A method for dry etching a polyimide sacrificial layer

Abstract

The invention discloses a method for dry etching a polyimide sacrificial layer, which comprises: S1, uniformly coating polyimide on a semiconductor wafer substrate and annealing and curing to form a polyimide layer film; S2, depositing a sputtered aluminum film on the polyimide layer film; S3, coating a photoresist on the aluminum film, and exposing and developing the photoresist; S4, continuing to etch the aluminum mask to transfer a predetermined pattern to the aluminum mask; S5, ion etching the sacrificial layer coated with the photoresist in two steps; S6, washing alternately with an acid solution and an alkaline solution in turn, corroding the aluminum mask, and removing the aluminum oxide micro-mask; S7, cleaning the residual colloid. The invention adopts metal Al and a thin layer of photoresist as a "double mask", which solves the problem that the current dielectric hard mask still cannot form a good etching morphology by drilling and cannot effectively control the key size, thereby affecting the performance of the device and the stability of the suspended structure.

Description

Technical Field

[0001] The invention belongs to the technical field of polyimide etching, and in particular relates to a method for dry etching a polyimide sacrificial layer. Background Art

[0002] Polyimide (PI) is a class of polymer chemicals containing a stable conjugated structure consisting of an imide ring and a benzene ring. It has some characteristics of deposited films widely used in microelectronics, such as low stress, low water absorption, high modulus and good ductility. At the same time, since PI is easily etched away by chemical reactions with oxygen, sodium hydroxide, etc., it is often used as a sacrificial layer to prepare suspended structures in microelectromechanical systems (MEMS), such as the microbridge structure in an uncooled infrared focal plane detector, which is obtained by using PI2600 series glue as a sacrificial layer material.

[0003] In order to reduce the adhesion effect, reduce the proportion and risk of failure, and improve the yield of MEMS systems, the PI sacrificial layer usually uses a dry plasma etching method to open holes. However, during the dry etching process, ions usually bombard and etch the photoresist and other materials on the silicon wafer at the same time. Especially in the oxygen RIE reactive ion etching system, the photoresist and polymer have similar reaction rates. Even if thick glue is used as a mask, the PI will often appear rough and have burrs around the through hole after ion etching, which is inconsistent with the theoretical design shape, seriously affecting the continuity of the subsequent deposited film, resulting in the breakage of the metal line of the detection unit support arm after the sacrificial layer is released, reducing the yield of the entire device.

[0004] In the RIE process, O2 and fluorine-based mixed gas are usually used as etchants for etching polymers, and the etching rate for PI is usually 0.13-0.2μm / min. When a hard mask such as a dielectric layer or a metal layer covers the PI layer, heat transfer will be blocked, and the temperature of the hard mask is higher than that of the PI sacrificial layer, resulting in a faster etching rate around the hard mask, thereby effectively improving the PI etching selectivity. However, commonly used dielectric hard masks, such as SiN or SiO2, still have problems such as being unable to form a good etching morphology during drilling and unable to effectively control critical dimensions, which affects the performance of the device and the stability of the suspended structure. Summary of the invention

[0005] The purpose of the present invention is to provide a method for dry etching a polyimide sacrificial layer, which solves the problem that the current dielectric hard mask still cannot form a good etching morphology during drilling and cannot effectively control the key dimensions, thereby affecting the performance of the device and the stability of the suspended structure.

[0006] In order to achieve the above object, the present invention provides a method for dry etching a polyimide sacrificial layer, the method comprising:

[0007] S1. Polyimide is uniformly coated on a semiconductor wafer substrate and annealed and cured to form a polyimide layer film.

[0008] S2. Depositing a sputtered aluminum film on the polyimide layer film obtained in step S1.

[0009] The aluminum film obtained by the magnetron sputtering method of the present invention is denser than the aluminum film obtained by other methods; the thickness of the aluminum film is 30-100nm. The aluminum film obtained by the magnetron sputtering method of the present invention is denser than the aluminum film obtained by other methods. The thickness of the aluminum film obtained by the magnetron sputtering method is related to the coating time, power, etc., but if the thickness of the aluminum film exceeds 100nm, the aluminum mask has not been formed yet, and the photoresist has been completely etched.

