Optical measurement method for ultra-precision machined surface microstructure

1. Overview

The surface processing quality of mechanical parts not only directly affects the performance of the parts, but also plays a vital role in the quality, reliability and life of the products. With the rapid development of ultra-precision machining technology, the micro-morphology measurement of ultra-precision machined surfaces has become a key issue to be solved in the field of ultra-precision machining.

Ultra-precision machined surfaces are extremely smooth, with a surface roughness Ra value ranging from a few nanometers to more than ten nanometers. The materials used to machine ultra-smooth surfaces mainly include optical materials such as optical glass, organic glass, and quartz glass, semiconductor materials such as germanium and silicon, and metal materials such as copper and aluminum. The traditional method for measuring surface micro-morphology is the mechanical stylus method, which can directly obtain the profile curve of a certain section of the measured surface through touch measurement. After data processing and analysis by computer, various surface feature parameters close to the real profile can be obtained. Although this type of instrument has a high resolution and a large range (such as the Talystep stylus profilometer with a resolution of up to 0.1nm and a measurement range of up to 100μm), its application in ultra-precision surface measurement is subject to certain restrictions because the sharp diamond stylus can easily scratch the ultra-smooth surface of the measured sample and cause measurement errors during measurement. In recent years, the emergence of scanning tunneling microscope (STM) and its derivative atomic force microscope (AFM) has revolutionized the surface micro-profile measurement technology. This type of instrument not only has ultra-high resolution up to the atomic scale (lateral resolution 0.1nm, vertical resolution 0.01nm), but also can obtain a large amount of information about the atomic structure and functional characteristics of the measured surface. However, STM and AFM have strict requirements on the measurement environment, and need to take good vibration isolation measures and be equipped with complex sensor motion servo control systems. In addition, the instruments are expensive and have a small measurement range. In practical applications, technical problems such as precision vibration isolation technology and piezoelectric ceramic control need to be solved. Since the advent of lasers in 1960, lasers have quickly become ideal light sources for precision optical measurements due to their monochromaticity, good coherence and directionality, and high light intensity. Various types of laser interferometers use the laser wavelength in a vacuum as the length measurement benchmark. The optical measurement method of surface micro-morphology, which mainly uses laser as the measurement light source, can not only achieve high-precision fast non-contact measurement, but also has a simple system structure and low cost. Therefore, it has developed rapidly in the field of ultra-precision surface non-contact measurement. At present, the more mature optical measurement methods mainly include the difference frequency method, scanning method, interference method, diffraction method, etc. At the same time, some new methods are being studied and developed. The following introduces several typical optical measurement methods.

II. Several typical optical measurement methods

1. X-ray interferometer

The structural principle of the X-ray interferometer is shown in Figure 1. The instrument is mainly composed of a beam splitter S, a mirror M and an analyzer A. They are three parallel wafers with cross sections of (111) or (220) made on the same crystal block. The material must be highly complete single crystal silicon, because the lattice spacing of single crystal silicon can be used as the basic measurement unit of nanometer precision. When X-rays are incident on the X-ray interferometer at the Bragg angle, macroscopic moiré interference fringes can be formed behind the analyzer. When the analyzer moves along the normal direction of its reflection crystal surface, the output light intensity changes by one cycle for each lattice spacing. By recording the number of cycles of the output light intensity change, micro displacement measurement can be achieved. Since the silicon lattice spacing is only 0.19nm, the measurement resolution can reach sub-nanometer level. The advantages of X-ray interferometry are high measurement resolution and accuracy, while its disadvantages are high environmental requirements and a relatively small measurement range.

