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An understanding ellipsometer

Overview of ellipsometer

Ellipsometer is a kind of optical measuring equipment used to detect film thickness, optical constant and material microstructure. Because it does not contact with the sample, does not damage the sample and does not need vacuum, the ellipsometer has become a very attractive measuring equipment. The materials that can be measured by ellipsometry include semiconductors, dielectrics, polymers, organics, metals and multilayers. Ellipsometry covers the fields of semiconductor, communication, data storage, optical coating, flat panel display, scientific research, biology, medicine, etc.

Advantages of ellipsometry

(1) It can measure very thin film (1nm) with high accuracy, which is 1 ~ 2 orders of magnitude higher than that of interferometry.

(2) It is a non-destructive measurement, which does not need to specially prepare samples or damage samples. It is simpler than other precision methods such as weighing method and quantitative chemical analysis method.

(3) The thickness, refractive index and absorption index of the film can be measured at the same time. Therefore, it can be used as an analysis tool.

(4) It is very sensitive to some surface structures, surface processes and surface reactions. It is a method to study surface physics.

In the field of semiconductor manufacturing, in order to monitor the process of film growth/etching on the surface of silicon wafer, its size needs to be measured. Generally, the measured objects are divided into two types:3D structure and 1D structure. 3D structure is the structure closest to the real device, and its measured results have the greatest correlation with electrical properties. The accuracy of 3D structure measurement is generally nano level. 1D structure is the stacking of several layers, dozens or even hundreds of layers of films, which is mainly used to escort the stability of the adjustment process in the early stage of R & D, and its measurement accuracy is generally in the order of Angstroms. As far as logic chips are concerned, the most important measurement object is the measurement of each layer of films at HKMG sites. Because the thickness of each layer of film at these sites is often only a few to more than a dozen angstroms, while the process window is more limited, which is often only 1-1.5 angstroms, that is, the process requirements are very high. These metal layers are closely related to electricity, so every Fab attaches great importance to the measurement of these sites.

How to verify these accuracy? In the Fab, a group of DOE wafers are generally sprinkled:baseline wafers and baseline +/- a few angstroms of wafers, and then each wafer is cut up to two points at the center and edge. Zai uses the TEM or XPS results as the reference value, which is linear with the ellipsometer measurement results. For example, if R-square reaches more than 0.9, it is qualified.

The most accurate way to verify the accuracy of the ellipsometer is the machine for depositing those films, such as the machine of Applied Materials and other companies. Films of different thickness can be deposited by adjusting the cycle number, and its nominal value is often extremely linear with the measured value of the ellipsometer (for example, R-square is above 0.95). But why not use the nominal value of these machines as the reference value? Because these machines themselves adjust their processes based on the values measured by the optical ellipsometer, of course, a third-party notary is required, that is, TEM or XPS.

The principle of optical ellipsometer has been in existence since the 1970s and has been very mature. The measurement of optical ellipsometer is not directly observed like TEM, but through the collection of optical signals and the reverse fitting through physical modeling (adjusting the optical dispersion parameters of the material itself and the 3D structure parameters of the film). What really determines the measurement accuracy is the hardware level, software algorithm, and the experience of physical modeling and parameter adjustment. The hardware level determines the strength of the signal, that is, the signal-to-noise ratio. The software algorithm determines the speed of parameter adjustment in physical modeling. Because physical modeling and parameter adjustment is the most time-consuming process:it is necessary to manually judge whether the calculation is over fitting or under fitting, whether the calculated 3D structure conforms to the manufacturing process, and whether the optical dispersion parameters of materials conform to the physical logic.

Instrument principle

Ellipsometer is an optical measuring instrument used to detect film thickness, optical constant and material microstructure. Due to the high measurement accuracy, suitable for ultra-thin film, non-contact with the sample, no damage to the sample and no vacuum, the ellipsometer has become a very attractive measurement instrument.

The elliptical polarization method involves the reflection of elliptically polarized light on the material surface. In order to characterize the characteristics of reflected light, it can be divided into two components:P and S polarization state. P component refers to linearly polarized light parallel to the incident plane, and s component refers to linearly polarized light perpendicular to the incident plane. Fresnel reflection coefficient r describes the reflection of incident light at an interface. The Fresnel reflection coefficient r of P and S polarization state components is the ratio of their reflected wave amplitude to the incident wave amplitude. In most cases, there will be multiple interfaces, and the light returning to the original incident medium has been reflected and transmitted for many times. The total reflection coefficients RP and RS are determined by the Fresnel reflection coefficient of each interface. RP and RS are defined as the ratio of the final reflected wave amplitude to the incident wave amplitude.

Ellipsometry, a non-contact and non-destructive film thickness and optical property detection technology, measures the change of polarization state when electromagnetic light waves obliquely enter the surface or the interface of two media. Ellipsometry only measures the electric field component of electromagnetic light wave to determine the polarization state, because when light interacts with materials, the effect of electric field on electrons is much greater than that of magnetic field.

Refractive index and extinction coefficient are physical quantities that characterize the optical properties of materials. Refractive index is the ratio of the speed of light in vacuum to the propagation speed of light in materials, n=C/V; Extinction coefficient indicates the absorption of light by materials. For transparent dielectric materials such as silica, light is not absorbed at all, and the extinction coefficient is 0. Both N and K are functions of wavelength, but independent of incident angle.

By measuring the change of polarization state, combined with a series of equations and material film model, the thickness T, refractive index n and absorption index (extinction coefficient) k of the film can be calculated by ellipsometry.

market size

According to gir (Global Info Research), in terms of revenue, the global revenue of ellipsometer in 2021 is about $40 million, and it is expected to reach $51 million in 2028. The Asia Pacific region will play a more important role. In addition to China, the United States and Europe, Japan, South Korea, India and Southeast Asia are still important markets that can not be ignored.