[0010] S3. Evenly coating the aluminum film obtained in step S2 with photoresist, and exposing and developing the photoresist to form a predetermined pattern on the photoresist.

[0011] The thickness of the photoresist is 1.0-3.0 μm (if it is less than 1 μm, it will not have a good masking effect during the etching process; the upper limit of the thickness of the photoresist is determined based on the comprehensive consideration of the photoresist and the coating machine. The commonly used thickness is generally not higher than 3 μm. Too thick a thickness causes waste and prolongs the etching time); the photoresist is used as a soft mask, and the hard mask aluminum film is a "double mask" for etching the polyimide sacrificial layer; due to the presence of the hard mask, oxygen atoms will form a passivation layer on the Al mask surface during the etching process, which will not cause severe damage to the mask. Therefore, there is no need to use the traditional thick photoresist process.

[0012] S4, after step S3, etching the aluminum mask again to transfer the predetermined pattern onto the aluminum mask;

[0013] The etching of the aluminum mask is performed by plasma dry etching, and the etching gas is a mixed gas of Cl2, BCl3 and N2; the Cl2 generates AlCl3 with aluminum; the BCl3 performs anisotropic etching and reduces the aluminum oxide layer to promote the continuation of the etching process.

[0014] The present invention uses plasma dry etching to etch the aluminum mask, and the etching gas is a mixed gas of Cl2, BCl3 and N2; Cl2 is the main etching gas, and the volatile AlCl3 generated by aluminum is carried out of the reaction chamber by the gas flow. BCl3 can provide BCl3 +(The mixed gas is excited by the RF power supply) and bombards the silicon wafer surface vertically to achieve anisotropic etching (different etching rates in the horizontal and vertical directions). At the same time, since an aluminum oxide passivation layer is easily formed on the aluminum surface, it hinders the etching process. BCl3 can reduce the oxide layer and promote the continuation of the etching process. The reaction equation is as follows: Al2O3+3BCl3→2AlCl3+3BOCl.

[0015] S5, after step S4 is completed, ion etching is performed on the sacrificial layer coated with the photoresist in two steps;

[0016] The two-step reaction uses fluorine-based gas and oxygen with different flow ratios for etching to form the required pattern. The fluorine-based gas is selected from any one of SF6, CF4, and CHF3; when the fluorine-based gas is SF6, the flow ratio of O2 to SF6 in the first step is not less than 3:1, and the flow ratio of O2 to SF6 in the second step is not less than 1:2; when the fluorine-based gas is CF4, the flow ratio of O2 to CF4 in the first step is not less than 2:1, and the flow ratio of O2 to CF4 in the second step is not less than 2:3; when the fluorine-based gas is CHF3, the flow ratio of O2 to CHF3 in the first step is not less than 3:4, and the flow ratio of O2 to CHF3 in the second step is not less than 2:5.

[0017] The first step is the main etching, and the second step is the modification of the required morphology. Therefore, for further etching, it is necessary to control the oxygen content in the first step to be higher than the oxygen content in the second step. If it is lower than the flow ratio, the etching morphology and corresponding parameters will not meet the requirements. Under the corresponding flow ratios, the three fluorine-based gases have similar performance results in key parameters such as the side wall slope, morphology and PI pore size after etching PI.

[0018] The gas pressure of the first step is 100-200 mTorr, the RF power is 200-450 W, and the etching time is 30-100 s; the gas pressure of the second step is 150-400 mTorr, the RF power is 350-600 W, and the etching time is 60-200 s;

[0019] Within the above pressure, power and etching time range, not only can the hard mask pattern be transferred well, but it is also conducive to the removal of residual photoresist and aluminum mask in the subsequent process. If etching is not performed within the above range, the following problems may occur: (1) If the RF power range is lower than the first and second steps, the etching machine will ignite abnormally; if the pressure or time range is lower than the first and second steps, the etching work cannot be completed; (2) If the RF power range is higher than the first and second steps, it will cause a certain amount of lateral corrosion of the Al mask, resulting in uneven aperture of the opening mask, and it may also be that the Al mask cannot be completely removed; if the pressure or time range is higher than the first and second steps, it will also affect the etching of the aluminum mask, PI film and etching morphology.