Figure 1 Schematic diagram of the X-ray interferometer structure

2. The optical principle of the differential interferometer

Wollaston prism dual-frequency laser interferometer is shown in Figure 2. The laser outputs light beams with frequencies of f1 and f2, which are left-handed and right-handed circularly polarized light, respectively. After passing through the λ/4 wave plate, the two circularly polarized light beams become linearly polarized light with polarization directions perpendicular to each other. The light beam is divided into two parts by the beam splitter 3. The upward reflected part is used as the reference beam and is focused on the photoelectric element 6 by the lens 5. The polarizer 4 is placed at 45°, so that the light beams of different frequencies that converge on the photoelectric element interfere because they have the same polarization direction. The photoelectric element then converts the change of the interference pattern into an electrical signal and sends it to the amplifier 7. The light beam passing through the beam splitter 3 is the measuring light beam. It passes through the telescope system composed of lenses 16 and 17, and is folded to the Wollaston prism 12 through the plane reflector 15. The Wollaston prism separates the two lights of different polarization directions in the measuring light beam, and then converges them at two points on the surface of the workpiece 14 to be measured through the objective lens 13. The reflected light beam is recombined into a light beam after passing through the objective lens 13, and the light beam is then converged to the photoelectric element 9 through the lens 10 and the polarizer 11. The photoelectric element 9 converts the change of the interference pattern into an electrical signal and sends it to the amplifier 8, and then compares it with the reference signal on the amplifier 7, and then obtains the height change of the measured surface profile through computer processing. The differential interferometer can be used to measure small displacements and small step heights, as well as to measure the microscopic profile of the surface. Since the two detection light spots fall on the workpiece and are very close to each other, they are insensitive to changes in vibration and temperature, and their resolution can reach the order of 0.1nm.

Figure 2 Optical schematic diagram of dual-frequency laser interferometer

3. Coaxial interferometer

The optical principle of the coaxial laser interferometer is shown in Figure 3. The instrument uses a dual longitudinal mode thermally stabilized laser 1 as the light source, and the wave plate 2 divides the laser beam into a reference beam and a measurement beam. The reference beam is projected onto the avalanche diode 3 that receives the reference signal through the polarizer P45° placed at 45° to the polarization direction; the measurement beam passes through the beam splitter 2 to the plane mirror 5, and then passes through the calcite prism 6. The central beam passing through the prism 6 becomes parallel light after being focused on the focal plane of the objective lens 11 by the lens 9. This beam is the reference arm. Through the adjustment of the objective lens 11 and the lens 9, the spot diameter of the reference arm on the surface of the specimen can be changed between 0.1 and 4 mm. The beam separated to the left by the calcite crystal is used as the measurement arm, and the minimum diameter of the beam focused on the surface of the specimen can reach 1 μm. Therefore, when the diameter of the reference spot is large enough, the reference arm is almost unaffected by the profile change, and the measurement arm can detect extremely small changes in the profile of the measured surface. The resolution of the instrument is about 0.5 nm.

Figure 3 Optical principle diagram of coaxial laser interferometer [page]

4. Double-focus interferometer

The optical principle of the double-focus laser interferometer is shown in Figure 4. The polarized light beam output by the He-Ne laser 1 enters the double-focus lens group 5 after passing through the beam expansion and collimation system 2 and the 1/2 wave plate 4. Through the special design of the double-focus lens, the focus of the ordinary light can be made to approach infinity, while the focus of the extraordinary light is located at a finite distance. After the two beams of light pass through the microscope objective 6 that is confocal with the extraordinary light, the ordinary light is focused on the measured surface, and the extraordinary light becomes a thin parallel beam after being collimated by the objective lens and also irradiates the measured surface. The two beams of light are respectively used as the measuring beam and the reference beam. After being reflected back from the measured surface, they are reunited through the double-focus lens, and interference is generated after passing through the half-reflecting mirrors 8, 9, the λ/4 wave plate and the analyzers P1 and P2. The two sets of interference fringes are received by the detectors D1 and D2 respectively. The analyzers P1 and P2 are perpendicular to each other and driven to rotate by the micro motor 11 to generate modulated interference fringe signals. The system can achieve a vertical resolution of Ra2nm. Its disadvantages are that the system structure is not compact, it is easily affected by the drift of electronic devices, and the return light adjustment is difficult.

Figure 4 Optical principle diagram of dual-focus laser interferometer

5. Optical heterodyne interferometer

Since Crane first proposed the principle of optical heterodyne interferometry in 1960, optical heterodyne interferometry technology has been widely used in the fields of displacement, vibration and surface measurement. Figure 5 shows the optical principle of the optical heterodyne Mach-Zehnder interferometer for nanometer measurement. In the figure, M1~M4 are reflectors; AOM1 and AOM2 are acousto-optic modulators; Mr and Mm are reference plane mirrors and measuring plane mirrors respectively; BE1 and BE2 are beam expansion systems; BS1 and BS2 are beam splitters; H1 and H2 are apertures; PD1 and PD2 are photoelectric receivers. The measurement principle of this instrument is to measure the phase difference change of the interference signal output by PD1 and PD2, so as to obtain the displacement of the measuring mirror Mm d = λΔφ/720 (where λ is the laser wavelength and the unit of Δφ is degree). The advantage of this method is that it has strong anti-interference ability and can achieve high measurement resolution through simple phase comparison technology. Its disadvantage is that the nonlinear error is large.

