Transfer printing method for large-area semiconductor material film

Abstract

The invention discloses a transfer printing method for a large-area semiconductor material film, which comprises the following steps: step 1, manufacturing a PDMS (Polydimethylsiloxane) seal with a corresponding size according to the area of a film to be transferred, and manufacturing a micro-column structure array on the surface of the PDMS seal; step 2, etching a corrosion channel surrounding an etching region on the Si thin film layer of the SOI; step 3, manufacturing a corrosion hole array in the etching area; 4, soaking the SOI wafer in HF acid, and corroding the silicon dioxide layer below the etching region through the corrosion channel and the corrosion hole array by HF acid corrosion; and step 5, after the silicon dioxide layer below the to-be-etched area is corroded to a certain degree, taking out the SOI wafer, cleaning the SOI wafer, and picking up the Si thin film layer by using a PDMS seal with a micro-column structure array. According to the method, the large-area Si thin film layer in the required area can be completely transferred and printed, the transfer printing process can be applied to chiplet and even wafer-level source and target substrates, and no extra protective layer needs to be added to the Si thin film layer.

Description

Technical Field

[0001] The present invention belongs to the technical field of transfer printing, and particularly relates to a transfer printing technology for large-area semiconductor material thin films. This technology can be applied to the transfer and printing integration of large-area semiconductor material thin films. Background Art

[0002] Transfer printing technology is a new type of assembly technology that can be used to transfer and assemble materials or devices on a donor substrate to a receiving substrate. This technology can be used for various materials and is applicable to devices ranging from millimeters to micrometers. When transferring small-sized materials, it can be called micro-transfer printing technology, thus realizing the high-precision and high-density integration of various micro-devices. The transfer medium used in the transfer process generally adopts an elastic material stamp, which is very suitable for processing thin film materials and thin film device samples. Therefore, in the fields of flexible wearable electronics, flexible semiconductors, and micro-nano optical device integration, this micro-transfer printing technology has great potential for wide use.

[0003] The specific implementation method of transfer printing technology generally adopts kinetic control transfer, which means that for a polydimethylsiloxane (PDMS) stamp with viscoelastic properties, hereinafter referred to as a PDMS stamp, the adhesion strength of the interface can be controlled by changing the peeling speed of the PDMS stamp.

[0004] Silicon on insulator (SOI), the structure is as Figure 1 shown, and it is composed of a bottom layer of 3 - thick Si substrate, an intermediate layer of 2 - SiO2 layer, and an uppermost layer of 1 - Si thin film layer. This material platform is rapidly becoming the standard platform for realizing large-scale photonic integrated circuits. The high refractive index contrast and CMOS manufacturing compatibility enable the high-yield production of compact devices using established and mature foundry processes, and can mass-produce a wide range of photonic components. Among them, the devices are generally fabricated on the uppermost Si thin film layer of SOI, with a thickness of about several hundred nanometers.

[0005] To achieve the heterogeneous integration of micro-nano optical devices, using transfer printing technology to transfer the 1 - Si thin film layer from the source substrate SOI to a target substrate, such as a glass substrate or other substrates, is an emerging deterministic material assembly method. The three-layer structure of SOI is an integral whole. If we want to transfer only the uppermost 1 - Si thin film layer separately, we need to use HF acid to remove the 2 - SiO2 layer as a sacrificial layer to disconnect the connection between the 1 - Si thin film layer and the 2 - SiO2 layer, and make the 1 - Si thin film layer adhere to the SOI structure. During the transfer process, it is necessary to maintain the integrity of the 1 - Si thin film layer.

[0006] Removing the 2-SiO2 layer requires wet etching with high concentration of hydrofluoric (HF) acid, which has a strong corrosive effect on SiO2, but reacts very slowly with Si. Generally, for the transfer printing of a small area (hundreds of microns) 1-Si thin film layer, it is only necessary to use photolithography technology at the edge of the square area where the thin film device is located to remove the surface 1-Si thin film layer, leaving an 8-channel for HF acid to flow into the SiO2 layer and contact it for wet etching. A mechanical connection structure needs to be made on the 8-channel to connect the square area where the 1-Si thin film layer is located with the Si thin film layer outside the area to prevent the 1-Si thin film layer from being disconnected from the overall SOI structure and falling off after the SiO2 layer is completely etched by HF acid.

