JP5871282B2 - A method for producing a piezoelectric oxide single crystal wafer. - Google Patents

Description

The present invention is a wafer, in particular lithium tantalate single crystal used in the SAW filter or the like, relates to a manufacturing method of a piezoelectric oxide single crystal wafers, such as lithium niobate single crystal.

  In the field of mobile communications such as cellular phones, piezoelectric oxide single crystal wafers such as lithium tantalate single crystals and lithium niobate single crystals are used as substrates for surface acoustic wave elements. The surface acoustic wave element is configured by providing an interdigital (comb-shaped) transducer (electrode) on one main surface of the oxide single crystal wafer exhibiting piezoelectricity.

  By the way, in the surface acoustic wave device having such a configuration, since the surface acoustic wave is excited and received by the transducer, the transducer forming surface of the single crystal wafer needs to be mirror-polished. If the back side is mirror-polished and has a similar mirror surface, unnecessary waves (disturbance waves) such as bulk waves are excited and received simultaneously with excitation and reception of surface acoustic waves, which causes spurious interference in the frequency characteristics. . Therefore, in the single crystal wafer for the surface acoustic wave device, the back side thereof is roughened by polishing (lapping) or honing with a rough abrasive, but conventionally, such a single crystal wafer, It is manufactured as follows.

  That is, a plane (orientation flat) is formed in a specified direction with respect to a single crystal ingot manufactured by the Czochralski method, etc., and is cut using a slicer with an inner peripheral blade or a wire saw, After being obtained, it is manufactured by lapping both sides of the obtained plate-like wafer to a predetermined thickness, then mirror-polishing only one side with a polishing apparatus and washing after completion of polishing.

  And since the piezoelectric oxide single crystal wafer manufactured in this way is used for a surface acoustic wave filter or the like, a metal electrode is formed on the surface according to its use. When forming a pattern by plate making, etc., if the wafer surface shape is irregularly wavy, it becomes difficult to form a metal electrode in accordance with the designed electrode pattern. That is, a single crystal wafer having a small arithmetic average roughness Ra must be provided.

  Therefore, conventionally, a technique for mirror-polishing a wafer having a surface shape with small undulations is known. For example, Patent Document 1 discloses that the wafer surface can be undulated or cracked by the influence of heat by enabling the wafer to be bonded and peeled off from the polishing disk at an appropriate temperature without rapid heating or rapid cooling. A wax-type wafer bonding and peeling apparatus is described that prevents this from occurring. Patent Document 2 discloses a waxless method that avoids unevenness and waviness of a wafer by making a positive pressure fluid seal layer uniform by using negative pressure and forming a positive pressure fluid seal layer to prevent entry of abrasives. A polishing apparatus is described.

  Moreover, since the unevenness | corrugation and waviness which are formed in the surface of the holding | maintenance packing of a wafer worsen the flatness of a wafer in patent document 3, a holding | packing packing, a reinforcing material, and a separator are interposed by an adhesive or an adhesive agent. A wax-less wafer holding laminate is described in which the uniformity of layer thickness is increased by laminating. In Patent Document 4, in the waxless wafer holding method described in Patent Document 3, bubbles or wrinkles are generated between the holder and the carrier plate, or the adhesive coating thickness is not constant, so that the fixing surface has irregularities. In order to reduce the flatness of the wafer that occurs, a wafer holding method is described in which the back surface of the plate-shaped holder is brought into close contact with a carrier plate having a large number of holes by vacuum suction through the holes.

  Furthermore, Patent Document 5 describes a double-side polishing method using a fiber-reinforced resin guide carrier that can easily improve the flatness and the like of a piezoelectric oxide single crystal wafer for surface acoustic wave elements. If the back side of the single crystal wafer has the same mirror surface, the surface acoustic wave is excited and received, and unnecessary waves such as bulk waves (disturbance waves) are also excited and cause spurious interference in the frequency characteristics. Then, as a method of roughening the back surface, a method of laminating two single crystal wafers with the back surface roughened through an adsorption plate or the like and mirror-polishing the outer surfaces of the two single crystal wafers. Is also described.

