JP6124452B2 - Method for reclaiming lithium tantalate crystal wafer - Google Patents

Description

The present invention relates to a method for reclaiming a lithium tantalate crystal wafer used for a surface acoustic wave device or the like.

  In the manufacturing process of surface acoustic wave elements, strict thickness standards are set for metal films, and not only the wafer center but also the periphery of the wafer is inspected, so defective products are likely to occur in terms of thickness. There is a reality.

  Such defective wafers are regenerated by a conventional regenerating method, and the regenerated wafer is used as a test wafer for confirming the manufacturing process, and the film thickness monitor and particle in each manufacturing process are used. It is used for checking of heat or dummy for heating device. In this way, the wafer that has been regarded as a defective product in the past is not reused for manufacturing the surface acoustic wave device after being regenerated, and is used as a test wafer for checking the conditions of the manufacturing process. This is just a situation.

  However, in recent years, the usage of lithium tantalate crystal wafers has increased rapidly for smartphones, but on the other hand, due to strict requirements for fine processing of wafers, the number of defective wafers is increasing. There is a situation. Therefore, since there is almost no change in the demand for test wafers, etc., of regenerated wafers that have been regenerated from this defective product, there is an increasing demand for reuse of this defective product wafer for manufacturing products such as surface acoustic wave devices. Yes.

  Since the conventional method for reclaiming defective wafers that meets this requirement uses the reclaim method developed for reclaiming semiconductor silicon crystal wafers as it is, the surface side of the diffusion layer of the semiconductor silicon crystal wafer is removed. Therefore, it is required to make the machining allowance a thickness of about several tens of μm.

Specifically, the conventional reproducing method is generally performed by sequentially performing the following steps (a) to (e).
(A) Step of removing a functional film such as a metal film by etching or double-sided lapping (b) Step of removing a work deformation layer by etching after double-sided lapping as necessary (c) Step of cleaning a wafer (d) Large size Polishing (polishing) the wafer surface while adding abrasive slurry with a single-side polishing machine having a surface plate (e) Finish cleaning step

  For example, in Patent Document 1, a semiconductor wafer having a metal film formed thereon is subjected to a metal film removal process, a lapping process, a surface grinding process, an etching process, a mirror polishing process, and a cleaning process, and the wafer thickness is cut by about 50 μm. A playback method for playback is described. When each process of such a conventional regeneration method is applied to a lithium tantalate crystal wafer as it is and the regeneration process is performed, the machining allowance becomes as thick as several tens of μm. There is a problem that the standard is not suitable for a wafer for manufacturing a surface acoustic wave device having a standard desirably in the range of ± 5 μm.

  That is, in the conventional recycling method, it is said that double-sided lapping is performed if necessary. However, if this machining allowance is 10 μm or more on one side, the allowance as a wafer is 20 μm or more, and the mechanical strength received during lapping In order to remove the processing strain layer due to a strong impact, a polishing allowance of about 10 μm is required. On the other hand, when mounting a surface acoustic wave device formed by forming an electrode pattern on a lithium tantalate crystal wafer, the solder bumps are usually fixed and mounted by applying ultrasonic waves. Since the thickness + electrode pattern thickness on the wafer + the thickness of the solder bump is about 20μm, including variations in the diameter of the solder bumps, the conventional reproduction method that increases the machining allowance to several tens of μm is a surface acoustic wave device. There is a problem that it is not suitable for the reproduction of a wafer for manufacturing the wafer.

  Recently, attempts have been made to reduce machining allowance as much as possible by adopting surface grinding. However, no matter how precise the surface grinding machine is, the grinding wheel is the normal # 400- # 1000. The grinding allowance is over 10μm, and the polishing allowance in the polishing process to remove the processing distortion due to this grinding is also about 10μm, and the total machining allowance is over 20μm, so this kind of surface grinding is adopted However, the conventional wafer reclamation method is also not suitable for the reclamation of lithium tantalate crystal wafers.

JP 2002-16023

The present invention has been made in view of such circumstances, and its purpose is to regenerate a used or defective lithium tantalate crystal wafer into a wafer for manufacturing a product such as a surface acoustic wave device. The present invention provides a method for reclaiming a lithium acid crystal wafer.

  Therefore, the present inventors have conducted intensive studies to achieve the above-mentioned object. As a result, the conventional silicon crystal reclaimed wafer has an arithmetic average surface roughness Ra of about 0.2 nm with very small irregularities. On the other hand, for wafers of lithium tantalate crystals, products such as surface acoustic wave filters can be manufactured by simply finishing to an appropriately rough surface roughness with an arithmetic average surface roughness Ra of 1 nm to 8 nm. It was found that it can be used as a wafer for this purpose, and previously filed as Japanese Patent Application No. 2013-66044. In the present invention, this knowledge is utilized for the regeneration of defective products of lithium tantalate crystal wafers.

