CN109001122B - Apparatus and method for measuring optical constants of gradient or graded refractive index films - Google Patents

Abstract

The invention relates to a device and a method for measuring optical constants of a gradient or graded index film. The device comprises a vacuum system, an ellipsometric parameter detection system, a film thickness monitoring system and an ion source, wherein the vacuum system comprises a vacuum chamber and a sample stage; the film thickness monitoring system comprises a quartz crystal oscillator film thickness meter; the sample stage is arranged at the top of the vacuum chamber, the ion source is arranged at the bottom of the vacuum chamber, and the ion beam output port of the ion source is arranged opposite to the sample stage. The measuring method comprises the following steps: firstly measuring initial ellipsometry parameters of a gradient refractive index film sample, thinning the film by using an ion beam, then measuring the ellipsometry parameters of the thinned film, repeating the thinning and testing steps until the film is thinned below a set step length, and obtaining the refractive index and extinction coefficient of the last film, on the basis, sequentially obtaining the optical constants of all the films on the last film, finally obtaining the optical constant distribution of the film sample along the thickness direction, and realizing the measurement of the optical constants of the gradient refractive index film.

Description

Apparatus and method for measuring optical constants of gradient or graded refractive index films

technical field

The invention relates to the technical field of measuring optical constants of thin films, in particular to a device and method for measuring optical constants of gradient or graded refractive index thin films.

technical background

Commonly used optical films such as anti-reflection films, high-reflection films, filter films, polarizing films, etc. are generally multi-layer film structures obtained by alternate plating with two (or more) coating materials with different refractive indices. In these films, although the refractive index of two adjacent films is different, for the same film, the internal refractive index is approximately uniform and does not change with the thickness, which is a uniform film with stable refractive index. Because there is an interface between the layers of the multilayer film, and the interface is the most likely to be enriched with defects and impurities, the absorption coefficient at the interface is often several orders of magnitude larger than the bulk absorption, and there is a large stress, which often leads to thin film Damage is easy to occur under strong laser, so the existence of the interface is an important reason why the damage threshold of the multilayer film system is not high in the laser system.

On the other hand, many studies have shown that the electric field intensity distribution inside the film is also an important factor affecting its laser damage capability: if the electric field intensity at the interface between the film and the film is large, it is difficult to increase the laser damage threshold of the entire film system . Therefore, it is necessary to continuously optimize the design, seek a reasonable film structure, and keep the peak value of the standing wave electric field away from the interface area, which can improve the laser damage resistance of the film to a certain extent, but due to the limitations of the layered dielectric film itself , it is difficult to keep all the peaks of the electric field away from the film interface, so the effect that can be obtained is very limited.

Since the functions of single-layer films are very limited, the commonly used optical thin films are generally multi-layer films, but the existence of interfaces between layers of multi-layer films limits the increase of the laser damage threshold of thin films. If a thin film with continuously changing refractive index can be prepared so that the interface between the film layers does not exist, the above problems can be avoided. Gradient-index films, or graded-index films, in which the refractive index and extinction coefficient change continuously along the thickness of the film without interfaces, may solve this problem. Therefore, the research on the preparation and application of gradient index thin films has already attracted extensive attention of researchers at home and abroad. However, research progress has been very slow for many years, and the main obstacle is that the optical constants (refractive index, extinction coefficient) of gradient index films cannot be detected. For a uniform single-layer film, there is no problem in the detection of optical constants, and accurate measurement results can be obtained by using an ellipsometer. However, for gradient films, there is still a lack of effective means and methods for detecting the refractive index, which makes it difficult to make breakthroughs in thin film process research. At present, two methods are mainly used for the detection of gradient refractive index films: one is to estimate the gradient distribution of the refractive index first, and then use the optical film system design software to calculate the optical transmittance or reflectance of the film, and then compare it with the actual test If the deviation is large, the distribution of the estimated refractive index is adjusted repeatedly until the calculated value is close to the measured value, that is, the estimated refractive index distribution is considered to be the actual distribution of the sample's refractive index. The second is to start from the process, first clarify the refractive index and extinction coefficient of the uniform single-layer film prepared under different process conditions. Since the preparation of the gradient film is realized by the change of the process parameters, when the gradient film is actually prepared, the process parameters are adjusted according to the pre-design, and the film optical constants corresponding to different process parameters are considered to be the optical constants of the gradient film. Obviously, neither of these two methods is a true reflection of the optical constants of thin film samples, so there is a large deviation from the actual situation.

Contents of the invention

The invention provides a device and method for measuring optical constants of a gradient or graded refractive index film to solve the problem that the prior art cannot measure the gradient or graded refractive index.