[0020] The present invention adopts RIE process to etch the sacrificial layer PI, and adopts fluorine-based gas and O2 mixed gas as etching gas. The first step of etching is to remove the photoresist and etch the sacrificial layer simultaneously. The fluorine element can promote the ionization of O2, so that the concentration of oxygen atoms in the system increases, and the oxidation reaction rate is accelerated; at the same time, the more active fluorine atoms will react with the hydrogen elements in the photoresist to generate volatile gas HF, and the photoresist stripped of hydrogen elements will obtain more free radical sites, and further react with oxygen atoms to obtain gas phase products to achieve the purpose of removing the photoresist. The second step is to further etch the sacrificial layer, wherein the oxygen atoms and the Al mask form a passivation layer to achieve the purpose of protecting the mask, and can form a side wall protection layer with the sputtered Al atoms; the fluorine atoms can activate the PI surface and destroy its molecular structure to generate more free radical sites to increase the etching rate.

[0021] S6, after step S5 is completed, washing with an acidic solution and an alkaline solution alternately in sequence, corroding the aluminum mask and removing the aluminum oxide micro-mask.

[0022] S7. Clean the residual colloid.

[0023] Preferably, the annealing and curing in step S1 is pre-baked at 100-180° C. for 30-120 min in an inert gas atmosphere, and then cured at 300-400° C. for 5-30 min; the cured thickness is 2.0-2.6 μm.

[0024] Preferably, the aluminum film in step S2 is formed at a pressure of 1×10 -4 Pa, and sputtered on a magnetron sputtering table with a radio frequency power of 200 W for 2 minutes to form an aluminum film with a thickness of 30 nm; the aluminum film is a hard mask.

[0025] Preferably, the fluorine-based gas is SF6, and the flow ratio of O2 to SF6 in the first step is 3:1, and the flow ratio of O2 to SF6 in the second step is 1:2; the fluorine-based gas is CF4, and the flow ratio of O2 to CF4 in the first step is 2:1, and the flow ratio of O2 to CF4 in the second step is 2:3; the fluorine-based gas is CHF3, and the flow ratio of O2 to CHF3 in the first step is 3:4, and the flow ratio of O2 to CHF3 in the second step is 2:5. After etching PI with the flow ratio, the key parameters such as the slope, morphology and PI pore size of the side wall can achieve the etching effect of the present invention. If it deviates from this range, problems such as PI side wall hollowing and PI residue at the bottom edge of the PI hole may occur.

[0026] Preferably, the aluminum mask is etched using HF solution, which will not damage the organic PI layer and can dissolve a portion of the aluminum oxide micro-mask.

[0027] Preferably, the alternate washing with an acidic solution and an alkaline solution is to first wash with an acidic solution and deionized water in sequence, blow dry with an inert gas, and then wash with a sodium hydroxide solution. The sodium hydroxide solution is used to remove aluminum oxide.

[0028] Preferably, the cleaning of the residual colloid is carried out using an organic chemical solvent EKC265, an isopropyl alcohol solution (IPA) and deionized water in sequence, with the purpose of removing the photoresist, the organic solvent and the inorganic solution in sequence.

[0029] The present invention provides a method for dry etching a polyimide sacrificial layer, which solves the problem that the current dielectric hard mask still cannot form a good etching morphology during drilling and cannot effectively control the key dimensions, thereby affecting the performance of the device and the stability of the suspended structure, and has the following advantages:

[0030] 1. The present invention uses metal Al and a thin layer of photoresist as a "double mask", which effectively avoids the problem that the selectivity of the sacrificial layer etching is not high in the oxygen RIE reactive ion etching system, resulting in the difficulty in controlling the etching morphology and key dimensions of the sacrificial layer, thereby affecting the subsequent thin film deposition and the stability of the suspended structure.

