METHOD FOR MECHANICAL GRINDING OF SOI WAFER AND MECHANICAL GRINDING APPARATUS

Abstract

The present invention provides a method for mechanical grinding of an SOI wafer and a mechanical grinding apparatus. The method includes a mechanical grinding process including a coarse grinding process, in which a feed speed of a coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition, thereby allowing for a damaged area proportion on a side of a carrier wafer away from a device wafer to be lower than a predetermined threshold during thinning of the device wafer on a side thereof away from the carrier wafer by mechanical grinding to a predetermined thickness. The present invention overcomes the problems that significant backside damage may be caused to an SOI wafer during its mechanical grinding, which may affect the performance and reliability of a device fabricated from the wafer.

Description

CROSS-REFERENCES TO RELATED APPLICATION

This application claims the priority of Chinese patent application number 202510058779.0, filed on Jan. 14, 2025, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the field of integrated circuit technology and, in particular, to a method for mechanical grinding of an SOI wafer and a mechanical grinding apparatus.

BACKGROUND

Silicon-on-insulator (SOI) wafers have been used in the field of electronics materials as one of the mainstream substrates for most electronic products. SOI, as an integrated circuit fabrication technique, involves growth of a thin layer of silicon over an insulator substrate in such a manner that the grown silicon layer is insulated from the substrate. On SOI wafers, integrated circuits can be fabricated with improved performance and increased reliability, which consume less power and generate reduced noise. Moreover, SOI enables further device miniaturization. With the rapid development of new energy vehicles, chips fabricated from SOI wafers for use on board such vehicles are gradually increasing their degree of integration. Accordingly, there is an increasing demand on the market for SOI wafers with higher uniformity, developing more uniform SOI wafers has attracted much interest in the art.

In the state of the art, an SOI wafer with a desired thickness is typically obtained from a mechanical grinding or polishing process. For these mechanical grinding and polishing processes, it is necessary to take into account the thinness of a top silicon layer. A surface-polarized top silicon layer with a non-uniform thickness may lead to degraded performance stability of a device fabricated from the wafer, local current density inconsistency in an electron channel, non-uniform thermal effects, or even line width variations. Since an SOI wafer has a very thin top silicon layer with a thickness generally of a few microns, mechanical grinding tends to cause more damage to its backside, possibly affecting the performance and reliability of a device fabricated from the wafer.

SUMMARY

It is an object of the present invention to provide a method for mechanical grinding of an SOI wafer and a mechanical grinding apparatus, which overcome the problem of higher proneness of the backside of an SOI wafer (with or without an insulator layer) to damage during a mechanical grinding process, which may affect the performance and reliability of a device fabricated from the wafer.

To this end, the present invention provides a method for mechanical grinding of an SOI wafer, which includes:

• • providing the SOI wafer, which includes a carrier wafer, a device wafer and a buried oxide layer between the carrier wafer and the device wafer; and
  • performing a mechanical grinding process to thin the device wafer on a side thereof away from the carrier wafer to a predetermined thickness, concurrently with a damaged area proportion on a side of the carrier wafer away from the device wafer being maintained lower than a predetermined threshold,
  • wherein the mechanical grinding process includes a coarse grinding process, in which a feed speed of a coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition.

Optionally, the satisfaction of the predefined condition by the feed speed of the coarse grinding wheel and the rotational speed of the grinding table may include: decreasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table exceeds a preset rotational speed; and increasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table is below a preset rotational speed.

Optionally, the predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy may be expressed as:

$Y=B-A*X,$

where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

Optionally, the coarse grinding wheel may have a mesh size of 50 to 800 and be driven by a spindle to rotate at a speed of 500 rpm to 4,000 rpm.

Optionally, the mechanical grinding process may further include a fine grinding process, in which a fine grinding wheel with a mesh size of 1,500 to 10,000 is driven by a spindle to rotate at a speed of 500 rpm to 4,000 rpm.

Optionally, after experiencing the mechanical grinding process, the device wafer in the SOI wafer may have a thickness of greater than 12 μm and a total thickness variation (TTV) of less than 1.5 μm.

Optionally, the carrier wafer in the SOI wafer may have a thickness of 772 μm to 777 μm and a TTV of less than 0.4 μm.