For achieving the purpose of the present invention, the technical solution provided by the present invention is:

A device for measuring optical constants of a gradient or graded refractive index film, comprising a vacuum system, an ellipsometric parameter detection system, a film thickness monitoring system and an ion source, wherein the vacuum system includes a vacuum chamber and a sample stage; the film thickness monitoring system includes Quartz crystal film thickness gauge;

The sample stage is installed on the top of the vacuum chamber, and the ion source is installed on the bottom of the vacuum chamber.

Further, the wavelength of the light source is 300-900 nm; the incident angle of the test light is 65°.

Further, the ion energy of the ion source is between 0-3000 eV.

The method for measuring according to the optical constant measuring device of the above-mentioned gradient or graded refractive index film, comprises the following steps:

Step 1. Calibration of the initial geometric thickness of the film sample: use a profiler to measure the geometric thickness of the film to obtain its initial thickness information, and determine the thickness step of each thinning according to the test requirements of the sample;

Step 2. Clamp the sample to be tested on the lower surface of the sample stage, with the coated surface facing the ion source, and vacuumize the vacuum chamber to make it reach the working vacuum of the ion source;

Step 3, using the ellipsometric parameter detection system to measure the initial ellipsometric information of the film sample, i.e. the ellipsometric parameters Ψ _n and Δ _n ;

Step 4. Turn on the ion source, etch the thin film with an argon ion beam to reduce the thickness by one step, and use a quartz crystal vibration film thickness meter to detect the thinned film thickness. The etched film thickness is recorded as d _n , corresponding to Layer L _n ;

The ellipsometric parameters Ψn _-1 and Δn _-1 of the thinned film sample are measured by the ellipsometric parameter detection system;

Step 5. Turn on the ion source again, use the argon ion beam to etch and thin the film, and use a quartz crystal film thickness meter to detect the thinned film thickness. The etched film thickness is recorded as d _n-1 (corresponding to the first L _n-1 layers);

The ellipsometric information Ψn _-2 and Δn _-2 of the thinned film sample are measured again by the ellipsometric parameter detection system;

Step 6. Repeat step 5 until the thickness of the film is reduced below the thickness step, and the film thickness corresponding to the _L1th layer is recorded as _d1 , and the ellipsometric information Ψ ₁ and Δ ₁ of the sample are measured by an ellipsometric parameter detection system;

Step 7, data processing and analysis: according to the values of Ψ ₁ , Δ ₁ and d ₁ , calculate the refractive index n ₁ and extinction coefficient k ₁ of the L ₁ film;

Step 8: Calculate the refractive index and extinction coefficient of L ₂ , L ₃ to L _n layers in sequence, so as to obtain the distribution of optical constants of the film sample along the thickness direction, and realize the measurement of the optical constants of the gradient refractive index film.

Compared with prior art, the advantage of the present invention is:

1. The present invention is not only applicable to the measurement of gradient or graded refractive index films, but also to the measurement of the refractive index distribution of ordinary multilayer dielectric films. It can be used for the analysis of the refractive index and extinction coefficient of multilayer dielectric films, and can truly reflect the optical properties of thin film samples. constant.

2. The present invention can obtain the distribution of optical constants (refractive index and extinction coefficient) of film samples along the thickness direction of the film, which provides a basis for determining the process control of each layer of film.

3. The present invention is not only applicable to the measurement of the gradient or graded refractive index of the medium thin film, but also suitable for the detection of the extinction coefficient and the thickness, and also suitable for the measurement of the distribution of the refractive index of the ordinary multilayer dielectric film.

Description of drawings

Fig. 1 is a schematic view of the device structure of the present invention;

Figure 2 is a schematic diagram of the thinning of gradient index films.

In the figure, 1-light source, 2-polarizer, 3-wave plate, 4-vacuum chamber, 5-sample stage, 6-sample to be tested, 7-ion source, 8-quartz crystal film thickness meter, 9-detection Polarizer, 10-detector, 11-computer.

Detailed ways

The present invention will be further described below in conjunction with the embodiments and accompanying drawings, but the protection scope of the present invention should not be limited thereby.

Referring to the optical constant measuring device of a kind of gradient or graded refractive index thin film shown in Fig. 1, comprise vacuum system, ellipsometric parameter detection system, film thickness monitoring system and ion source 7, described vacuum system comprises vacuum chamber 4 and sample stage 5; The film thickness monitoring system includes a quartz crystal film thickness meter 8;

The sample stage 5 is installed on the top of the vacuum chamber 4, and the ion source 7 is installed on the bottom of the vacuum chamber 4, both facing each other, and the ion beam output port of the ion source 7 is arranged facing the sample stage.

The wavelength of the light source is 300-900 nm; the incident angle of the test light is 65°.

The ion energy of the ion source 7 is between 0-3000 eV.

