An Auto-focusing System for White Light Microscopic Measurement
Ming Chang
a,b, Juti Rani Deka
b ,Pei Jung Chen
b, Yu Kuan Chen
b, Changcai Cui
aa
College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou, China 362021;
bDepartment of Mechanical Engineering, Chung Yuan Christian University, Chungli,
Taiwan 320
ABSTRACT
With the rapid development of semiconductor technology the demand for high resolution measuring system is evolving at an ever-increasing pace. Microscope was initially used to detect the defect by connecting charge couple device (CCD) as an auxiliary device. In general, for microscopic measurement human eyes are used to focus on the sample. The adjustment depends on the operator’s astute measurement ability, which affected the repeatability and accuracy of the readings. There is a need of high-speed microscope auto focusing system for industrial applications. The present investigation describes about the development of an autofocus system to carry out microscopic measurement more precisely and accurately with less time.
The measurement system consists of a light source, two beam splitters, a movable sample stage and a Mirau’s interferometer, a photo-detector and 8051 microcontroller (MCU89C51). The light reflected from the sample surface interferes with the light reflected from the reference and produce an interference pattern, which is imaged onto a CCD array. In the setup developed for the autofocus one extra beam splitter is placed in the path of interfered beam to the CCD. The beam splitter is placed at equal distances from the CCD and the photodetector. The focus position is determined from the voltage developed in the photo-detector due to the movement of sample stage of the microscope.
The maximum voltage that obtained at the focus position is confirmed with the CCD image. Microcontroller is used to stop the controller at the focus position immediately once the sample stage reaches it. Software is developed to locate the maximum intensity position. The design may autofocus the interferometer within 4mm distance in 1 second. The auto- focusing not only provides enhanced repeatability and accuracy of the results at a faster rate but also minimizes operator involvement.
Keywords: Autofocus, Mirau’s interferometer, photodetector, microcontroller
1. INTRODUCTION
White light interferometry has been used for many years as a reliable non-contact optical profiling system for measuring step heights and surface roughness in many precision engineering applications. The recent developments in both instrumentation and in measuring software have increased the vertical resolution of these instruments to give a capability of better than 0.1 nm, which makes it a potential practical tool for assessing good semiconductor surfaces. This interferometry is extensively used to determine in field of the surface shape metrology of the semiconductor and TFT- LCD industries. The surface roughness and flatness of wafers, size and co-planarity of stannum-balls and bumps of flip- chips, size and height of spacers in the progress of CF and cell in manufacturing of TFT-LCD are measured with white light interferometry. The surface profile of the fiber and micro optics are also monitored with white light interferometry now a day.
* Corresponding author: Tel: +886(3)2654303; Fax: +886(3)265439; Email: [email protected]
As mentioned, white light interferometric technique is developed giving priority in the automatic optical inspection (AOI) field. Many organizations in Taiwan such as ITRI, CHROMA, CARMAR-TECH, HIROSE-TECH etc have been
using this technique and developed interferometers. Autofocussing techniques is one of the most important issue in these systems and has been using in many fields, such as a multilayered optical disk for high density storage,microscopic image analysis of defect areas in optical disks, multiple blurred objects image restoration, and surveillance camera system [1]. As a consequence, development of a high speed autofocussing system in microscopic measurement is necessary for industrial applications. Autofocussing systems in general can be divided into two categories. One type deal with the development of algorithm to obtain the focus from the captured image and the other type set a hardware system on the microscope to obtain the focus position. Although the frameworks of these systems developed by those companies have been integrated but lack of the autofocus system and use of autofocus algorithm make the measurement procedure slow.
The field of view and operating distance is small in microscopic measurement systems. At such small operating distance the focus point should be detected precisely as it affects the quality of the image and measurement precision to great extent. Most of the industries develop inspection system on the microscope which uses the signal captured by CCD and transfer the light intensity to measurable voltages by proper timer and 1D signal to 2D image followed by visual processing algorithm to reach the correct focal point. But this method suffers from the back and forth motion error near the focus point and can not meet the fast autofocussing condition as required for the industrial applications. Besides, the sampling rate of common commercial CCD can just reach 30 fps and hence not suitable for high speed autofocussing system. There is need of a measuring system which may autofocus on the sample surface at high speed and low cost. Use of proper hardware may increase focus speed and precession drastically. Utilizing the characteristics of light and proper photo detector the maximum intensity peak position through the motion can be detected precisely which substantially increases the speed and precision of detection of the exact focus position.
Development of a high speed low cost autofocussing system for white light microscopic measurement is presented in this investigation. The system consists of broad bandwidth light source, two beam splitters, a motorized sample stage, a Mirau’s interferometer, a photo detector and a microcontroller. When broad bandwidth light is used as a source in an interferometer, the modulation or visibility of the fringes drops off rapidly from its maximum value at the minimum OPD and diminishes to zero at distances greater than the coherence length of the source. For autofocussing a stepper motor is used to tune the sample stage for focus position of the microscope. The Z stage position of the microscope is calibrated to the corresponding voltage developed in the photo detector. The maximum intensity position of the interferogram is determined from the successive subtraction of the voltage developed at two consecutive sample stage position. The designs can autofocus the microscope within 4mm distance in 1 second, which is independent on how far it was initially from focus.