  However, with such conventional technology, without sufficient evidence, we focus exclusively on making the wafer surface waviness and irregularities as small as possible to improve the flatness of the wafer, and for mirror polishing. The actual situation is that, without knowing the degree of processing strain that occurs until now, the thickness of the processing strain layer, etc., the emphasis has been on increasing the polishing allowance and increasing the flatness.

  As a result, in the conventional piezoelectric oxide single crystal wafer, the arithmetic average roughness Ra of the surface roughness is as fine as 0.1 nm to 0.3 nm, which is 1 nm or less, and the metal of the electrode pattern formed on this surface The problem of film peeling phenomenon has begun to attract attention. This is also because the frequency in which the surface acoustic wave element is used is a high frequency band exceeding 1 GHz, and the interelectrode width and the electrode width are being narrowed to the submicron unit accordingly.

  Further, in the conventional mirror polishing process, the polishing allowance is increased in order to increase the flatness. On the other hand, this is effective for removing the mirror surface processing strain, but the material of the piezoelectric oxide crystal. From the standpoint of removing more than necessary, there is a problem of increasing the wafer cost.

Japanese Utility Model Publication No. 58-129658 Japanese Patent Laid-Open No. 2-98926 JP-A-62-297064 JP-A-63-93562 JP-A-11-309665

In view of the above circumstances, an object of the present invention is that a piezoelectric oxide such as a lithium tantalate single crystal or a lithium niobate single crystal used for a surface acoustic wave filter or the like does not cause peeling of an electrode pattern with an appropriate surface roughness. It is to provide a method for manufacturing a single crystal wafer.

  As a result of intensive studies to achieve the above object, the present inventors have lost the characteristics as a surface acoustic wave device in a specular state with an appropriate surface roughness of arithmetic average roughness Ra of 1 nm to 8 nm. It was found that a single crystal wafer with a small processing strain to such an extent that it can be used for applications such as surface acoustic wave filters. In addition, since it has been newly found that peeling of the electrode pattern does not occur even if such a single-crystal wafer in a mirror state is subsequently made into a device, it is possible to manufacture a single-crystal wafer having such a surface roughness with good reproducibility. The present invention has been made by further devising a polishing method that can be used.

The present invention relates to a method for producing a piezoelectric oxide single crystal wafer in which a surface finish of a wafer comprising a sliced lithium tantalate single crystal or a lithium niobate single crystal is subjected to surface finish grinding. After the first surface rough grinding with a roughness Ra of 10 to 30 nm, the final surface finish grinding with a grinding amount of 1 to 5 μm is performed using a # 6000 or more grinding wheel, and the wafer The surface side of the substrate is characterized by an arithmetic average roughness Ra in the range of 1 nm to 8 nm.

  Furthermore, in the production method of the present invention, after the surface finish grinding, a single-side polishing process may be further performed to make the arithmetic average roughness Ra in the range of 1 nm to 4 nm.

  The piezoelectric oxide single crystal wafer of the present invention is preferably used for applications such as a surface acoustic wave filter, and has an excellent effect that peeling of an electrode pattern does not occur in a subsequent device fabrication process. In addition, according to the manufacturing method of the present invention, the mirror polishing process requires less polishing allowance than the conventional method, so that the yield can be greatly improved.

  Hereinafter, although the form for implementing this invention is demonstrated, the manufacturing process of a piezoelectric oxide single crystal wafer is demonstrated first. The piezoelectric oxide single crystal is grown, for example, by the Czochralski method, and its ingot is manufactured. After a plane (orientation flat) is formed in the specified direction for this ingot, the ingot is sliced to a predetermined thickness to obtain a plate-like wafer, and this sliced wafer is subjected to double-sided lapping as necessary. Grind to the desired thickness. The slicing and lapping in this case may be performed in the same manner as in the past.