The vacuum system is mainly used to provide an environment for the normal operation of the ion source, and its vacuum chamber can reach a vacuum degree of 1×10 ^-4 Pa.

The ellipsometric parameter detection system is composed of a light source 1, a polarizer 2, a wave plate 3, an analyzer 9 and a detector 10. The polarizer 2 and the wave plate 3 are sequentially arranged on the incident light path of the light source 1, and the outgoing light An analyzer 9 and a detector 10 are arranged in sequence on the top, and this system is used for measuring ellipsometric parameters of thin film samples. The natural light (wavelength range 300-900 nm) emitted by the light source 1 becomes linearly polarized light after passing through the polarizer 2 , after passing through a quarter-wave plate, through the window set on the vacuum chamber, incident on the surface of the film sample at 65°, reflected by the surface of the film, and then through the window on the other side of the vacuum chamber, through the analyzer 9 After that, it is received by the photodetector 10. According to the relevant theory of polarization and extinction, the ellipsometric parameter detection system can measure the ellipsometric parameters Ψ and Δ of the film, where,

和 />

分别为p光和s光在薄膜表面的反射系数， />

和 />

为p光和s光的位相差。In the formula

and />

are the reflection coefficients of p-light and s-light on the surface of the film, respectively, />

and />

is the phase difference between p light and s light.

The quartz crystal film thickness gauge 8 is used to measure the geometric thickness information of the thin film. After the sample to be tested 6 is clamped on the sample stage 5, the electrodes on its surface are connected to the quartz crystal film thickness gauge, and the film to be tested faces the ion source. After the quartz crystal vibrating film thickness meter starts to work, it is calibrated with the measured film thickness according to the oscillation frequency of the quartz wafer.

The ion source 7 is a cold cathode ion source 7, which constitutes an ion beam etching system. The ion source 7 can emit an argon ion beam, and the ion energy is adjustable from 0-3000 eV. In said embodiment, the output ion energy of the ion source is adjusted to 800eV, and the beam current density is 2mA/cm ² , so as to obtain the output ion beam, which is used to etch and thin the thin film on the surface of the sample.

The computer control system 11 is connected with various functional components to perform data analysis and processing. The geometric thickness of the film is obtained by the quartz crystal film thickness monitoring system. Through the cooperation of the detector 10, the polarizer 2, the wave plate 3 and the analyzer 9, the ellipsometric parameter information of the film surface can be obtained. At the same time, it is used to control the ion energy and output parameters of the ion source 7, as well as the horizontal state of the sample stage and the like.

Referring to Fig. 2, the method for measuring using the optical constant measuring device of the gradient or graded refractive index film provided by the present invention comprises the following steps:

(1) Calibration of the initial geometric thickness of the film sample: before starting to test the optical constants of the gradient film, first measure the geometric thickness of the film with a profiler, the initial thickness is 358nm, and the thickness step of each thinning can be determined to be 10nm . The thickness step can be set according to the specific requirements of the sample. The smaller the step, the higher the resolution, but the lower the test efficiency.

Requirements for sample 6 to be tested: the graded refractive index film to be tested is deposited on a quartz wafer in the same furnace as the workpiece during preparation. Au electrodes are pre-prepared on both surfaces of the quartz wafer, one of which is completely covered by the Au film, and the other surface The periphery is an Au electrode, and the central part is a bare quartz wafer, and the gradient refractive index film to be measured is deposited on the surface. The film to be tested by this device must be plated on a quartz wafer, which is standard on the market and has been pre-plated with electrodes. In order to measure the refractive index distribution of the coating film under a certain process, use a standard quartz wafer that has been coated with electrodes, and then deposit a thin film on the quartz wafer as the sample to be tested 6 .

(2) Clamp the sample 6 to be tested on the sample table 5 in the vacuum chamber, the coating surface on the sample 6 to be tested is facing the ion source 7, and connect the two electrodes on the upper and lower surfaces of the sample 6 to be tested with the quartz crystal film thickness meter 8 connections; the adjustment of the sample stage 5 is driven by two stepping motors, and the levelness of the clamping surface of the sample stage 5 is slightly adjusted, so that the light beam reflected by the film surface completely enters the detector 10, and the sample stage has water cooling, which can Ensure that the temperature of the sample is constant during the test.

Vacuumize the vacuum chamber to make it reach the working vacuum degree of 1×10 ^-3 Pa of the ion source.

(3) Use the ellipsometric parameter detection system to measure the initial ellipsometric information of the film sample, that is, the ellipsometric parameters Ψ _n and Δ _n , which are the comprehensive ellipsometric parameters of the initial gradient film.

