A method for measuring the thickness of an ultra-thin polyimide film

Abstract

The present invention relates to a method for measuring the thickness of a polyimide film. Step 1: Spin-coat a polyimide precursor solution on a SiO2 / Si substrate, and perform a first heating to achieve semi-curing of the film to obtain a semi-cured film substrate. Step 2: Cut the semi-cured film substrate to obtain a cross-section, perform a second heating to obtain a fully cured film, and then perform thermal imidization to form a polyimide film. Step 3: Observe the cross-section under a scanning electron microscope and measure the film thickness. When measuring the thickness of a polyimide film according to the method of the present invention, different from traditional film testing methods, the sample preparation is simple, the testing method is convenient, and the data presentation is intuitive and accurate.

Description

Technical Field

[0001] The present invention belongs to the field of microscopic measurement, and particularly relates to a method for measuring the thickness of an ultra-thin polyimide film. Background Art

[0002] Polyimide is one of the organic polymer materials with the best comprehensive performance. Due to the unique imide ring in its structure, polyimide has good high and low temperature resistance, solvent resistance, and excellent mechanical properties, and has been widely used in many fields such as aviation, aerospace, microelectronic devices, displays, gas separation, fuel cells, etc.

[0003] With the development of the microelectronics and electronic device fields towards miniaturization, people pay more attention to the manufacture of devices with characteristics such as flexibility, thinness, and rollability. Therefore, the research demand for ultra-thin and uniform polyimide films with controllable thickness is increasing day by day.

[0004] There are many methods for measuring the thickness of a film. For example, Patent CN111336932B discloses a microscopic differential reflection spectroscopy method for measuring the thickness of a nano-film. Patent CN113405486B discloses a method for measuring the thickness of a film based on the time-frequency domain analysis of white light interference. Patent CN12361972A discloses a method for measuring the thickness of a multi-layer film using an ellipsometer. Patent CN112729133A discloses a method for measuring the thickness of a film based on the diffraction intensity of a detection grating. The above methods all belong to the category of optical measurement methods. The required measuring instruments are relatively complex and precise, and software is needed to establish a model for data analysis, and then the film thickness data is obtained. The thickness data depends on the model, sometimes there will be deviations, and the result presentation form is not intuitive.

[0005] A scanning electron microscope can very intuitively perform microscopic imaging on the film, and use software to accurately measure the film thickness of the sample and display the measured film thickness. The accuracy of its measurement mainly lies in whether the sample itself is effectively prepared. Often, for many samples, if they are not prepared by appropriate electron microscopy sample preparation means, the structure of its cross-section is false, then the film thickness measured based on the scanning electron microscope image is not real. Summary of the Invention

[0006] The present invention aims to provide a method for measuring the thickness of an ultra-thin polyimide film, effectively solving the problem that the existing measurement methods cannot accurately measure the film thickness.

[0007] To solve the above problems, the present invention is implemented by adopting the following technical solutions:

[0008] A method for measuring the thickness of an ultra-thin polyimide film, comprising:

[0009] Step 1: Spin-coat the polyimide precursor solution on the SiO2 / Si substrate, and perform a one-time heating to achieve semi-curing of the film to obtain a semi-cured film substrate;

[0010] Step 2: Cut the semi-cured film substrate to obtain a cross-section, perform a secondary heating to obtain a fully cured film, and then perform thermal imidization to form a polyimide film;

[0011] Step 3: Observe the cross-section under a scanning electron microscope and measure the film thickness.

[0012] Optionally, the temperature of the one-time heating is lower than 100°C, and the duration does not exceed 60 s.

[0013] Preferably, the temperature of the one-time heating is 50°C, and the duration is 40 s;

[0014] or 55°C for 35 s;

[0015] or 60°C for 30 s.

[0016] Optionally, the temperature of the secondary heating is lower than 100°C, and the duration is not less than 60 s.

[0017] Preferably, the temperature of the secondary heating is 60°C, and the duration is 60 s;

[0018] or 70°C for 50 s;

[0019] or 80°C for 40 s.

[0020] Optionally, the polyimide precursor solution is obtained by polycondensation of diamine and dianhydride monomers in a polar solvent to obtain a polyamic acid solution with a viscosity of 2000 - 30000 cP.