[0031] 2. In the two steps of the present invention, the mixed gas etching of fluorine-based gas and oxygen atoms with different flow ratios can not only completely remove the photoresist, but also effectively control the etching size and etching morphology of the sacrificial layer, further improving the device yield.

[0032] 3. The present invention performs reactive ion etching on the sacrificial layer in two steps. During the etching process, there is no need to perform a separate photoresist ashing or wet stripping process, which simplifies the sacrificial layer etching process and improves the stability and repeatability of the process. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The present invention is a schematic flow chart of a method for dry etching a polyimide sacrificial layer.

[0034] Figure 2 Schematic diagram of the polyimide film etching process of the present invention; wherein (a) is "double-layer mask" deposition; (b) is "double-layer mask" patterning; (c) is the first etching of the sacrificial layer; (d) is the second etching of the sacrificial layer; and (e) is the removal of the hard mask.

[0035] Figure 3 This is a morphology diagram obtained after etching and cleaning the sacrificial polyimide layer in an embodiment of the present invention. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] The model of the polyimide (PI) used in the present invention is Hitachi 2610; the model of the organic chemical solvent EKC is DuPont EKC265.

[0038] like Figure 1 As shown, a schematic flow chart of a method for dry etching a polyimide sacrificial layer of the present invention; Figure 2 As shown, the schematic diagram of the polyimide film etching process of the present invention; Figures 1-2 , the etching method of the polyimide sacrificial layer of the present invention is described in detail below:

[0039] A method for dry etching a polyimide sacrificial layer, the method comprising:

[0040] S1. PI is uniformly coated on a semiconductor wafer substrate and annealed and cured to form a polyimide layer film.

[0041] First, clean the wafer surface, and then evenly deposit a PI sacrificial layer on the coating equipment. Pre-bake at 100-180°C for 30-120 minutes in a nitrogen atmosphere, and then cure at 300-400°C for 5-30 minutes in a nitrogen atmosphere. The thickness after curing is measured to be 2.0-2.6μm.

[0042] S2. Deposit a sputtered Al film on the PI layer thin film obtained in step S1.

[0043] S3. Evenly coating the Al film obtained in step S2 with photoresist, exposing and developing the photoresist, and forming a predetermined pattern on the photoresist.

[0044] After the sputtering of the hard mask Al film is completed, a photoresist is further coated, and the thickness of the photoresist is 1.0 to 3.0 μm.

[0045] S4. After step S3, the aluminum mask is etched again to transfer the predetermined pattern to the Al mask.

[0046] S5. After step S4 is completed, the sacrificial layer coated with the photoresist is subjected to reactive ion etching in two steps, wherein a mixed gas of fluorine-based gas and oxygen atoms with different flow ratios is used for etching in the two steps to form a desired pattern without removing the photoresist.

[0047] The present invention adopts RIE process to perform sacrificial layer PI etching, and adopts fluorine-based gas and O2 mixed gas as etching gas, and the fluorine-based gas is selected from any one of SF6, CF4, and CHF3; when the fluorine-based gas is SF6, in the first step, the flow ratio of O2 to SF6 is not less than 3:1, and in the second step, the flow ratio of O2 to SF6 is not less than 1:2; when the fluorine-based gas is CF4, in the first step, the flow ratio of O2 to CF4 is not less than 2:1, and in the second step, the flow ratio of O2 to CF4 is not less than 2:3; when the fluorine-based gas is CHF3, in the first step, the flow ratio of O2 to CHF3 is not less than 3:4, and in the second step, the flow ratio of O2 to CHF3 is not less than 2:5.

[0048] S6, after step S5 is completed, washing with an acidic solution and an alkaline solution alternately in sequence, corroding the Al mask and removing the aluminum oxide micro mask.

[0049] After the sacrificial layer is etched, the hard mask Al layer and the aluminum oxide micro mask formed by Al and O atoms need to be removed.

[0050] S7. Clean the residual colloid.

