented so that they can sense stress in the direction of their current axes and two are placed to sense stress perpendicular to their current flow. Therefore, resistance change of the first two piezoresistors will always be opposite to that of the other two. This is achieved by placing two piezoresistors parallel to opposite edges of the diaphragm and the other two perpendicular to the other two edges. When the is diaphragm bent downwards, causing the tensile stress on the diaphragm surface at the edges, the parallel resistors are under lateral stress and show a decrease in resistance while the perpendicular ones are under longitudinal stress and show an increase in resistance. If the resistors are correctly positioned with respect to the stress field over the diaphragm, the absolute value of the four resistors changes can be made equal.

Figure 11 : Wheatstone Bridge Configuration

The sensor has been designed and simulated using MEMSPRO and ANSYS software tools.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor Dimension of Silicon membrane = 2500 μm × 2500 μm × 50 μm

Dimension of piezoresistors = 20 μm × 75 μm × 2 μm

Dimension of the metal contact pads = 20 μm × 20 μm × 0.5 μm Applied pressure = 0.01MPa perpendicular to the surface. Applied voltage on the contact pads of the piezoresistor = 5V

Figure 12 : Design of the piezoresistive pressure sensor.

a = 2500 µm u = 50 µm x = 75 µm t = 50 µm

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

4.5 Results and discussions :

The design and simulation of the sensor is done by MEMSPRO and ANSYS 10.0 software. Initially the area of the pressure sensor was chosen as 900 µm × 900 µm and the distance between the temperature and pressure sensor was taken as 100 µm to avoid cross-coupling effect.

It is observed from the simulation results of the temperature distribution of the sensor chip that when the pressure sensor is thermally heated to a temperature of say 350˙C due to ambient temperature, then due to conduction of heat through the lateral direction, the temperature sensor senses a temperature of 335˙C as shown in the graph. But the position of the temperature sensor has to be optimized so that it can sense approximately the same temperature as the ambient temperature. So as per the simulation results of the temperature distribution of the sensor , it is placed at a distance of 50 µm from the pressure sensor then it can sense a temperature of 345˙C as shown in figure.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor Figure 13 : Top view of the nanocrystalline porous silicon based integrated MEMS pressure and temperature sensor as designed by MEMSPRO

Figure 14 : Back side of the nanocrystalline porous silicon based integrated MEMS pressure and temperature sensor as designed by MEMSPRO

Chapter 4

:

Figure 16 : Temperature distribution graph, when the distance between the pressure and temperature sensor is 100 μm.

Chapter 4

:

Figure 17 : Stress distribution on the sensor chip on applied pressure on the membrane

Figure 18 : Temperature distribution on the sensor, distance between pressure and temperature sensor is 50 μm.

Chapter 4

:

Figure 19 : Stress distribution on the sensor chip on applied pressure on the membrane Distance between the temperature and pressure sensor is 50 μm.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

Figure 20 : Model of the piezoresistive pressure sensor membrane As done by ANSYS 10.0

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

Figure 21 : Current density distribution on applied voltage on the piezoresistor

Figure 22 : Current density distribution on applied pressure and voltage on the piezoresistor

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

Figure 23 : x-component of stress(σl) on the membrane of pressure sensor

Figure 24 : x-component of stress(σl) along the two sides of the piezoresistors, which are perpendicular to the edges.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor Figure 25 : y-component of stress (σt) on the membrane of pressure sensor

Figure 26 : y-component of stress (σt) along the two sides of the piezoresistors, which are perpendicular to the edges.

Chapter 4

:

The variation of the stress along the other two piezoresistors (i.e., the one parallel to the edges) were found to be equal but opposite to the ones shown in Figure-24. Using the values of x-component of stress and the y-component of stress, it is possible to work out a fractional change in the value of resistance of a P-type piezoresistors below:

∆R

π

(σ

– σ

)

R

2

The Wheatstone bridge configuration converts the resistance change directly to a voltage signal. The differential output voltage (∆V) of an ideally balanced bridge with assumed identical (but opposite in sign) resistance change, ∆R, in response to a differential pressure change ∆P on the sensor, is given by:

∆V

∆R V

R

where R is the zero-stress resistance and Vbias, the bridge supply voltage. The

pressure sensitivity (S) is then defined as the relative change of output voltage per unit of applied differential pressure and is given by:

S = ∆ V 1

∆R 1

∆ P V

∆P R

Note that stress is maximum at the at the edges and minimum at the centre of the diaphragm. The stress at the edges is maximum because the edges are constrained (i.e., no displacement along x, y and z- direction)[14].

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

Figure 27 : x-component of the stress on the surface of the diaphragm in a direction perpendicular to the applied pressure. Note that the stress is maximum at the edges.

Figure 28 : Displacement of the piezoresistors perpendicular to the direction of applied pressure. Note that displacement varies from one end of the piezoresistor to the other.

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

Figure 29 : Current density distribution on the nanoporous silicon pressure sensor On applied voltage on the piezoresistor

Figure 30 : Current density distribution on the nanoporous silicon pressure sensor with applied pressure on the membrane

Chapter 4 : Design And Simulation Of MEMS Pressure And Temperature Sensor

References

[1] Optimized technology for the fabrication of piezoresistive pressure sensors Merlos A, Santander J, Alvarz M D and Campabadal F 2000 J. Micromech. Microeng. 10 204–8

[2] A suspended microchannel with integrated temperature sensor for high pressure Flow studies Wu S, Mai J, Zohar Y, Tai Y C and Ho C M 1997–1998

[3] Fabrication of microdiaphragm pressure sensor utilizing micromachining Fujii T, Gotoh Y and Kuroyanagi S 1992 Sensors Actuators A 34 217–24

[4] Single crystal silicon piezoresistive nanowire bridge Toriyama T and Sugiyama S 2003 Sensors Actuators A 108 244–9

[5] Piezoresistive pressure sensing by porous silicon membrane Pramanik C and Saha H 2006 IEEE Sensors 6 301–9

[6] Design, fabrication, testing and simulation of porous silicon based smart MEMS pressure sensor Pramanik C, Islam T, Saha H, Bhattacharya J, Banerjee S and Dey S 2005 18th Int. Conf. on VLSI Design

[7] Effects of uniaxial stress on hole subbands in semiconductor quantum wells Lee J and Vassell M O 1988 I. Theory Phys.Rev. B 37 8855

[8] Temperature compensation of piezoresistive porous silicon pressure sensor using ANN Pramanik C, Islam T and Saha H 2006 Microelectron. Reliab.46 343–51

[9] An integrated pressure and temperature sensor based on nanocrystalline porous silicon C Pramanik, H Saha and U Gangopadhyay, J. Micromech. Microeng.16 (2006) 1340–1348

[10] A Silicon Piezoresistive Pressure Sensor Ranjit Singh, Low Lee Ngo,Ho Soon Seng, Frederick Neo Chwee Mok

[11] ANSYS 10.0 Help File and Tutorial. [12] MEMSPRO Software.