That is, the present invention is a method for reclaiming a lithium tantalate crystal wafer characterized by including at least the following steps (a) to (d).
(A) A step of sorting the used or defective wafers by their thicknesses and rearranging the sorted wafers by their thicknesses (b) A step of removing functional films on the wafers by etching (c) the mirror side of the wafer is roughly ground to the arithmetic mean roughness Ra10~30nm grinding allowance of 2 to 3 [mu] m, after the crude grinding, arithmetic mean roughness grinding allowance of 0.5~2μm by # 6000 or more grinding Ra1~8nm (D) A step of cleaning and drying the wafer.

  Further, according to the present invention, it is preferable to perform finish polishing with a polishing slurry having an arithmetic average roughness Ra of 0.5 nm to 2 nm after the step (c), if necessary.

  According to the present invention, a conventional used or defective wafer can be recycled and reused as a wafer for producing a product such as a surface acoustic wave device, which greatly contributes to the effective use of resources. can do.

  Hereinafter, although one embodiment of the present invention is described, the present invention is not limited to this embodiment. The diameter of the lithium tantalate crystal wafer targeted by the present invention is usually 100 mm or 150 mm in diameter.

The regeneration method of the present invention includes at least the following steps (a) to (d), and these steps will be specifically described below.
(A) A step of sorting the used or defective wafers by their thicknesses and rearranging the sorted wafers by their thicknesses (b) A step of removing functional films on the wafers by etching (c) the mirror side of the wafer is roughly ground to the arithmetic mean roughness Ra10~30nm grinding allowance of 2 to 3 [mu] m, after the crude grinding, arithmetic mean roughness grinding allowance of 0.5~2μm by # 6000 or more grinding Ra1~8nm (D) A step of cleaning and drying the wafer.

  In the step (a) of the present invention, first, a used or defective wafer is sorted by its thickness. The product thickness of lithium tantalate crystal wafers is 150μm, 180μm, 200μm, 250μm, 350μm, and this product (the product before it becomes a defective product) is sorted by thickness to be a post process ( This is because the machining allowance in the surface grinding machine in the step c) can be managed with higher accuracy.

  Specifically, since the second orientation flat differs depending on the thickness of the lithium tantalate crystal wafer, the wafer is first sorted by product thickness with reference to the second orientation flat, and then the thickness of a micro gauge or the like is selected. The wafers sorted by the thickness measuring device are rearranged according to their thicknesses and subjected to grinding. Since the processing allowance for grinding at this time is a small amount, if the wafer is arranged in advance in the order of its thickness and then ground while changing the finished thickness, for example, every 1 μm, the grinding machine Since the load (machining allowance) can be reduced, it becomes possible to manage with high accuracy.

In the step (b), the metal film is removed by etching. A metal film is usually attached to the wafer selected in the step (a), and there are various kinds of metal films. For example, metal films such as Al, Al—Cu, Ti, Nb, Ta, W, etc. can be usually removed with hydrochloric acid. However, Ti must be removed with KSMF-series etchant manufactured by Kanto Chemical, and Ta and Nb must be removed with 30-40% sodium hydroxide solution and hydrofluoric acid. In addition, some of the defective wafers have a SiO ₂ film formed on the above metal film. In this case, the wafer is immersed in a 30-40% caustic soda solution before The metal film may be removed with an etchant.

  As described above, the functional film formed on the defective wafer has various types such as a metal film. Usually, the functional film is immersed in 30-40% caustic soda for one day and night and then immersed in 1N hydrochloric acid. Then, almost all functional films including metal films can be removed by immersing in KSF-series solution manufactured by Kanto Chemical.

  Next, in the step (c), polishing is performed. The surface grinding machine (equipment capable of rough grinding and finishing grinding with two or more axes) used in this polishing processing has a rough grinding wheel. A finish grinding wheel is mounted, and can be driven to rotate at an arbitrary rotational speed by a drive system (motor). Specifically, after setting a predetermined finished plate thickness, grinding is performed by rotating a grindstone at a high speed on a wafer held by a ceramic chuck. At this time, the table is also rotated and the wafer is ground by fine cutting. Grinding conditions such as the rotational speed of the grindstone, the cutting speed, and the rotational speed of the table during this grinding are set to optimum conditions. The ceramic chuck of the surface grinder to be used is a so-called multi-porous vacuum suction type in which the wafer can be clamped by pulling the back surface into a vacuum.

  In the step (c) of the present invention, one surface of the piezoelectric oxide single crystal wafer is mirror-finished using a surface grinding machine. Specifically, the wafer from which the metal film has been removed by etching is held by vacuum suction. And grinding. In order to efficiently perform this grinding process, it is preferable to first perform grinding with a rough grindstone, and then perform finish grinding using a finely defined grindstone.

  In this case, the grinding wheel may be determined based on the arithmetic average roughness Ra of lithium tantalate. However, in the case of rough grinding, the grinding allowance is 2 to 3 μm, and the arithmetic average roughness Ra at this time It is preferable to select a grindstone having a thickness of 10 to 30 nm, for example, a # 4000 grindstone, because the grinding time is short and the processing strain layer can be kept small.