(4) Turn on the ion source 7 and etch the thin film with an argon ion beam to reduce its thickness by 10 nm. During the etching process, a quartz crystal vibrator film thickness meter 8 is used to monitor the thinned film thickness, and the removed layer is recorded as the L _n layer, and the corresponding film thickness is d _n (10 nm in this example). The ellipsometric parameters Ψ _n-1 and Δ _n-1 of thinned film samples were measured by an ellipsometric parameter detection system.

(5) Turn on the ion source again and use an argon ion beam to etch and thin the film to reduce its thickness by another 10 nm. During the etching process, a quartz crystal vibrating film thickness meter is used to detect the thinned film thickness, and the thinned layer is recorded as the L _n-1 layer, and the corresponding film thickness is d _n-1 (10 nm in this example). The ellipsometric information Ψ _n-2 and Δ _n-2 of the thinned film samples were measured again by the ellipsometric parameter detection system.

(6) Repeat step 5 until the thickness of the film is reduced to less than 10nm, this layer is recorded as L ₁ layer, and the film thickness is d ₁ (remaining 8 nm). The ellipsometric information Ψ ₁ and Δ ₁ of the sample were measured by an ellipsometric parameter detection system.

(7) Data processing and analysis. According to the values of Ψ ₁ , Δ ₁ and d ₁ , calculate the refractive index n ₁ and extinction coefficient k ₁ of the L ₁ film.

(8) On the basis of the known optical constant and thickness of _L1 layer, calculate the refractive index and extinction coefficient of _L2 , _L3 until _Ln layer in sequence, so as to obtain the optical constant distribution of the film sample along the thickness direction, and draw the refractive index , The variation curve of the extinction coefficient with the thickness of the film realizes the measurement of the optical constant of the gradient refractive index film.

Claims (1)

1. An optical constant measurement method of a gradient or graded index film, which is characterized in that: the method comprises the following steps:

step 1, calibrating initial geometric thickness of a film sample: measuring the geometric thickness of the film by adopting a profiler to obtain initial thickness information of the film, and determining the thickness step length of each thinning according to the test requirement of a sample;

step 2, clamping a sample (6) to be tested on the lower surface of a sample table (5), wherein a coating surface of the sample is opposite to an ion source (7), and vacuumizing a vacuum chamber (4) to enable the vacuum chamber to reach the working vacuum degree of the ion source (7);

step 3, measuring initial ellipsometry information of the film sample by using an ellipsometry parameter detection system, namely an ellipsometry parameter ψ _n And delta _n ；

Step 4, turning on an ion source (7), etching and thinning the film by one step by using an argon ion beam, detecting the thinned film thickness by using a quartz crystal diaphragm thickness gauge (8), and marking the etched film thickness as d _n Corresponds to the L _n A layer;

measuring ellipsometry parameters psi of thinned film sample by using ellipsometry parameter detection system _n-1 And delta _n-1 ；

Step 5, turning on the ion source (7) again, and etching and thinning the film by using an argon ion beam while adopting stoneThe thickness of the film etched by the quartz crystal diaphragm thickness gauge (8) is recorded as d _n-1 Corresponds to the L _n-1 A layer;

re-measuring ellipsometry information ψ of thinned film sample by adopting ellipsometry parameter detection system _n-2 And delta _n-2 ；

Step 6, repeating the step 5 until the thickness of the film is reduced to be lower than the thickness step length, corresponding to the L ₁ The film thickness of the layer is denoted as d ₁ Ellipsometric information ψ of a sample is measured by an ellipsometric parameter detection system ₁ And delta ₁ ；

Step 7, data processing and analysis: according to ψ ₁ ，Δ ₁ And d ₁ Calculating L ₁ Refractive index n of layer film ₁ And extinction coefficient k ₁ ；

Step 8, calculating L in turn ₂ 、L ₃ Up to L _n The refractive index and extinction coefficient of the layer, so as to obtain the optical constant distribution of the film sample along the thickness direction, and realize the measurement of the optical constant of the gradient refractive index film;

the optical constant measuring device of the gradient or graded index film used by the measuring method comprises a vacuum system, an ellipsometry parameter detecting system, a film thickness monitoring system and an ion source (7), wherein the vacuum system comprises a vacuum chamber (4) and a sample table (5); the film thickness monitoring system comprises a quartz crystal oscillator film thickness meter (8);

the sample table (5) is arranged at the top of the vacuum chamber (4), the ion source (7) is arranged at the bottom of the vacuum chamber (4) and is opposite to the vacuum chamber, and an ion beam output port of the ion source (7) is arranged opposite to the sample table;

the light source wavelength of the ellipsometry parameter detection system is 300-900 nm; the incidence angle of the test light is 65 degrees;

the ion energy of the ion source (7) is between 0 and 3000 and eV.

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