[0021] Optionally, the SiO2 / Si substrate also needs to be pretreated. The process conditions of the pretreatment are: first use an air gun to blow off the surface dust, and then use O2 Plasma and UV-ozone for surface hydrophilic treatment.

[0022] Optionally, the process parameters of the spin-coating are: the rotation speed in the first stage is 500 rpm, and the duration is 10 - 30 s; the rotation speed in the second stage is 1000 - 9000 rpm, and the duration is 50 - 120 s.

[0023] Optionally, the thermal imidization to form a polyimide film is as follows: under atmospheric conditions, the heating rate is 2 - 10°C, heat from room temperature to 70°C for 1 - 4 h, 150°C for 2 - 4 h, 200°C for 1 - 4 h, 250°C for 1 - 24 h, and then cool to room temperature.

[0024] A method for preparing an ultra-thin polyimide film, comprising:

[0025] Step 1: Spin-coat a solution of a polyimide precursor on a SiO2 / Si substrate, and perform a primary heating to achieve semi-curing of the film to obtain a semi-cured film substrate; the primary heating temperature is lower than 100 °C, and the duration does not exceed 60 s;

[0026] Step 2: Cut the semi-cured film substrate to obtain a cut surface, perform a secondary heating to obtain a fully cured film, and then perform thermal imidization to form a polyimide film; the secondary heating temperature is lower than 100 °C, and the duration is not less than 60 s;

[0027] The thickness of the polyimide film is 2-22 μm, the tensile strength is 120-164 MPa, the elongation at break is 28-140%, and the uniformity is 2%-6%.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] The method for measuring the film thickness of the present invention can very intuitively perform microscopic imaging on the film, the sample preparation process is simple, effective stress-free "cutting" is achieved, the true cross-sectional structure is obtained, and it is convenient to accurately measure the film thickness. Description of the Drawings

[0030] The drawings are used to provide a further understanding of the present invention and constitute a part of the specification, and are used together with the following specific embodiments to explain the present invention, but do not constitute a limitation to the present invention. In the drawings:

[0031] Figure 1 is a flow chart of the film thickness measurement method of the present invention;

[0032] Figure 2 is a measurement diagram of the film of Example 1 based on the present invention under a scanning electron microscope;

[0033] Figure 3 is a measurement diagram of the film of Example 2 based on the present invention under a scanning electron microscope;

[0034] Figure 4 is a measurement diagram of the film of Example 3 based on the present invention under a scanning electron microscope;

[0035] Figure 5 is a measurement diagram of the film of Example 4 based on the present invention under a scanning electron microscope;

[0036] Figure 6 is a measurement diagram of the film of the present invention by ordinary cutting sample preparation under a scanning electron microscope. Specific Embodiments

[0037] The content of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made. These all belong to the protection scope of the present invention.

[0038] The method for measuring the thickness of the ultra-thin polyimide film of the present invention comprises the following steps: (1) Spin-coating the polyimide precursor polyamic acid solution on the SiO2 / Si substrate; the process parameters of spin-coating are: the rotation speed in the first stage is 500 rpm, and the duration is 10 - 30 s, the rotation speed in the second stage is 1000 - 9000 rpm, and the duration is 50 - 120 s; (2) Place the SiO2 / Si substrate on a hot plate, and heat it once at a temperature below 100 °C for a duration not exceeding 60 s; for example, 50 °C - 40 s or 55 °C - 35 s or 60 °C - 30 s, to semi-cure the film; (3) Use a silicon wafer knife to cut the SiO2 / Si substrate carrying the semi-cured film; (4) Place the cut SiO2 / Si substrate on the hot plate while keeping the cross-section non-contact and non-destructive, and heat it twice at a temperature below 100 °C for a duration not less than 60 s; for example, 60 °C - 60 s or 70 °C - 50 s or 80 °C - 40 s, to form a uniform and stable film; (5) Put it into an oven for thermal imidization to form a polyimide film; (6) Observe the cross-section of the SiO2 / Si substrate carrying the film under a scanning electron microscope and measure the film thickness. When measuring the thickness of the polyimide film according to the method of the present invention, different from the traditional film testing method, the sample preparation is simple, the testing method is convenient, and the data presentation is intuitive and accurate.