[0007] However, for transfer printing of Si thin film layers with a large area (millimeter to centimeter level) and extremely thin (hundreds of nanometers level), the method of etching only by 8-channels flowing HF acid at the edge of the square area is difficult to completely etch the 2-SiO2 layer. It takes a lot of time and is prone to over-etching at the edge where etching starts earlier, that is, Si also reacts with HF acid to a certain extent, resulting in damage to the Si film. In addition, it is difficult to achieve complete printing using a traditional transfer printing stamp, and wrinkles, fragmentation, and large-scale fractures are very likely to occur. Traditional transfer printing stamps use unstructured PDMS planes for transfer printing. Therefore, it is necessary to improve the structure and stamps used for wet etching, and then use transfer technology for transfer printing of large-area, extremely thin materials. Summary of the invention

[0008] The technical problems to be solved by the present invention are:

[0009] In order to solve the problems of SiO2 layer being difficult to corrode and stamp adhesion being inappropriate in the transfer printing of Si thin films on insulators with large areas (millimeter to centimeter level) and extremely thin areas (hundreds of nanometers), holes are introduced into the Si thin film layer and their distribution is designed to optimize the etching process so that the etching time at the edge of the square area and the etching time within the area are synchronized to prevent over-etching as much as possible. The micro-column structure is improved to solve the problem of too small an adhesion control range for the unstructured PDMS plane.

[0010] The technical solution of the present invention is:

[0011] A transfer printing method for a large-area semiconductor material film, comprising:

[0012] Step 1: Prepare a PDMS stamp of corresponding size according to the area of the film to be transferred, and make a micro-column structure array on the surface of the PDMS stamp;

[0013] Step 2, etching a corrosion channel surrounding the etching area on the Si thin film layer of SOI;

[0014] Step 3: Fabricate an array of etching holes in the etching area;

[0015] Step 4: Immerse the SOI wafer in HF acid. The HF acid corrodes the silicon dioxide layer below the etching area through the etching channels and the array of etching holes;

[0016] Step 5: After the silicon dioxide layer below the etching area is corroded to a certain extent, take out the SOI wafer, wash it, and pick up the Si thin film layer with a PDMS stamp with an array of micro-column structures.

[0017] In Step 1: The PDMS stamp consists of three parts: a glass substrate, a PDMS boss, and PDMS micro-columns. The glass substrate is a flat structure. The PDMS boss is installed in the center of the glass substrate. The PDMS boss is a two-layer stepped structure, and an array composed of multiple PDMS micro-columns is distributed on the surface of the upper step. The PDMS boss and the PDMS micro-columns are made of the same material and have a certain elasticity. Increasing the spacing of the PDMS micro-columns will reduce the adhesion force of the PDMS stamp.

[0018] In Step 2: Make a tether structure with a certain interval on the etching channels. The interval of the tether structure is 90 - 110 μm, the length is 18 - 22 μm, and the narrowest area of the tether is 1 - 2 μm.

[0019] In Step 3: The diameter of each hole in the array of etching holes is 2.5 - 3.5 μm, the interval is 25 - 35 μm, and the edge holes are 25 - 35 μm away from the etching channels.

[0020] In Step 4: The SOI wafer is immersed in 40% concentrated HF acid after being wetted in clean water once.

[0021] In Step 5: The PDMS stamp contacts the Si thin film layer in the form of an inclined angle, and then gradually reduces the inclination angle to ensure that the micro-column structure is in full contact with and adheres tightly to the Si thin film layer to be transferred. After all the forces are applied, lift it at high speed, then contact the target substrate again, and finally slowly lift the stamp.

[0022] Advantages of the present invention:

[0023] This invention can completely transfer and print a large-area (millimeter to centimeter level) and extremely thin (hundred-nanometer level) 1-Si thin film layer in the required area, can apply the transfer printing process to chiplet and even wafer-level source and target substrates. The structure etched by semiconductor processes is suitable for industrial mass production, and no additional protective layer needs to be added to the Si thin film layer.