At present, ellipsometry is widely used in OLED, integrated circuit, solar photovoltaic, chemistry and other fields. Some experts believe that with the outbreak of domestic flat panel display, photovoltaic and other industries, the domestic ellipsometer will form a large market of 3 billion yuan to 5 billion yuan. According to expert estimates, about 67%of the global display panel manufacturing is produced in China.

Spectroscopic ellipsometer and laser ellipsometer

根据不同产品类型,椭圆偏振仪细分为:Spectroscopic ellipsometer and laser ellipsometer。

The laser ellipsometer uses a very narrow bandwidth laser as the light source to characterize the surface and interface of nano film samples at a single wavelength. As a conventional measuring tool of nano film, laser ellipsometer

Compared with spectroscopic ellipsometry, it has the following characteristics:

1. More accurate measurement of the optical constants of materials:This is determined by the narrow-band monochromatic properties of the laser. The laser bandwidth is usually much less than 1nm, so the material parameters of materials at the laser wavelength can be obtained more accurately.

2. Fast measurement of dynamic process:the good directivity of laser makes its intensity very high, so it is very suitable for real-time measurement of dynamic process.

However, the ability of laser ellipsometer to analyze multilayer films is insufficient, which is not as good as spectral ellipsometer.

Development of ellipsometry

In 1887, Drude first put forward the ellipsometry theory and established the first set of experimental device to successfully measure the optical constants of 18 metals. In 1945, Rothen first proposed the term”ellipsometer”. Since then, ellipsometry has made great progress and has been widely used in the field of thin film measurement. According to the working principle, ellipsometry is mainly divided into extinction type and photometric type. On the basis of ordinary ellipsometer, ellipsometer, infrared ellipsometer, imaging ellipsometer and generalized ellipsometer are developed.

A typical extinction ellipsometer includes light source, polarizer, compensator, polarizer and detector. The extinction ellipsometer finds out a set of azimuth angles (P, C, a) of the polarizer, compensator and polarizer by rotating the polarizer and polarizer, so as to minimize the light intensity incident on the detector. From these extinction angles, the ellipsometry parameters y and D are obtained.

In the early stage of the development of ellipsometry, as the only light detector, human eyes can only detect the existence or disappearance of signal light, so the type of ellipsometry in the early stage is extinction type.

The measurement accuracy of extinction ellipsometer mainly depends on the positioning accuracy of polarizer, and there are few systematic error factors. However, the azimuth of polarizer needs to be read or calculated during measurement, which affects the measurement speed. Therefore, the extinction ellipsometer is mainly suitable for occasions where the measurement speed is not too high, such as university laboratories. In industrial applications, photometric ellipsometry is mainly used.

The photometric ellipsometer carries out Fourier analysis on the light intensity received by the detector, and then deduces the ellipsometry parameters from the Fourier coefficient. Photometric ellipsometry is mainly divided into rotary polarization device ellipsometer and phase modulation ellipsometer. The rotary polarization device ellipsometer includes rotary polarizer ellipsometer, rotary compensator ellipsometer and rotary polarizer ellipsometer.

The photometric ellipsometer can directly analyze the light intensity signal received by the detector without measuring the azimuth of the polarization device. Therefore, the measurement speed is faster than that of the extinction ellipsometer. It is especially suitable for industrial applications such as on-line detection and real-time measurement.

The research of ellipsometry in China began in the 1970s. In the mid-1970s, China’s first single wavelength extinction ellipsometer tp-75 was designed and manufactured by Professor Mo Dang of Sun Yat sen University. In 1982, the rotary polarizer wavelength scanning photometric ellipsometer (tpp-1) was also invented. Then, in the middle and late 1980s, Xi’an Jiaotong University developed a laser light source ellipsometer, and realized the automation of the ellipsometer in the same period. In 1994, Professor Chen Liangyao of Fudan University developed a new type of automatic ellipsometer that rotates both polarizer and polarizer. This type of ellipsometer has been successfully commercialized and sold to many domestic and foreign units, including Germany. In 1998, Huang Zhiming and Chu Junhao, academicians of Shanghai Institute of Technical Physics, Chinese Academy of Sciences, developed an infrared elliptical polarization spectrometer with rotating polarizer and polarizer at the same time. In 2000, researcher Jin Gang of the Institute of mechanics of the Chinese Academy of Sciences developed the first ellipsoidal microscope imager in China. The instrument can realize nano scale measurement and real-time observation of biomolecular dynamic changes and their interactions. In 2000, Chen Liangyao and Zhang Rongjun of Fudan University developed an infrared ellipsometry system based on double Fourier transform. In 2013, Zhang Chuanwei’s team of Huazhong University of science and technology successfully developed the ellipsometer prototype. In 2014, Professor Liu Shiyuan of Huazhong University of science and technology and others used Muller matrix ellipsometry to test the resist pattern of nano imprint lithography, and also detected the foot asymmetry encountered in the process. The theoretical and experimental results show that the instrument has good sensitivity. In 2015, the first commercial high-end Muller matrix ellipsometer in China was finally successfully launched.

Mainstream manufacturers

Enterprise name

domestic

RuiliScientific instruments

Heneng sunshine

Shared optics

Quantity extensionscience and technology

Saifan photoelectric

WuhanEnjoy光science and technology

abroad

Accurion GmbH

K-Mac

Angstrom Advanced

SeMilliber

J.A.Woollam

HORIBA

Photonic Lattice

Angstrom Sun

Otsuka Electronics

Gaertner

Film Sense

Holmarc Opto-Mechatronics

Onto Innovation Inc.

AQUILA

PARISA TECHNOLOGY

DigiPol Technologies

Sentech Instruments

海洋光学