[0039] The polyimide precursor solution in step (1) is a polyamic acid solution with a viscosity of 2000 - 30000 cP obtained by polycondensation reaction of diamine and dianhydride monomers in a polar solvent;

[0040] The process parameters of spin-coating in step (1) are: the rotation speed in the first stage is 500 rpm, and the duration is 10 - 30 s, the rotation speed in the second stage is 1000 - 9000 rpm, and the duration is 50 - 120 s;

[0041] The process parameters of heat treatment for film formation in step (5) are: under atmospheric conditions, the heating rate is 2 - 10 °C, heating from room temperature to 70 °C for 1 - 4 h, 150 °C for 2 - 4 h, 200 °C for 1 - 4 h, 250 °C for 1 - 24 h, and finally slowly cooling to room temperature.

[0042] The inventors also found that the method of the present invention can also be used to prepare ultrathin films. The thickness of the polyimide film is 2 - 22 μm, the tensile strength is 120 - 164 MPa, the elongation at break is 28 - 140%, and the uniformity is 2% - 6%.

[0043] A method for preparing an ultrathin polyimide film, comprising:

[0044] Step 1: Spin-coat a polyimide precursor solution on a SiO2 / Si substrate, and perform a first heating to semi-cure the film to obtain a semi-cured film substrate; Step 2: Cut the semi-cured film substrate to obtain a cut surface, perform a second heating to fully cure the film, and then perform thermal imidization to form a polyimide film; The first heating is carried out at a temperature below 100°C for a duration not exceeding 60 s. It can be 50°C for 40 s, or 55°C for 35 s, or 60°C for 30 s. The second heating is carried out at a temperature below 100°C for a duration not less than 60 s. It can be 60°C for 60 s, or 70°C for 50 s, or 80°C for 40 s.

[0045] Example 1:

[0046] A method for measuring the thickness of a polyimide film using a scanning electron microscope, adopting the following method:

[0047] (1) Spin-coat a polyamic acid solution of a polyimide precursor with a viscosity of 13000 cP on a Si substrate with 285 nm SiO2 and 100 crystal phases; The spin-coating speed is 500 rpm for 30 s and 3000 rpm for 60 s;

[0048] (2) Place the SiO2 / Si substrate on a hot plate, keep it at 50°C for 40 s, and semi-cure the film;

[0049] (3) Use a silicon wafer knife to cut the SiO2 / Si substrate carrying the semi-cured film;

[0050] (4) Place the cut SiO2 / Si substrate on a hot plate while keeping the cut surface non-contact and non-destructive, keep it at 60°C for 60 s to form a uniform and stable film;

[0051] (5) Place the SiO2 / Si substrate carrying the film in an oven for staged heat treatment to form a film by thermal imidization. The heat treatment process parameters are: the heating rate is 4°C, heat up from room temperature to 70°C and keep it for 1 h, 150°C for 2 h, 200°C for 1 h, 250°C for 1 h, and slowly cool to room temperature.

[0052] (6) Attach the SiO2 / Si substrate carrying the film to the vertical stage of a scanning electron microscope, keeping the cross-section in step (3) above as the observed surface. Observe the cross-section under the scanning electron microscope with an acceleration voltage of 3.00 kV, a magnification of 3.00 Kx, and an imaging mode of SE2, as Figure 2 shown, and measure the film thickness to be 9.230 μm.

[0053] Example 2:

[0054] A method for measuring the thickness of a polyimide film using a scanning electron microscope, which adopts the following method:

[0055] (1) Spin-coat a polyimide precursor polyamic acid solution with a viscosity of 13000 cP on a 285 nm SiO2, 100 crystal phase Si substrate; the spin-coating speed is 500 rpm for 30 s and 2000 rpm for 60 s;

[0056] (2) Place the SiO2 / Si substrate on a hot plate and keep it at 50 °C for 40 s to semi-cure the film;

[0057] (3) Use a silicon wafer knife to cut the SiO2 / Si substrate carrying the semi-cured film;

[0058] (4) Place the cut SiO2 / Si substrate on a hot plate at 60 °C for 60 s while keeping the cross-section non-contact and non-destructive to form a uniform and stable film;

[0059] (5) Place the SiO2 / Si substrate carrying the film in an oven for staged heat treatment to form a film by thermal imidization. The heat treatment process parameters are: the heating rate is 4 °C, heating from room temperature to 70 °C and holding for 1 h, 150 °C and holding for 2 h, 200 °C and holding for 1 h, 250 °C and holding for 1 h, and slowly cooling to room temperature.