  Next, the grinding process will be described. The surface grinder to be used (a device capable of rough grinding and finishing grinding with two or more axes) is equipped with a rough grinding wheel and a finishing grinding wheel, and is driven to rotate at an arbitrary rotational speed by a drive system (motor). be able to. 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 that can clamp the wafer by pulling the back surface to a vacuum. Further, the single-side polishing machine to be added as necessary may be one commonly used in semiconductors, and can clamp a wafer on a plate by vacuum suction and polish it with a colloidal silica abrasive.

  In the present invention, an actual mirror processing of one surface of a piezoelectric oxide single crystal wafer is performed using a surface grinder. Specifically, a lapped piezoelectric oxide single crystal wafer is held by vacuum suction. 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. For the determination of the whetstone, it may be selected based on the arithmetic average roughness Ra of lithium tantalate or lithium niobate, but in the case of rough grinding, a whetstone whose Ra is 10 to 30 nm, for example, # 4000 It is preferable to select a grindstone because the grinding time is short and the working strain layer can be kept small. Also, in the case of finish grinding, if a fine grinding wheel of # 6000 or more, preferably # 8000- # 10000, is used, the arithmetic average roughness Ra can be a substantial mirror surface in the range of 1 to 8 nm. It is preferable because it is possible. And if it is such a regular grindstone, it becomes possible to suppress the distortion layer produced by mirror surface processing to such an extent that it does not affect the operation of the surface acoustic wave element. When the arithmetic average roughness Ra is less than 1 nm, the metal film is likely to be peeled off, and when it exceeds 8 nm, the processing strain becomes apparent because it approaches the rough surface, and the characteristics of the surface acoustic wave device, for example, insertion loss This is not preferable because a problem such as a large value occurs.

As for the grinding amount in the grinding process, the grinding amount is set to 1 to 5 μm in the final finish grinding in consideration of the residual depth of grinding distortion. If the grinding amount is less than 1 μm, there is a risk that a strain layer generated by rough grinding may remain, and if it exceeds 5 μm, it will lead to the use of a piezoelectric crystal that is more expensive than necessary, which increases the cost of the wafer. Therefore, it is not preferable.
The polishing allowance by a single-side polishing machine is determined in consideration of the arithmetic average roughness Ra and the level of the processing strain layer in the surface grinding stage, but about 2μ is sufficient at most. By doing so, the mirror surface Ra can be suppressed to 1 to 4 nm.

Hereinafter, although an example and a comparative example are explained concretely, the present invention is not restricted to the following examples.
<Example 1>
In Example 1, a lithium tantalate (LiTaO ₃ ) single crystal having a diameter of 4 inches was used as a sample. This single crystal ingot manufactured by the Czochralski (CZ) method is cylindrically ground and OF processed, then sliced to 0.25 mm using a wire saw, and appropriate so that the arithmetic surface roughness Ra of the front and back surfaces is 0.12 μm The sliced wafer was lapped with a double-sided lapping to a desired thickness of 200 μm using a suitable abrasive (average grain size of free abrasive grains 7 μm). Next, the single crystal wafer thus lapped (roughening the front and back surfaces) was subjected to substantial mirror polishing using a surface grinder. Specifically, after 10 μm grinding with a rough grinding wheel and 2 μm grinding with finish grinding, this wafer was washed, and the flatness and arithmetic average roughness were measured.

  The same process was performed on 100 wafers and the flatness was measured. The TTV (total thickness variation) representing the wafer thickness variation was 0.30 to 1.02 μm, LTVmax at 5 mm x 5 mm site. (Local thickness variation) was 0.10 to 0.21 μm, and PLTV (percent local thickness variation) was 99% or more. Moreover, when arithmetic mean roughness was measured with five wafers, the result was 2.3 nm to 7.5 nm.