[0051] According to a specific embodiment of the present invention, in step S1, the PI sacrificial layer is first pre-baked at 180° C. for 1 hour in a nitrogen atmosphere, and then cured at 400° C. for 30 minutes in a nitrogen atmosphere. The thickness after curing is measured to be 2.1 μm.

[0052] According to a specific embodiment of the present invention, in step S2, a magnetron sputtering table is used to perform thin film Al sputtering, wherein the pressure of the reaction chamber is 1×10 -4 Pa, RF power is 200W; sputtering time is 2min, and the measured film thickness is 30nm.

[0053] According to a specific embodiment of the present invention, plasma dry etching is used in step S4, and the etching gas is a mixed gas of Cl2, BCl3 and N2. Cl2 is the main etching gas, and the volatile AlCl3 generated by the reaction with aluminum is carried out of the reaction chamber by the gas flow. BCl3 can provide BCl3 +, vertically bombarding the silicon wafer surface to achieve anisotropic etching. At the same time, since an aluminum oxide passivation layer is easily formed on the aluminum surface, which hinders the etching process, BCl3 can reduce the oxide layer and promote the continuation of the etching process.

[0054] According to a specific embodiment of the present invention, in step S5, the gas pressure of the first etching is 150mTorr, the RF power is 300W, and the etching time is 50s; the gas pressure of the second etching is 300mTorr, the RF power is 450W, and the etching time is 100s.

[0055] According to a specific embodiment of the present invention, in step S5, the gas pressure of the first etching is 100mTorr, the RF power is 200W, and the etching time is 30s; the gas pressure of the second etching is 150mTorr, the RF power is 300W, and the etching time is 60s.

[0056] According to a specific embodiment of the present invention, in step S5, the gas pressure of the first etching is 200mTorr, the RF power is 450W, and the etching time is 100s; the gas pressure of the second etching is 400mTorr, the RF power is 600W, and the etching time is 200s.

[0057] According to a specific embodiment of the present invention, the fluorine-based gas in step S5 is SF6, the flow ratio of O2 to SF6 in the first step is 3:1, and the flow ratio of O2 to SF6 in the second step is 1:2.

[0058] According to a specific embodiment of the present invention, the fluorine-based gas in step S5 is CF4, the flow ratio of O2 to CF4 in the first step is 2:1, and the flow ratio of O2 to CF4 in the second step is 2:3.

[0059] According to a specific embodiment of the present invention, the fluorine-based gas in step S5 is CHF3, the flow ratio of O2 to CHF3 in the first step is 3:4, and the flow ratio of O2 to CHF3 in the second step is 2:5.

[0060] According to a specific embodiment of the present invention, in step S6, the aluminum mask is etched using a 0.55% HF solution, which will not damage the organic PI layer and can dissolve a portion of the aluminum oxide micro-mask.

[0061] According to a specific embodiment of the present invention, in step S6, the alternating washing with an acidic solution and an alkaline solution is first washed with an acidic solution and deionized water in sequence, then dried with an inert gas, and then washed with a sodium hydroxide solution, the sodium hydroxide solution is used to remove aluminum oxide, and this washing process is repeated three times.

[0062] According to a specific embodiment of the present invention, the residual colloid is cleaned in step S7 by sequentially using an organic chemical solvent EKC265 (DuPont EKC265), an organic solution IPA (isopropyl alcohol) and deionized water, with the purpose of sequentially removing the photoresist, the organic solvent and the inorganic solution.

[0063] like Figure 3 As shown in FIG. 1 , the morphology obtained after etching and cleaning the sacrificial polyimide layer in an embodiment of the present invention. Figure 3 It can be concluded that the PI hole has a bottom diameter of 4.5 μm and a top diameter of 5.0 μm, and has a smooth sidewall morphology with a slope of more than 75°. If the method of the present invention is not used, the top opening may be smaller than the bottom opening, or the PI may be hollowed out during the etching process, which is very unfavorable for the subsequent thin film deposition process.

[0064] The present invention uses a hard mask Al method to complete the etching of PI, and the process of removing the hard mask Al after the etching is completed will not affect the underlying PI and other subsequent processes. In addition, with the help of the Al mask and the two-step etching method, the side wall of the PI hole can be etched relatively smoothly, and its slope and hole diameter meet the requirements of the manufacturing process.