On the basis of the same inventive concept, the present invention also provides a mechanical grinding apparatus for implementing a method for mechanical grinding of an SOI wafer as defined in any of the preceding paragraphs, which includes a coarse grinding wheel and a grinding table, wherein a feed speed of the coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition, which enables a device wafer to be thinned on a side thereof away from a carrier wafer to a predetermined thickness, with a damaged area proportion on a side of the carrier wafer away from the device wafer being maintained below a predetermined threshold.

Optionally, the satisfaction of the predefined condition by the feed speed of the coarse grinding wheel and the rotational speed of the grinding table may include: decreasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table exceeds a preset rotational speed; and increasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table is below a preset rotational speed.

Optionally, the predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy may be expressed as:

$Y=B-A*X,$

where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

BRIEF DESCRIPTION OF THE DRAWINGS

Those of ordinary skill in the art will understand that the following drawings are presented to enable a better understanding of the present invention and not intended to limit the scope thereof in any sense, in which:

FIG. 1 shows a graph plotting the dependence of backside damage to an SOI wafer on a feed speed and a rotational speed of a grinding table, according to an embodiment of the present invention;

FIG. 2 shows backside damage to an SOI wafer in Group 1, according to an embodiment of the present invention;

FIG. 3 shows backside damage to an SOI wafer in Group 2, according to an embodiment of the present invention;

FIG. 4 shows backside damage to an SOI wafer in Group 3, according to an embodiment of the present invention;

FIG. 5 shows backside damage to an SOI wafer in Group 4, according to an embodiment of the present invention;

FIG. 6 is a flowchart of a method for mechanical grinding of an SOI wafer, according to an embodiment of the present invention;

FIGS. 7 to 8 show schematic representations of structures corresponding to steps in a method for mechanical grinding of an SOI wafer, according to an embodiment of the present invention; and

FIG. 9 shows a schematic diagram of a mechanical grinding apparatus, according to an embodiment of the present invention.

In these figures, 10 denotes a carrier wafer; 11, a first buried oxide layer; 20, a device wafer; 21, a second buried oxide layer; 30, a mechanical grinding apparatus; 31, a grinding table; and 32, a coarse grinding wheel.

DETAILED DESCRIPTION

Objects, advantages and features of the present invention will become more apparent upon reading the following more detailed description of specific embodiments thereof in conjunction with the accompanying drawings. Note that the figures are rather simplified and not necessarily to scale, with the only intention to help explain embodiments of the invention disclosed herein in a more convenient and clearer way. In addition, the illustrated structures are usually part of their real-world counterparts. In particular, as the figures tend to have distinct emphases, they are sometimes drawn to different scales.

As used herein, the singular forms “a”, “an” and “the” include plural referents, and the term “or” is generally employed in the sense of “and/or”, “a number of” of “at least one” and “at least two” of “two or more”. Additionally, the use of the terms “first”, “second” and “third” herein is intended for illustration only and is not to be construed as denoting or implying relative importance, or as implicitly indicating the number of referenced items. Accordingly, defining an item with “first”, “second” or “third” is an explicit or implicit indication of the presence of one or at least two such items. When an element is referred herein to as being “disposed” on another element, this is generally intended to only mean that there is a connection, coupling, engagement or transmission relationship between the two elements, which may be either direct or indirect with one or more intervening elements, and should not be interpreted as indicating or implying a particular spatial position relationship between them. That is, the element may be located inside, outside, above, under, beside, or at any other location relative to the other element, unless the context clearly dictates otherwise. Those of ordinary skill in the art can understand the specific meanings of the above-mentioned terms herein, depending on their context.

Research conducted by the inventors indicates that, since a top silicon layer of an SOI wafer has a very small thickness, typically of several microns, it would take a very long time for a mechanical grinding process to thin an approximately 775 μm device wafer therein to a desired thickness, due to a significant amount of material to be removed by mechanical grinding. Throughout this process, a backside carrier wafer in the SOI wafer remains in contact with a grinding table and is therefore prone to irreversible damage to its contacted backside surface, which would create a high risk of peeling off and cracking in subsequent processes. Once this happens, the SOI wafer may be contaminated, or may even have to be scrapped. In worse cases, extremely heavy losses may be incurred in subsequent fabrication of devices.

