CN106546368A - A kind of method for characterizing film residual stress - Google Patents

Abstract

The invention relates to a method for characterizing the residual stress of a film, comprising: using a film/substrate structure sample, controlling a laser to emit a short pulse laser beam with a certain frequency and energy, converging into a linear laser beam on the surface of the sample, and generating ultrasonic waves on the surface of the sample Surface waves; detection by piezoelectric sensors, and acquisition of discrete time-domain voltage signals; obtaining experimental dispersion curves; modeling and obtaining theoretical dispersion curves of film/substrate models in the state of no residual stress; comparing experimental dispersion curves with theoretical dispersion The curves are compared to qualitatively determine the magnitude and arrangement of the residual stress of the sample film. The invention can quickly and non-destructively detect the residual stress of the film.

Description

A Method for Characterizing Residual Stress of Thin Films

technical field

The invention belongs to the field of thin film characteristic characterization, and relates to a method for non-destructive detection of thin film residual stress by ultrasonic surface wave technology.

Background technique

The residual stress in the film is divided into compressive stress and tensile stress in terms of the external effect of force. When the compressive stress is too large, the film will buckle, which will weaken the adhesion of the film and separate the film from the substrate; When the tensile stress is too large, the film will wrinkle or even crack. If the excessive residual stress in the thin film cannot be detected in time during the manufacturing process of the integrated circuit and the next step of work is continued, the performance of the integrated circuit will inevitably be seriously affected. Therefore, the detection of residual stress in thin films is of great significance. The principle of ultrasonic surface wave method to measure the mechanical parameters of thin film is that ultrasonic surface wave is dispersed when propagating in the layered structure of film/substrate, and the surface wave velocity is not only related to frequency, but also related to the thickness, density and elastic constant of the film. , residual stress and the density and elastic constant of the base material. By approximating and matching the dispersion curves obtained from the theoretical model and experimental signal processing, the parameters of the film sample can be measured. Based on this, the present invention provides a method for non-destructively characterizing the residual stress of a sample film, that is: based on the theory of acoustoelasticity, a primary theoretical calculation model is established to study the variation law of the dispersion curve under different residual stresses, and through the excitation of the laser with the acoustic The dispersion curve obtained from the experiment of measuring the properties of the film by the surface wave is fitted to qualitatively characterize the magnitude of the residual stress of the film.

Contents of the invention

The purpose of the present invention is to provide a method for quickly and non-destructively detecting the residual stress of a thin film, and effectively characterize the residual stress of a low-k thin film. The technical solution is as follows:

A method for characterizing film residual stress, comprising the steps of:

1) Using a sample with a film/substrate structure, the laser is controlled to emit a short pulse laser beam of a certain frequency and energy, which is finally converged into a linear laser beam on the surface of the sample through an optical adjustment system, and ultrasonic surface waves are generated on the surface of the sample;

2) The surface wave is detected by the piezoelectric sensor after propagating a certain distance on the surface of the sample, and then the discrete time-domain voltage signal is obtained after signal conditioning and data acquisition;

3) Perform mathematical processing including Fourier transform on the collected discrete time-domain voltage signal to obtain the experimental dispersion curve;

4) Substituting other parameters such as the density, Young's modulus, Poisson's ratio, and thickness of the sample film and substrate into the MATLAB theoretical model,

5) In the theoretical model, the stress t is used to represent the size of the residual stress, and t=0, and the program is run to obtain the theoretical dispersion curve of the film/substrate model under the state of no residual stress;

6) Comparing the experimental dispersion curve with the theoretical dispersion curve when the stress t=0, it is stipulated that when t<0, it is expressed as compressive stress, and when t>0, it is expressed as tensile stress, so as to judge the type of residual stress, and then pass Change the value of stress t to find the value of theoretical curve stress t that is closest to the experimental dispersion curve, repeat the test to measure the stress t value of a group of samples, and sort them, so as to qualitatively determine the size and arrangement of the residual stress of the sample film.

Description of drawings

FIG. 1 is a schematic diagram of a laser-excited surface acoustic wave detection system for film residual stress adopted in the present invention.

Fig. 2 The effect of different compressive stresses on the dispersion curve when the surface wave propagates along the Si[100] direction.

Fig. 3 The effect of different tensile stresses on the dispersion curve when the surface wave propagates along the Si[100] direction.

Fig. 4 The effect of different compressive stresses on the dispersion curve when the surface wave propagates along the Si[110] direction.