  Also, in the case of finish grinding, if you use a fine standard of # 6000 or more, preferably # 8000- # 10000, the grinding allowance is 0.5-2 μm and the arithmetic average roughness Ra is in the range of 1-8 nm. Since it can be set as a substantial mirror surface, it is preferable. And if it grinds with such a standard grindstone, it will become possible to suppress the distortion layer produced by mirror surface processing to the extent which does not affect the operation of a surface acoustic wave element.

  In the present invention, the arithmetic average roughness Ra in the finish grinding is 1 to 8 nm, but when a finer surface roughness is required, while adding a slurry such as colloidal silica, arithmetic average roughness Ra0. You may add the single side grinding | polishing which carries out final grinding | polishing to 5-2 nm. The polishing allowance at this time is preferably about 1 μm.

  In the conventional regeneration method, the polishing process of (c) is performed for the purpose of removing the processing strain layer in the previous step, but in the present invention, the polishing process is performed only for the purpose of adjusting the surface roughness. The polishing allowance can be suppressed to 5 μm or less, preferably 1 to 5 μm or less. As a polishing method, if the wafer subjected to finish grinding is vacuum-held on a ceramic chuck in the same manner as the holding method with a surface grinder, the polishing allowance can be reduced to the minimum.

  Finally, in the step (d), cleaning and drying are performed, and the processing such as cleaning is the same as the conventional one.

Next, specific examples of the present invention will be described. After forming an electrode pattern consisting of 50 wafers of 100mm diameter lithium tantalate crystals and Al-Cu film, which became defective in the Al-Cu metal film deposition process with Ti film formed on the backside of the wafer, an SiO ₂ film is formed As a result of measuring the thickness of 50 wafers with a diameter of 100 mm that were defective in the process, 95 were wafers with a product thickness of 200 μm and the remaining 5 were wafers with a product thickness of 180 μm. In the following experiment, 95 wafers having a product thickness of 200 μm were used.

  These 95 wafers were soaked in 30-40% caustic soda for one day, then soaked in 1N hydrochloric acid for one day and night, and further soaked in a KSF-series solution manufactured by Kanto Chemical for one day. It was completely removed, and the functional film could be removed to the extent that traces due to the electrode pattern were slightly seen on the surface side.

  Next, the surface side of these wafers was ground with a biaxial surface grinder. In the primary grinding, a # 40000 grindstone was used, and the wafer was vacuum chucked and ground on a ceramic plate with many holes so that an alumina vacuum chuck could be formed. The grinding allowance of 95 wafers at this time was in the range of 1.8 to 2.9 μm. Thereafter, the wafer was similarly held on an alumina ceramic plate and subjected to finish grinding with a # 10,000 grindstone. At this time, the grinding allowance of 95 wafers was in the range of 0.8 to 1.5 μm. It was. As a result of spin cleaning and drying the ground wafer, all 95 wafers could be regenerated without cracking.

  When the surface roughness of the regenerated wafer was measured by the AFM method, the arithmetic average roughness of 95 wafers was in the range of 3 nm to 8 nm. Moreover, as a result of inspecting the cleanliness of the surface with a condensing lamp, it was confirmed that the surface was finished at the same level as a wafer manufactured in a normal process. In addition, when the thickness of 95 wafers was measured, the reduction was only about 3-4 μm, and the surface roughness and cleanliness were within the normal wafer specifications. It was confirmed that there was no problem as a wafer for manufacturing such products.

Next, when 50 wafers were wiped from 95 wafers and the surface side was slightly polished with colloidal silica using a single-side polishing machine, the surface roughness of the 50 wafers was further reduced from 0.7 nm to 1.2 nm. We were able to. The polishing cost for 50 wafers at this time was about 1 μm.

Claims (2)

In a recycling method for reclaiming a wafer by removing the functional film from a used or defective wafer of lithium tantalate crystals on which a functional film is formed, the method includes at least the following steps (a) to (d): A method for reclaiming a lithium tantalate crystal wafer.
(A) A step of sorting the used or defective wafers by their thicknesses and rearranging the sorted wafers by their thicknesses (b) A step of removing functional films on the wafers by etching (c) the mirror side of the wafer is roughly ground to the arithmetic mean roughness Ra10~30nm grinding allowance of 2 to 3 [mu] m, after the crude grinding, arithmetic mean roughness grinding allowance of 0.5~2μm by # 6000 or more grinding Ra1~8nm washed surface grinding, the total step (d) wafer grinding allowance is grinding 5μm below, the drying step. 2. The method for reclaiming a lithium tantalate crystal wafer according to claim 1 , wherein after the step (c), finish polishing is performed with a polishing slurry having an arithmetic average roughness Ra of 0.5 nm to 2 nm.