A mechanical grinding process involves coarse grinding and fine grinding. Since an SOI wafer is maintained in closer contact with a grinding table at a higher pressure during coarse grinding, damage may be particularly in this step caused to a backside of the SOI wafer (i.e., a side of a device wafer away from a carrier wafer). In a conventional coarse grinding process, backside damage to an SOI wafer may arise from a feed speed of a coarse grinding wheel and a rotational speed of a grinding table, and optimization is usually achieved by separately adjusting the feed speed of the coarse grinding wheel or the rotational speed of the grinding table. However, when attempting to accelerate the process by increasing the feed speed, significant damage may be still caused to a backside insulator layer in the SOI wafer. Therefore, the inventors hypothesize that there might be a process window, in which the feed speed of the coarse grinding wheel and the rotational speed of the grinding table cooperate synergistically to prevent significant backside damage to the SOI wafer, and have made attempts to expand such a process window.

The inventors have conducted an experiment designed using a DoE (Design of Experiments) approach to investigate a feed speed of a coarse grinding wheel and a rotational speed of a grinding table used in a coarse grinding process as two factors that might affect the performance of the process. As indicated by a large amount of data obtained from the experiment, the presence of backside damage to ground SOI wafers is remarkably correlated with the feed speed of the coarse grinding wheel and the rotational speed of the grinding table. When the feed speed of the coarse grinding wheel is controlled within the range of 3 μm/s to 10 μm/s during grinding process, the rotational speed of the grinding table Y and the feed speed of the coarse grinding wheel X satisfy the formula Y=B−A*X, where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

Specifically, FIG. 1 shows a graph plotting the dependence of backside damage to an SOI wafer on a feed speed and a rotational speed of a grinding table according to an embodiment of the present invention. We have verified the presence of backside damage to SOI wafers after they were coarsely ground on a grinding table 31 rotating at different speed with a coarse grinding wheel 32 fed at a speed controlled in the range of 3 μm/s to 10 μm/s. The results are shown in FIG. 1, in which the abscissa represents the feed speed, measured in μm/s, and the ordinate represents the rotational speed of the grinding table, measured in revolutions per minute (rpm). In the graph of FIG. 1, SOI wafers without significant backside damage are marked with green dots, while those with significant backside damage are marked with red dots. The results show that the presence of backside damage to the ground SOI wafers is remarkably correlated with the feed speed of the coarse grinding wheel and the rotational speed of the grinding table. When the feed speed of the coarse grinding wheel is controlled within the range of 3 μm/s to 10 μm/s during coarse grinding, the rotational speed of the grinding table Y and the feed speed of the coarse grinding wheel X satisfy the formula Y=B−A*X, where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

The inventors have also investigated and tested four groups in an experiment. In all the groups, each carrier wafer 10 exhibits the same thickness of 773 μm, and each top silicon layer (i.e., each device wafer) is mechanically ground to the same thickness of 13 μm. The groups are ground on a grinding table 31 rotating at different speeds with a coarse grinding wheel 32 fed at different speeds. For Group 1, the coarse grinding wheel 32 is fed at a speed of 4 μm/s, and the grinding table 31 is rotated at a speed of 120 rpm. For Group 2, the coarse grinding wheel 32 is fed at a speed of 5 μm/s, and the grinding table 31 is rotated at a speed of 200 rpm. For Group 3, the coarse grinding wheel 32 is fed at a speed of 7 μm/s, and the grinding table 31 is rotated at a speed of 20 rpm. For Group 4, the coarse grinding wheel 32 is fed at a speed of 9 μm/s, and the grinding table 31 is rotated at a speed of 100 rpm. SOI wafers in the four groups are prepared using the same process. Specifically, the process includes: providing two silicon wafers, one as a device wafer 20, and the other as a carrier wafer 10. The carrier wafer 10 has a thickness of 773 μm. One or both of the silicon wafers are thermally oxidized, obtaining an SOI buried oxide layer. The device wafer 20 and the carrier wafer 10 are bonded to each other at a reduced pressure of 0.01 mbar to 100 mbar, e.g., 5 mabr. The resulting SOI wafer with the buried oxide layer is further subjected to a heat treatment for strength enhancement, chamfering, edge etching and mechanical grinding. After being mechanically ground, each SOI wafer is overall dished in shape along its thickness and has a top silicon layer (i.e., the device wafer 20) with a thickness of 13 μm.