  Next, a metal film mainly made of aluminum is attached to the mirror side of the wafer thus obtained, and an electrode having a line width of 0.5 microns (comb type) is formed on the wafer surface by photolithography, and an adhesive tape is used. When the peeling test was performed, no peeling of the metal film was observed. Further, when the loss, which is a characteristic of the surface acoustic wave device, was examined for the chip on which the electrode pattern was formed, no increase in the loss was observed, so a substantially undistorted mirror surface was formed on the wafer surface. It was confirmed that

<Example 2>
In Example 2, the ground wafer obtained in Example 1 was further polished using colloidal silica with a single-side polishing machine. The machining allowance in this polishing process was 1.7 microns. The same process was performed on 100 wafer substrates, and the flatness was measured. As a result, the TTV was 0.35-1.32 μm, the LTVmax at 5 mm × 5 mm site was 0.14-0.28 μm, and the PLTV was 99% or more. there were. Moreover, when arithmetic mean roughness was measured with five wafers, the result was 1.3 nm to 3.5 nm.

  Next, a metal film mainly made of aluminum is attached to the mirror surface side of the substrate thus obtained, an electrode having a line width of 0.5 microns (comb type) is formed on the substrate surface by photolithography, and an adhesive tape is used. When the peeling test was performed, no peeling of the metal film was observed. Further, when the loss, which is a characteristic of the surface acoustic wave device, was examined for the chip on which the electrode pattern was formed, no increase in the loss was observed, so a substantially undistorted mirror surface was formed on the wafer surface. It was confirmed that

<Comparative example>
In the comparative example, the ground wafer obtained in Example 1 was further polished using colloidal silica with a single-side polishing machine. The machining allowance in this polishing process was 10 microns. The same process was carried out on 100 wafers and the flatness was measured. As a result, the TTV was 0.50 to 2.32 μm, the LTVmax at 5 mm × 5 mm site was 0.34 to 0.48 μm, and the PLTV was 99% or more. It was. Moreover, when arithmetic mean roughness was measured with five wafers, the result was 0.18 nm to 0.24 nm.

  Next, a metal film mainly made of aluminum is attached to the mirror surface side of the substrate thus obtained, an electrode having a line width of 0.5 microns (comb type) is formed on the substrate surface by photolithography, and an adhesive tape is used. When a peeling test was performed, peeling was observed in some metal films. Further, when the loss, which is a characteristic of the surface acoustic wave device, was examined for the chip on which the electrode pattern was formed, no increase in the loss was observed. Therefore, even in the comparative example, the surface of the wafer was substantially unstrained. It was confirmed that the mirror surface was made.

  As described above, in Examples 1 and 2 in which the arithmetic average roughness Ra of the surface is in the range of 1 nm to 8 nm, no peeling of the metal film was confirmed. Therefore, the metal film forming the electrode pattern than the comparative example It became clear that the adhesiveness of the was excellent. Further, since the processing accuracy of the wafer flatness is excellent, the device yield can be improved.

  In Examples 1 and 2, the description has been made by taking a 4-inch diameter lithium tantalate wafer as an example, but the present invention also applies to a piezoelectric oxide single crystal wafer such as lithium niobate used for a surface acoustic wave filter or the like. This is also effective, and the thickness and diameter of the wafer are not limited to those in the first and second embodiments.

According to the present invention, since a piezoelectric oxide single crystal wafer having a high yield and excellent flatness can be manufactured, this is a great contribution to the device manufacturing industry in the field of mobile communication.

Claims (2)

In a method for manufacturing a piezoelectric oxide single crystal wafer in which surface finishing of a wafer comprising a sliced lithium tantalate single crystal or lithium niobate single crystal is subjected to surface finish grinding, the surface side of the wafer is expressed by an arithmetic average roughness Ra. After the first surface rough grinding of 10 to 30 nm, the final surface finish grinding with a grinding amount of 1 to 5 μm is performed using a # 6000 or more grinding wheel, and the wafer surface side A method for producing a piezoelectric oxide single crystal wafer, characterized in that the arithmetic average roughness Ra is in the range of 1 nm to 8 nm. 2. The piezoelectric oxide single layer according to claim 1 , wherein after the surface finish grinding, a single-side polishing process is further performed on the surface side of the wafer to obtain an arithmetic average roughness Ra in a range of 1 nm to 4 nm. Crystal wafer manufacturing method.