[0024] The 4 - glass substrate and 5 - PDMS bosses are used to ensure the thickness and stiffness of the stamp, while ensuring uniform force when the PDMS contacts the 1 - Si thin film layer to be transferred during the transfer printing process, which is beneficial to the transfer printing of complete large - area and extremely thin materials.

[0025] The SiO2 under the hole - free area left by the 7 - hole distribution design can be appropriately retained, which can support the Si thin film without reducing the transfer success rate.

[0026] By introducing an equally - spaced tether structure around the etching area, the Si thin film above the completely etched middle - layer SiO2 can be stabilized, which not only plays a connecting role but also prevents irregular fracture or breakage of the Si thin film.

[0027] The 6 - PDMS micro - pillars are a periodically arranged cylindrical structure. Its structural characteristic is that the adhesion force decreases when the period of the micro - pillars distributed on the surface is large, and the adhesion force increases when the period is small. This property can be used to further adjust the adhesion force of the PDMS stamp surface. Compared with the stamp without the micro - pillar structure, since this structure can reduce the adhesion force of the PDMS stamp surface by adjusting the spacing period, the success rate of the printing step in the transfer printing of the 1 - Si thin film layer can be greatly increased. Brief Description of the Drawings

[0028] Figure 1 It is a side view of the SOI structure.

[0029] Figure 2 It is a side view of the transfer - printing PDMS stamp.

[0030] Figure 3 It is a top view of the transfer - printing PDMS stamp.

[0031] Figure 4 It is a hole design diagram of the Si thin film layer on the SOI surface. Detailed Implementation Manner

[0032] For the transfer printing of large-area (millimeter to centimeter scale) and extremely thin (hundred-nanometer scale) Si-on-insulator thin films, by using traditional semiconductor processing techniques such as photolithography and dry etching processes, 7-holes are introduced into the 1-Si thin film layer and their distribution is designed. The etching process is optimized to synchronize the etching time at the edges of the square region and the etching time within the region, and over-etching is prevented as much as possible. At the same time, a tether structure is designed at the edge part and arranged at equal intervals to suspend the Si thin film, which can effectively prevent the Si thin film from cracking and collapsing. The tether structure is the same as the tether structure mentioned in Section 5.2 of the article: Transfer Print Integration of Waveguide-Coupled Germanium Photodiodes Onto Passive Silicon Photonic ICs. In addition, it is difficult to achieve complete printing using a traditional stamp for transfer printing, and wrinkles, fractures, and large-area breaks are prone to occur. The reason is that the adhesion force regulation range of the unstructured PDMS plane is too small to achieve printing operations; while the thin-film-based transfer printing technology cannot ensure uniform stress between the stamp and the thin film when used for the transfer printing of large-area thin films, making it difficult to achieve complete transfer printing. Therefore, the structure and stamp for wet etching need to be improved, and then the transfer printing technology is used for the transfer printing of large-area and extremely thin materials, and the application scope of the transfer printing process is extended to chiplets and even wafer-level source substrates and target substrates. By utilizing the viscoelastic properties of the elastic stamp itself and the property that the adhesion force of the micro-column structure can be changed according to the periodic interval, the periodic interval and the adhesion force are inversely proportional within a certain range, and the adhesion force is adjusted by changing the period of the cylindrical structure on the elastic stamp.

[0033] Example 1:

[0034] To transfer print a 3-mm 2 sized, 220-nm-thick 1-Si thin film layer on SOI, this area is the etching area. Using traditional semiconductor processing techniques, namely photolithography and dry etching, circular 7-holes with a diameter of 3 μm are etched on the Si thin film in this area. The distribution of the 7-holes is as Figure 4, the 7-holes are evenly distributed periodically with an interval of 30 μm. The distance between the edge 7-holes and the 8-channel is 30 μm. The areas without holes are reserved for fabricating micro-nano photon devices. At the edge of the etching area, the surface Si thin film is etched hollow with a width of about 20 μm using traditional semiconductor processing technology to form an 8-channel surrounding the entire etching area. And a tether structure with an interval of 100 μm is fabricated on the 8-channel around the perimeter, with a length of 20 μm. The narrowest area of the tether is about 1.5 μm. The SOI wafer is immersed in clean water once and then soaked in 40% concentration of HF acid. The 2-SiO2 layer is removed as a sacrificial layer by HF acid through the 7-holes and the 8-channel. The 2-SiO2 layer in the middle layer of the SOI structure is almost completely etched. After taking it out, it is successively placed in clean water and isopropyl alcohol to wash away the HF acid for subsequent transfer printing.

