The LIGA technique uses X-rays from a synchrotron source to expose photoresist. The typical peak wavelength used is 3Â, and at these short wavelengths the X-rays can
CHAPTER 1. INTRODUCTION 43
Figure 1.22: 100pm deep, 1mm diameter SOI gyroscope [69]
penetrate deep into the resist with little diffraction. This allows for high aspect ratio structures to be formed in the resist, and structures up to several millimetres are possible. If a metallic base is used as a substrate to the resist the defined cavities can then be electroplated with metal and thus a replication process is possible.
This process only allows for only one copy to be made because the surrounding resist must be removed before the metallic structure is freed. However if the electroplating is allowed to continue above the height of the resist a hot embossing or an injection moulding die can be made, allowing for mass reproduction.
The LIGA process was originally developed at the Nuclear Research Centre in Karlsruhe, Germany as a technique for manufacturing separation nozzles for the enrichment of uranium [70]. However it was later realised that the technique could be used for microstructure fabrication [71].
LIGA Fabrication Steps
Fig(1.23) shows the LIGA process with the following steps:
1. metallise substrate if silicon is used or prepare solid metal substrate; 2. apply resist;
3. expose resist to short wavelength radiation (X-rays from a synchrotron source) using a mask;
CHAPTER 1. INTRODUCTION 44
5. electroform with metal (usually nickel);
6. overplate and then remove metal plug to create an embossing tool; 7. apply suitable base and then injection mould;
8. release;
9. replate with metal.
Figure 1.23: The LIGA process
The LIGA process seems an ideal method for creating microstructures as tall structures with almost vertical side-walls can be achieved. However there are several drawbacks with LIGA that have prevented it from being widely adopted:
I. The cost. A typical wafer that undergoes aU the steps in the LIGA process costs up to £10,000, including the mask fabrication and the use of a synchrotron source. The LIGA mask needs to be made of an x-ray absorbing material such as gold or tungsten with a thickness of I2pm to I5pm [72] and often cost around £4,000. The x-ray source is usually obtained from a synchrotron and obtaining time on a synchrotron is expensive. In the UK as of March 2002 there is only
CHAPTER 1. INTRODUCHON 45
one available synchrotron lab Synchrotron Radiation Sources (SRS), Daresbury. A second synchrotron ring. Diamond at the Rutherford A ppleton Laboratory, Oxford, is due for completion by September 2006.
2. Incompatibility w ith other integrated processes. Integrating electronics and me chanical devices onto the same wafer is one of the prim ary aims of MEMS, and the high energy radiation sources used in LIGA make this difficult.
Low Cost UV LIGA
LIGA processes based on UV lithography [5, 12, 71-78] have the advantage of being much simpler to im plement than the original x-ray process. They can be carried out using commercially available materials and standard clean room equipment, ideal conditions for an academic environment. This low-cost LIGA approach forms the main fabrication technique used in this thesis. However there are some drawbacks of using optical lithography. In particular the longer wavelength and lower pow er of the source mean that the pattern resolution and the exposure dep th limited. For example, optical absorption limits the maximum depth to around 50pm in conventional positive photoresists, while line w idths are difficult to control to better than -5 p m at this depth due to dffiraction. Using optical lithography rather than x-rays to define a mould in a polyimide Frazier and AUen [79, 80] were the first to transfer the x-ray process to a UV one, though their structures were limited to thickness of -40pm . Despite these problems the consumables are widely available and make up the standard materials found in semiconductor technology.
The low cost LIGA approach also allows for multi-level structures. By repeated steps of lithography and electroforming using a spin-on m ethod of positive photoresist, micro coils have been formed [74]. A multilevel structure was necessary in their design of a planar coil, which required a contact from the centre of the spiral coil to be brought to outside of the device. The process is not limited to w et positive resists as negative dry film resists have also been used [77,81].
M oving Structures U sing Photoresist Sacrificial Layers
As early as 1967 photoresist sacrificial layer processing was used to fabricate resonant gate transistor (RGT) devices [82] (fig(1.24)). In these initial experiments gold was used
as the structural material to create a cantilever beam as a resonant gate for a field-effect
transistor. During the fabrication steps photoresist was used as the sacrificial material, onto which the gold beam was plated. To release the structure the photoresist was removed using an organic solvent, leaving a suspended device.
CHAPTER 1. INTRODUCTION 46 Canlilever - C i te electrode ram B ia s ^ Voltage ^ Output Load Resistor Output Oram Diffusion Input Signal C flannel • Input Force Plate Diffusion Oxide Silicon substrate
(a) geometry and circuit connections of an RGT (b) fabrication steps
Figure 1.24: The Resonant Gate Transistor
A more recent example of a movable structure, which uses a two layer sacrificial photoresist, is described in [83, 84]. The first layer consisted of a thin layer of photoresist (-3pm of Shipley SI828) and was used as the sacrificial layer while the exposed areas were electroplated to form anchors. On top of this sacrificial layer was the device layer created by electroforming a thicker layer of photoresist (12pm of AZ4562). Once the sacrificial layer was removed a suspended structure was formed which could then be actuated by driving its comb electrodes.
m com b electrodes central mass support frame electrode fixed supports
Figure 1.25: Resonator fabricated using a two layer sacrificial process
Other Methods for Structuring Moulds
There are several other methods of structuring polymer moulds. These include reactive ion etching [85] see fig(1.26), excimer [86] and YAG laser micromachining (fig(1.27)). Excimer lasers have also been used in conjunction with halftone masks to create resist moulds with continuous relief fig(1.28).
CHAPTER!. INTRODUCTION 47
W I A A W A
10
Figure 1.26: High aspect ratio polyimide etching. 32pm thick polyimide with 1pm gaps
using O2 plasma using an electron cyclotron resonance source
CHAPTER!. INTRODUCTION 48
•,V H K - - H p K
(a) halftone principle
rm m enni'
(b) stepped relief (c) continuous relief
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