[0060] (6) Attach the SiO2 / Si substrate carrying the film to the vertical stage of a scanning electron microscope, keeping the cross-section in step (3) above as the observed surface. Observe the cross-section under the scanning electron microscope with an acceleration voltage of 3.00 kV, a magnification of 3.50 Kx, and an imaging mode of SE2, as Figure 3 shown, and measure the film thickness to be 10.02 μm.

[0061] Example 3:

[0062] A method for measuring the thickness of a polyimide film using a scanning electron microscope, which adopts the following method:

[0063] (1) Spin-coat a polyimide precursor polyamic acid solution with a viscosity of 20000 cP on a Si substrate with 285 nm SiO2 and 100 crystal phase; the spin-coating speed is 500 rpm for 30 s and 2500 rpm for 60 s;

[0064] (2) Place the SiO2 / Si substrate on a hot plate and keep it at 50 °C for 40 s to semi-cure the film;

[0065] (3) Use a silicon wafer knife to cut the SiO2 / Si substrate carrying the semi-cured film;

[0066] (4) Place the cut SiO2 / Si substrate on a hot plate while keeping the cut surface non-contact and undamaged, and keep it at 60 °C for 60 s to form a uniform and stable film;

[0067] (5) Place the SiO2 / Si substrate carrying the film in an oven for staged heat treatment to form a film by thermal imidization. The heat treatment process parameters are: the heating rate is 4 °C, heating from room temperature to 70 °C and holding for 1 h, 150 °C and holding for 2 h, 200 °C and holding for 1 h, 250 °C and holding for 1 h, and slowly cooling to room temperature.

[0068] (6) Attach the SiO2 / Si substrate carrying the film to the vertical stage of a scanning electron microscope, keep the cut surface in step (3) as the observed surface, observe the cut surface under the scanning electron microscope, the acceleration voltage is 3.00 kV, the magnification is 3.50 Kx, and the imaging mode is SE2. As Figure 4 shown, the measured film thickness is 10.30 μm.

[0069] Example 4:

[0070] A method for measuring the thickness of a polyimide film using a scanning electron microscope, which adopts the following method:

[0071] (1) Spin-coat a polyimide precursor polyamic acid solution with a viscosity of 20000 cP on a Si substrate with 285 nm SiO2 and 100 crystal phase; the spin-coating speed is 500 rpm for 30 s and 2000 rpm for 60 s;

[0072] (2) Place the SiO2 / Si substrate on a hot plate and keep it at 50 °C for 40 s to semi-cure the film;

[0073] (3) Use a silicon wafer knife to cut the SiO2 / Si substrate carrying the semi-cured film;

[0074] (4) Place the cut SiO2 / Si substrate on a hot plate while keeping the cut surface non-contact and undamaged, and keep it at 60 °C for 60 s to form a uniform and stable film;

[0075] (5) Place the SiO2 / Si substrate with the film on it in an oven for staged heat treatment to form a film by the method of thermal imidization. The heat treatment process parameters are as follows: the heating rate is 4 °C, heat preservation for 1 h when heating from room temperature to 70 °C, heat preservation for 2 h at 150 °C, heat preservation for 1 h at 200 °C, heat preservation for 1 h at 250 °C, and then slowly cool to room temperature.

[0076] (6) Attach the SiO2 / Si substrate with the film on it to the vertical stage of a scanning electron microscope, keeping the cross-section in step (3) above as the observed surface, observe the cross-section under the scanning electron microscope, with an acceleration voltage of 3.00 kV, a magnification of 3.50 Kx, and an imaging mode of SE2. As Figure 5 shown, the measured film thickness is 11.71 μm.

[0077] Comparative example:

[0078] A method for measuring the thickness of a polyimide film using a scanning electron microscope, which adopts the following method:

[0079] (1) Spin-coat a polyimide precursor polyamic acid solution with a viscosity of 20000 cP on a 285 nm SiO2, 100 crystal phase Si substrate; the spin-coating speed is 500 rpm for 30 s and 2000 rpm for 60 s;

[0080] (2) Place the SiO2 / Si substrate with the film on it in an oven for staged heat treatment to form a film by the method of thermal imidization. The heat treatment process parameters are as follows: the heating rate is 4 °C, heat preservation for 1 h when heating from room temperature to 70 °C, heat preservation for 2 h at 150 °C, heat preservation for 1 h at 200 °C, heat preservation for 1 h at 250 °C, and then slowly cool to room temperature;