2. SYSTEM MECHANISM
2.1 Experimental setup
Speed is a very important attribute for autofocussing systems. The experimental system broadly consists of a broad band width light source, two beam splitters, a motorized sample stage, a Mirau’s interferometer, a photo detector and one 8051 microcontroller. Figure 1 shows the schematic diagram of the experimental system. The shaded block in Figure 1 represents the new components attached to the existing setup. The photo detector (THORLABS DET10A) is used to determine the voltage developed at the focus position and one 8051 microcontroller to determine the exact focus position during the measurement. A brief description of the different apparatus used in the development system is given below.
2.1.1 Mirau interferometer
A two beam Mirau interferometer (Nikon, 403492) of magnification 50X is used at the present study. In general, the incoming light from the light source is incident on the interferometer and get split inside it. One of the beams goes to the reference surface inside the interferometer and the other to the sample. After reflection from the sample surface the beams recombine inside the interferometer, undergoing constructive and destructive interference and produces light and dark fringe pattern corresponding to the optical path difference. A typical interferogram due to the variation of the optical path difference is shown in Figure 2. White light interferometer uses a broad band width light source and a CCD camera as a detector. It uses vertical scanning technique for surface profile measurement of an object [2-3]. Due to the
large spectral bandwidth of the source, its coherence length is short and a good contrast fringes will be obtained only when the two paths of the interferometer are closely matched. The surface profile can be obtained by changing the position of the reference mirror in the interferometer. With the change in position the intensity changes and produces an interferogram and the relative surface height is detected by eliminating the modulated peak from the interferogram at that point [4-6]. Modifications of this typical interferometer design have been done with the addition of a photo detector and a microcontroller for rapid determination of the focus position.
2.1.2 Photo detector (THORLABS DET10A)
Photo detector is a device that generates an analog electronic focus signal proportional to the intensity of reflected beam.
THORLABS DET10A photo detector is a high speed silicon detector of 1ns rise/fall time which has peak wavelength 750nm and peak sensitivity 0.45 A/W. The output signal is the direct photocurrent out of the photo diode anode and is a function of the incident light power (P) and wavelength (λ). The amount of photocurrent is estimated from the spectral
responsivity S(λ).
2.1.3 8051 microcontroller
In a microcontroller, the program, code and data are stored in different memory areas.8051 contains internal ROM and RAM, I/O ports with programmable pins, timers and counters, serial data communication. 8051 architecture consists of the following specific features:
· 8 bit CPU with registers A and B
· 16 bit PC &data pointer (DPTR)
· 8 bit program status word (PSW)
· 8 bit stack pointer (SP)
· Internal ROM or EPROM (8751) of 0(8031) to 4k (8051)
· Internal RAM of 128 bytes.
· 4 register banks, each containing 8 registers
· 80 bits of general purpose data memory
· 32 input/output pins arranged as four 8 bit ports: P0-P3
· Two 16 bit timer/counters: T0-T1
· Two external and three internal interrupt sources
· Oscillator and clock circuits
Fig.2: Interferogram produced by white light interferometry
Photo detector
Fig.1: Schematic diagram of the autofocus system
Light
Sample CCD
Movable stage
Fig. 3(a) Optical profile and CCD image of
a disc Fig. 3(b) Sample stage movement along Z direction
versus voltage developed at the photodetector
0.1000 0.2000 0.3000 0.4000 0.5000 0.6000
900.0 930.0 960.0 990.0 1020.0 1050.0 Stage distance (mm)
Photodector voltage ( V )
0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000
998.0 1000.0 1002.0 1004.0
2.2 Auto focusing
The microscope used in the present investigation consists of a white light source as illuminator, eyepiece, objectives and a stage, which supports the specimen being viewed. The microscope has a CCD camera, which is attached to a computer.
The stage position has to be adjusted properly to get the focus position. The adjustment depends on the operator’s astute measurement ability, which affected the repeatability and accuracy of the readings. Moreover, this is a time consuming process and required constant operator intervention. The microscope uses a stepper motor for its stage movement. In the modified design one extra beam splitter is placed in the imaging radiation path of the optical microscope. The beam splitter routes the reflected beam towards the photodetector and the transmitted beam towards the CCD camera of the microscope. The position of the beam splitter determines the degree of focusing as the change in the brightness of the image is directly determined as its function. To obtain high degree of focusing and make sure of the focus position the beam splitter is placed exactly at equal distance (d) from both the CCD and photodetector. The intensity of the reflected beam is acquired from the photocurrent developed in the photodetector.. This photocurrent is converted to a voltage (Vout) by adding an external load resistance, R
Load. The output power is observed with a digital voltmeter (DVM). The output voltage is derived using the relation V
Out = P * S(λ) * R
Load .