[0065] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A method for dry etching a polyimide sacrificial layer, characterized in that: The method includes: S1, uniformly coating polyimide on a semiconductor wafer substrate and annealing and curing to form a polyimide layer film; The annealing and curing is performed by pre-baking at 100-180°C for 30-120 minutes in an inert gas atmosphere, and then curing at 300-400°C for 5-30 minutes; the cured thickness is 2.0-2.6 μm; S2, depositing a sputtered aluminum film on the polyimide layer film obtained in step S1; The thickness of the aluminum film is 30 to 100 nm; S3, uniformly coating the aluminum film obtained in step S2 with a photoresist, and exposing and developing the photoresist to form a predetermined pattern on the photoresist; The thickness of the photoresist is 1.0 to 3.0 μm; the photoresist is used as a soft mask; S4, after step S3, etching the aluminum mask again to transfer the predetermined pattern onto the aluminum mask; The etching of the aluminum mask adopts plasma dry etching, and the etching gas is a mixed gas of Cl2, BCl3 and N2; the Cl2 and aluminum generate AlCl3; the BCl3 performs anisotropic etching and reduces the aluminum oxide layer to promote the continuation of the etching process; S5, after step S4 is completed, ion etching is performed on the sacrificial layer coated with the photoresist in two steps; The two-step reaction uses fluorine-based gas and oxygen with different flow ratios for etching to form the required pattern; the fluorine-based gas is selected from any one of SF6, CF4, and CHF3; when the fluorine-based gas is SF6, the flow ratio of O2 to SF6 in the first step is not less than 3:1, and the flow ratio of O2 to SF6 in the second step is not less than 1:2; when the fluorine-based gas is CF4, the flow ratio of O2 to CF4 in the first step is not less than 2:1, and the flow ratio of O2 to CF4 in the second step is not less than 2:3; when the fluorine-based gas is CHF3, the flow ratio of O2 to CHF3 in the first step is not less than 3:4, and the flow ratio of O2 to CHF3 in the second step is not less than 2:5; The gas pressure of the first step is 100-200 mTorr, the RF power is 200-450 W, and the etching time is 30-100 s; the gas pressure of the second step is 150-400 mTorr, the RF power is 350-600 W, and the etching time is 60-200 s; S6, after step S5 is completed, washing with an acidic solution and an alkaline solution alternately in sequence, corroding the aluminum mask and removing the aluminum oxide micro-mask; S7. Clean the residual colloid.

2. The method for dry etching a polyimide sacrificial layer according to claim 1, characterized in that: The aluminum film in step S2 is placed in a reaction chamber at a pressure of 1×10 -4 Pa, and sputtered on a magnetron sputtering table with a radio frequency power of 200 W for 2 min to form an aluminum film with a thickness of 30 nm.

3. The method for dry etching a polyimide sacrificial layer according to claim 1, characterized in that: The fluorine-based gas is SF6, and the flow ratio of O2 to SF6 in the first step is 3:1, and the flow ratio of O2 to SF6 in the second step is 1:2; the fluorine-based gas is CF4, and the flow ratio of O2 to CF4 in the first step is 2:1, and the flow ratio of O2 to CF4 in the second step is 2:3; the fluorine-based gas is CHF3, and the flow ratio of O2 to CHF3 in the first step is 3:4, and the flow ratio of O2 to CHF3 in the second step is 2:

5.

4. The method for dry etching a polyimide sacrificial layer according to claim 1, characterized in that: The aluminum mask is etched using HF solution.

5. The method for dry etching a polyimide sacrificial layer according to claim 1, characterized in that: The alternate washing with acidic solution and alkaline solution in sequence is to first wash with acidic solution and deionized water in sequence, blow dry with inert gas, and then wash with sodium hydroxide solution.

6. The method for dry etching a polyimide sacrificial layer according to claim 1, characterized in that: The residual colloid is cleaned by sequentially using an organic chemical solvent EKC 265, an isopropanol solution and deionized water.

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