[0081] (3) Transfer the sample to deionized water at 50 °C, make the water surface completely submerge the sample, and use tweezers to assist in peeling to obtain the polyimide film;

[0082] (4) Use a scalpel to cut the film by comparing with a ruler;

[0083] (5) Attach the film to the vertical stage of a scanning electron microscope and observe under the scanning electron microscope, with an acceleration voltage of 5.00 kV and an imaging mode of SE2. As Figure 6 shown, it is measured that the cross-section of the film is very uneven, with problems such as curling and warping, which affect the cross-section observation and the measurement of the true film thickness.

[0084] The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for measuring the thickness of an ultra-thin polyimide film, characterized in that, including: Step 1: Spin-coat the polyimide precursor solution on the SiO2 / Si substrate and perform one-time heating to achieve semi-curing of the film to obtain a semi-cured film substrate; Step 2: Cut the semi-cured film substrate to obtain a cross-section, perform secondary heating to obtain a fully cured film, and then perform thermal imidization to form a polyimide film; Step 3: Observe the cross-section under a scanning electron microscope and measure the film thickness; The temperature of the one-time heating is lower than 100 °C and the duration does not exceed 60 s; the temperature of the secondary heating is lower than 100 °C and the duration is not less than 60 s; The polyimide precursor solution is a polyamic acid solution with a viscosity of 2000 - 30000 cP obtained by polycondensation of diamine and dianhydride monomers in a polar solvent; The process parameters of the spin-coating are: the rotation speed in the first stage is 500 rpm and the duration is 10 - 30 s; the rotation speed in the second stage is 1000 - 9000 rpm and the duration is 50 - 120 s; The formation of the polyimide film by thermal imidization is as follows: under atmospheric conditions, the heating rate is 2 - 10 °C, heat from room temperature to 70 °C for 1 - 4 h, 150 °C for 2 - 4 h, 200 °C for 1 - 4 h, 250 °C for 1 - 24 h, and then cool to room temperature.

2. The method for measuring the thickness of the ultra-thin polyimide film according to claim 1, wherein The temperature of the one-time heating is 50 °C and the duration is 40 s; or 55 °C with a duration of 35 s; or 60 °C with a duration of 30 s.

3. The method for measuring the thickness of the ultra-thin polyimide film according to claim 1 or 2, characterized in that, The temperature of the secondary heating is 60 °C and the duration is 60 s; or 70 °C with a duration of 50 s; or 80 °C with a duration of 40 s.

4. The method for measuring the thickness of the ultra-thin polyimide film according to claim 1 or 2, characterized in that, The SiO2 / Si substrate also needs to be pretreated, and the process conditions of the pretreatment are: first use an air gun to blow off the surface dust, and then use O2 Plasma and UV-ozone for surface hydrophilic treatment.

5. A method for preparing an ultra-thin polyimide film, characterized in that, including: Step 1: Spin-coat the solution of the polyimide precursor on the SiO2 / Si substrate and perform one-time heating to achieve semi-curing of the film to obtain a semi-cured film substrate; the temperature of the one-time heating is lower than 100 °C and the duration does not exceed 60 s; Step 2: Cut the semi-cured film substrate to obtain a cross-section, perform secondary heating to obtain a fully cured film, and then perform thermal imidization to form a polyimide film; the temperature of the secondary heating is lower than 100 °C and the duration is not less than 60 s; The thickness of the polyimide film is 2 - 22 μm, the tensile strength is 120 - 164 MPa, the elongation at break is 28 - 140%, and the uniformity is 2% - 6%; The polyimide precursor solution is a polyamic acid solution with a viscosity of 2000 - 30000 cP obtained by polycondensation of diamine and dianhydride monomers in a polar solvent; The process parameters of the spin-coating are: the rotation speed in the first stage is 500 rpm and the duration is 10 - 30 s; the rotation speed in the second stage is 1000 - 9000 rpm and the duration is 50 - 120 s; The formation of the polyimide film by thermal imidization is as follows: under atmospheric conditions, the heating rate is 2 - 10 °C, heat from room temperature to 70 °C for 1 - 4 h, 150 °C for 2 - 4 h, 200 °C for 1 - 4 h, 250 °C for 1 - 24 h, and then cool to room temperature.

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