Fig. 5 The effect of different tensile stresses on the dispersion curve when the surface wave propagates along the Si[110] direction.

detailed description

The experimental curve of surface acoustic wave propagation on the sample surface was measured by the laser-excited surface acoustic wave experimental system. The schematic diagram of the laser-excited surface acoustic wave detection film residual stress system is shown in Figure 1, in which the MNL 801S nitrogen molecular laser is used with a wavelength of 337.1 nm, the average pulse energy is 400uJ. The piezoelectric sensor consists of a polyvinylidene fluoride film (PVDF) and a self-made wedge probe. The amplifier is MITEQ AU-1338 type high frequency broadband voltage amplifier, the digital oscilloscope adopts Tektronics TDS3000B type, the bandwidth is 300MHz, and the highest sampling rate is 2.5GS/s. The sample used in the experiment is a film/substrate structure. In order to make the results more accurate, it is necessary to ensure that the measured samples belong to the same batch and have highly similar Young's modulus, density and Poisson's ratio. During the measurement process, the same set of data should be guaranteed to be the same crystallographic measurements. The specific measurement process is as follows:

(1) The computer controls the laser to emit a short pulse laser beam with a certain frequency and energy. After the optical adjustment system finally converges into a linear laser beam on the surface of the sample, due to the thermoelastic effect, ultrasonic surface waves will be generated on the surface of the sample;

(2) The surface wave is detected by the piezoelectric sensor after propagating for a certain distance on the surface of the sample, and is sampled and stored in the digital oscilloscope after being amplified by the amplifier.

(3) Perform a series of mathematical processing including Fourier transform on the collected discrete time-domain voltage signal to obtain the experimental dispersion curve.

The other parameters of the sample were substituted into the MATLAB theoretical model, and the relevant parameter settings of the film and the substrate are shown in Table 1. In the theoretical model, the stress t is used to characterize the magnitude of the residual stress. Let t=0, run the program to get the theoretical dispersion curve of the film/substrate model in the state of no residual stress.

Comparing the experimental dispersion curve with the theoretical dispersion curve at stress t=0, it is stipulated that when t<0, it is expressed as compressive stress, and when t>0, it is expressed as tensile stress, so the type of residual stress can be judged. When the surface wave propagates along different crystal directions, the analysis is as follows:

(1) It is assumed that the surface wave propagates along the [100] direction of the Si surface. When t is the compressive stress, the frequency-velocity dispersion curve is shown in Figure 2. At a fixed frequency, it decreases with the increase of the compressive stress; When t is the tensile stress, the frequency-velocity dispersion curve is shown in Figure 3. At a fixed frequency, it increases with the increase of the tensile stress. And when propagating in the [100] direction, the change of the curve with a difference of 200MPa is relatively small;

(2) It is assumed that the surface wave propagates along the [110] direction of the Si surface. When t is the compressive stress, the frequency-velocity dispersion curve is shown in Figure 4. At a fixed frequency, it decreases with the increase of the compressive stress; When t is the tensile stress, the frequency-velocity dispersion curve is shown in Figure 5. At a fixed frequency, it increases with the increase of the tensile stress. And when propagating in the [110] direction, the curve with a difference of 200MPa changes relatively greatly.

According to the above rules, the residual stress of the sample film can be sorted. By changing the value of stress t to find the theoretical curve closest to the experimental dispersion curve, so as to determine the value of t corresponding to the tested sample. Repeat the test to measure the stress t values of a group of samples and sort them, so as to qualitatively determine the size and arrangement of the residual stress of the sample films.

Table 1 Parameter setting of film/substrate structure

Claims (1)

1. a kind of method for characterizing film residual stress, comprises the following steps：

1) using the print of film/substrate structure, the short pulse laser beam that laser instrument launches certain frequency and energy, Jing are controlled Crossing optical adjustment system and finally linear beam being pooled on print surface, print surface produces ultrasonic surface wave；

2) surface wave is detected by piezoelectric transducer after certain distance is propagated on print surface, then through signal condition and data acquisition Discrete time-domain voltage signal is obtained afterwards；

3) the discrete time-domain voltage signal to collecting is carried out including the Mathematical treatment including Fourier transform, obtains testing frequency dispersion Curve；

4) will be the density of the film and substrate of print, Young's modulus, Poisson's ratio, thickness theoretical in interior other specification substitution MATLAB In model,

5) in theoretical model with stress t characterizing the size of residual stress, make t=0, operation program then obtain without residual stress Film under state/substrate model theory dispersion curve；

6) experiment dispersion curve is compared with theoretical dispersion curve during stress t=0, it is stipulated that t<Compression is shown as when 0, t>Tension is shown as when 0, its residual stress type is thus judged, then by change stress t value come find out and test frequency The value of the most close theoretical curve stress t of non-dramatic song line, repeats the stress t values that test measures one group of print, is ranked up, depending on Property determines the big minispread of print film residual stress.