Figure 5 Optical principle diagram of optical heterodyne interferometer

3. Development of optical measurement technology for surface micro-morphology

Since the 1980s, optical measurement methods based on various measurement principles have emerged one after another, such as light sectioning, optical probes, and interference microscopes. Optical probes use focused beams as measurement probes and use different optical principles to detect the small spacing changes of the measured surface micro-morphology relative to the focusing optical system; interference microscopes use the principle of light wave interference to detect surface micro-morphology, and have the advantages of good intuitive surface information and high measurement accuracy. In particular, the application of phase-shift interferometry technology in interference microscopes in recent years has greatly improved its measurement accuracy and speed. Its resolution has exceeded 1A, and its measurement repeatability has reached 0.1A. The technical indicators of the light sectioning method and several optical probes and interference microscope measurement systems are shown in the table below.

In recent years, optical measurement methods for surface micro-morphology have received increasing attention and have been widely used in the field of non-destructive testing. Products have also been gradually commercialized, including color-grade fringe measuring instruments such as FECO interforemeter, Wyko's Mirau fringe scanning interferometer, and Zego's heterodyne interferometer. In 1984, Huang of Lockheed Missile Company in the United States successfully developed an optical heterodyne profiler using optical common-mode suppression technology. In 1985, M. J. Downs of the National Physical Laboratory of the United Kingdom used birefringent crystals to make focusing lenses and successfully developed a dual-focus profiler. These two optical profilers can obtain extremely high resolution, but the disadvantage is that the reference spot size is small, which is easy to cause errors during measurement. In 1986, Panter et al. of the Royal Institute of Technology in Sweden used a collimated reference beam to obtain a reference spot with a larger diameter, solving the problem of too small a reference spot. In 1990, the optical profiler developed by Offide of the University of London in the United Kingdom achieved a vertical resolution of 0.3nm. Many domestic scientific research units have also made some breakthroughs in the research and development of non-contact measurement methods and instruments for ultra-precision surfaces. In 1986, Professor Zhou Zhaofei and others from Chengdu University of Science and Technology successfully developed a coaxial laser profiler, which solved the contradiction between large reference spot and high resolution. In 1990, Gu Lirong and others from Tsinghua University used acousto-optic modulation heterodyne interferometer to measure the disk surface, and obtained a resolution of 1nm with a measurement range of ±30μm. In 1992, You Zheng from Huazhong University of Science and Technology applied differential interferometer to obtain a resolution of Ra1nm. In 1993, Zhuo Yongmo and others from Zhejiang University developed a dual-focus profiler with a vertical resolution of Ra2nm. However, the current research on interferometers in China is basically still a follow-up study. Some of the instruments developed have not yet been commercialized. The measurement resolution is 1 to 2 orders of magnitude lower than the international advanced level, which is far from meeting the needs of ultra-precision machining surface detection in my country.

In summary, compared with stylus profilometers and scanning probe microscopes, the optical measurement method for ultra-precision machining surfaces has the advantages of high resolution, large measurement range, and high measurement accuracy. However, it also has obvious shortcomings, such as easy changes in surface phase, high sensitivity to surface tilt, small range, and difficulty in calibration. In practical applications, there are still problems such as drift, low-frequency response, and vibration recognition that need to be solved. Since the use of optical methods to measure surface topography requires the use of complex and high-precision mechanical scanning mechanisms, the measurement resolution is also affected by mechanical vibration, circuit noise, and motion errors of the mechanical scanning mechanism. In addition, the measurement speed of the optical method is slow, and the adjustment time of the optical system is long. At present, the main development direction of ultra-precision machining surface topography measurement technology is to improve the lateral resolution of the measurement system, realize three-dimensional topography measurement, and online detection. Relevant experts predict that in the next ten years, the optical structure and mechanical structure of optical measuring instruments will not change much, and the main research focus should be on the development of measurement software. Only by paying attention to the development and application of software can the level of ultra-precision surface micro-topography measurement technology be continuously improved. (end)