A compact disc (CD) is placed on the sample stage and allowed to move along the Z-direction for initial automatic detection of focus position. The change in voltage acquired by the photodetector is monitored with the change in stage position. The corresponding CCD image is also viewed in the computer monitor. It is observed that at the position of the sample stage where the CCD forms the clear image, the photodetector also gives the maximum voltage i.e. the maximum voltage position corresponds to the focus position of the system. Figure 3(a) shows the optical profile and the CCD image of the disc and Figure 3(b) shows the corresponding voltage obtained at the focus position. In the Figure 3(b) a sharp peak is observed at the position of 1001µm followed by two weak peaks in both sides of the sharp peak. The inset of Figure 3(b) shows two peaks of less intensity in both sides of the peak position. These peaks may appear due to the change in field of view with the change in the stage position. Find out the maximum voltage position during the stage movement is the prime intention for the determination of focus position. The difference in voltage at consecutive stage position is noted and the position where this difference becomes zero is considered as the focus position. To reduce the time required by the system to search out the focus position hardware is developed using one 8051microcontroller.
Figure 4 shows the block diagram of the development of the autofocussing system. Software has been developed to detect the focus position. The approach is to make maximum use of the existing/available hardware although it required customized design of the additional circuitry involving manipulation of the corresponding voltage levels of the photodetector.
The voltage developed at the photodetector output is amplified using a low power dual operational amplifier LM358.
This is used as it has a low input offset voltage of 2 mV and large output voltage swing. The amplified analog voltage is
1 Lo,,e Profile
60,000 00,000 40,000- 30,000- 20,000- 10,000-
0 20 30 40
(pm)
IL
00 60 70 00 90 100 110 120 130 140 100 160 170 160 190 200
MgIe: -21.5
converted into digital using an 8-Bit A/D converter ADC0804. ADC0804 has differential analog voltage and can be easily interfaced to all microprocessors and controller as it does not need interfacing logic or operates stand alone mode.
The output of the ADC0804 is used as the input of the microcontroller. The main goal of using 8051 is to stop the sample stage of the microscope once it reaches the focus position and should be at very high speed. Port 1 of the 8051is used as the output port, which is connected to the stepper motor control circuit of the microscope. Port 0 is used as in/out port, it takes the input from the ADC0804 and gives the output to the stepper motor control circuit. This work has considered three seconds for 4mm adjustable distance. As the sample stage moves on either side of the focus position, the photodetector detects the voltage and program is written using 8051microcontroller to stop the sample stage at the focus position. The program is written in such a way that it also compares the consecutive input voltage and commands the output port to stop when this difference becomes zero.
3. RESULTS AND DISCUSSION
The experimental system is used to find the step height and the surface profile of a CCD chip. It is observed from the CCD image of Figure 5(a) that the surface consists of three layers. Hence the sample stage needs three different focus positions. Figure 5(b) shows three sample stage positions corresponding to the three layers. The focus positions of the three layers are found to be at 810 µm, 833µm and 858µm respectively. The heights of the three layers are measured with Chroma 7502 interferometer. The line profile of the CCD chip image with Chroma 7502 is shown in Figure 6. The heights of the second and third layers are measured to be 22.85µm and 47.13µm respectively with respect to the top layer.
Fig. 6: Line profile of the CCD Fig. 5(a): CCD image of the CCD
Fig. 5(b): Focus position of three layers
0.8600 0.8800 0.9000 0.9200 0.9400 0.9600
800.0 820.0 840.0 860.0 880.0 900.0
Distance (µm)
Voltage ( V )
Fig. 4: Block diagram of autofocus system Photo
Detector
R C Filter Circuit
Operation Ampifier
R C Filter Circuit
ADC Converter
RS232 Serial Z-axis
Stage
CONTROLLER MCU
89C51
These heights are approximately equal to the difference in focus position of the three layers which are 23µm and 48µm respectively.
The experimental results shows that the system developed can not only adjust the focus position at high speed but also can measure the surface height of unknown samples within 1 second. Comparison of autofocussing system and manual measurement facility is given in table 1.
Table 1: Comparison of auto-focusing system and manual measurement setup
Performance Auto-focus system Manual setup
Scanning range 4mm 4mm
Time 1 second 3-4 minutes
Repeatability Excellent (Average difference of two data taken at different times <1%)
Good (after averaging three sets)
4. CONCLUSIONS
An autofocus system is developed in this study using Mirau’s interferometer, beam splitters, photodetector and a microcontroller. The system is calibrated using a disc whose surface profile is already known. The surface profile and the heights between three different layers in a CCD chip are measured with the developed system. It is observed that the surface height can be determined from the difference in focus position of the system directly. The results obtained are verified with the Chroma 7502 interferometer which shows quiet similar results. The system can automatically adjust the focus position depending on how far it was initially. The repeatability of the results is examined by measuring the surface height of the disc at the same locations at ten different times. The system can detect the focus position within three seconds and allow rapid acquisition of surface profile. The customized design and integration of microcontroller in the development of the system reduces the data acquisition time and makes the system faster for locating the focus position. Besides rapid detection of focus position automatically, low cost of the system components is also an added advantage of using this technique in comparison to other techniques.
5. ACKNOWLEDGEMENT
This research is supported by the science and technology key project of Fujian Province, China, under project number 2008I0020, and the project of the specific research fields in the Chung Yuan Christian University, Taiwan
.
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