[0035] Using the elastic stamp of the present invention, the area of the 4-glass substrate is 76.2×76.2 mm 2 , and the thickness is 3 mm; the lower surface area of the 5-PDMS boss is 6×6 mm 2 , and the upper surface area is 3×3 mm 2 , and the total thickness of the boss is 2.4 mm; the 6-PDMS micro-columns are fabricated on the surface of the 5-PDMS boss. The areas of all the micro-column regions are the same, with an upper surface area of 3×3 mm 2 , and the 6-PDMS micro-column layer is in direct contact with the 1-Si thin film layer. After the 2-SiO2 layer in the SOI substrate is etched, the SOI with the 1-Si thin film layer to be transferred is placed obliquely, so that the stamp and the thin film to be transferred are in contact in an inclined angle rather than parallel contact, and it must be ensured that all the 6-PDMS micro-columns are in contact with and closely attached to the 1-Si thin film layer to be transferred. After all the forces are applied, it is lifted at high speed. Due to the increased adhesion, the 1-Si thin film layer can be picked up onto the stamp. Then it is contacted with any flat and clean target substrate, and finally the stamp is slowly lifted. The 1-Si thin film layer can be printed onto the target substrate, thus realizing the transfer printing of a large-area (mm 2 ) and extremely thin (220 nm) 1-Si thin film layer.

Claims

1. A transfer printing method for a large-area semiconductor material thin film, characterized in that Including: Step 1: Fabricate a PDMS stamp with a corresponding size according to the area of the thin film to be transferred, and fabricate an array of microcolumn structures on the surface of the PDMS stamp. Step 2: Etch a corrosion channel surrounding the etching area on the Si thin film layer of the SOI. Step 3: Fabricate an array of corrosion holes in the etching area. Step 4: Immerse the SOI wafer in HF acid. The HF acid corrodes the silicon dioxide layer below the etching area through the corrosion channel and the array of corrosion holes. Step 5: After the silicon dioxide layer below the etching area is corroded to a certain extent, take out the SOI wafer, wash it, and pick up the Si thin film layer with the PDMS stamp with an array of microcolumn structures.

2. The transfer printing method of the large-area semiconductor material thin film according to claim 1, characterized in that In Step 1: The PDMS stamp consists of three parts: a glass substrate, a PDMS boss, and PDMS microcolumns. The glass substrate is a flat structure. The PDMS boss is installed in the center of the glass substrate. The PDMS boss is a two-layer stepped structure, and an array composed of multiple PDMS microcolumns is distributed on the surface of the upper step.

3. The transfer printing method of the large-area semiconductor material thin film according to claim 2, characterized in that: The PDMS boss and the PDMS microcolumns are made of the same material and have a certain elasticity. Increasing the spacing of the PDMS microcolumns will reduce the adhesion force of the PDMS stamp.

4. The transfer printing method of the large-area semiconductor material thin film according to claim 1, characterized in that Step 2: Fabricate a tether structure with a certain interval on the corrosion channel. The interval of the tether structure is 90 - 110 μm, the length is 18 - 22 μm, and the narrowest area of the tether is 1 - 2 μm.

5. The transfer printing method of the large-area semiconductor material thin film according to claim 1, characterized in that Step 3: The diameter of each hole in the array of corrosion holes is 2.5 - 3.5 μm, the interval is 25 - 35 μm, and the edge hole is 25 - 35 μm away from the corrosion channel.

6. The transfer printing method of the large-area semiconductor material thin film according to claim 1, characterized in that Step 4: Immerse the SOI wafer in 40% concentrated HF acid after wetting it once in clean water.

7. The transfer printing method of the large-area semiconductor material thin film according to claim 1, wherein Step 5: The PDMS stamp first contacts the Si thin film layer in the form of an inclined angle, and then gradually reduces the inclination angle to ensure that all the microcolumn structures are in contact with and closely attached to the Si thin film layer to be transferred. After all the forces are applied, lift it up at high speed; then contact the target substrate again, and finally slowly lift the stamp.

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