TABLE 1
Presence of backside damage to SOI wafers at different
feed speeds of coarse grinding wheel and different
rotational speeds of grinding table
	Thick-	Feed Speed	Rotational	Thickness	Presence of
	ness of	of Coarse	Speed of	of Ground	Backside
	Carrier	Grinding	Grinding	Top Silicon	Damage to
	Wafer	Wheel	table	Layer	Grounded
Group	(μm)	(μm/s)	(rpm)	(μm)	Wafer
1	773	4	120	13	No
2	773	5	200	13	Yes (50%)
3	773	7	20	13	Yes (10%)
4	773	9	100	13	No

FIG. 2 shows backside damage to an SOI wafer in Group 1, according to an embodiment of the present invention. FIG. 3 shows backside damage to an SOI wafer in Group 2, according to an embodiment of the present invention. FIG. 4 shows backside damage to an SOI wafer in Group 3, according to an embodiment of the present invention. FIG. 5 shows backside damage to an SOI wafer in Group 4, according to an embodiment of the present invention. As shown in Table 1 and FIGS. 2 to 5, the SOI wafers in Groups 1 and 4 are absent of significant backside damage, because their damaged area proportions are below a predetermined threshold, which is 5%. For Group 1, the coarse grinding wheel 32 is fed at a speed of 4 μm/s, and the grinding table 31 is rotated at a speed of 120 rpm. For Group 4, the coarse grinding wheel 32 is fed at a speed of 9 μm/s, and the grinding table 31 is rotated at a speed of 100 rpm. The feed speed of the coarse grinding wheel 32 and the rotational speed of the grinding table 31 for each of Groups 1 and 4 satisfy the predefined condition. In contrast, significant backside damage to the SOI wafers of Groups 2 and 3 can be confirmed because their damaged area proportions are 50% and 10%, respectively, both higher than the predetermined threshold (5%). For Group 2, the coarse grinding wheel 32 is fed at a speed of 5 μm/s, and the grinding table 31 is rotated at a speed of 200 rpm. For Group 3, the coarse grinding wheel 32 is fed at a speed of 7 μm/s, and the grinding table 31 is rotated at a speed of 20 rpm. The feed speed of the coarse grinding wheel 32 and the rotational speed of the grinding table 31 for each of Groups 2 and 3 do not meet the predefined condition.

On the basis of the above, it is a principal object of the present invention to provide a mechanical grinding method including a coarse grinding process, in which a feed speed of a coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition, thereby allowing for a damaged area proportion on a side of a carrier wafer away from a device wafer to be lower than a predetermined threshold during thinning of the device wafer on a side thereof away from the carrier wafer by mechanical grinding to a predetermined thickness. The present invention overcomes the problems that significant backside damage may be caused to an SOI wafer during its mechanical grinding, which may affect the performance and reliability of a device fabricated from the wafer. That is, with the present invention, damage to a carrier wafer in an SOI wafer on a side thereof away from a device wafer in the SOI wafer can be remarkably reduced, promising good performance and reliability of a device fabricated from the wafer.

Specifically, FIG. 6 is a flowchart of a method for mechanical grinding of an SOI wafer, according to an embodiment of the present invention. As shown in FIG. 6, embodiments of the present invention provide a method for mechanical grinding of an SOI wafer, which includes the steps of:

• • S10) providing the SOI wafer, which includes a carrier wafer, a device wafer and a buried oxide layer between the carrier and device wafers; and
  • S20) performing a mechanical grinding process to thin the device wafer on a side thereof away from the carrier wafer to a predetermined thickness, concurrently with a damaged area proportion on a side of the carrier wafer away from the device wafer being maintained lower than a predetermined threshold,
  • wherein the mechanical grinding process includes a coarse grinding process, in which a feed speed of a coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition.

The satisfaction of the predefined condition by the feed speed of the coarse grinding wheel and the rotational speed of the grinding table may include: decreasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table exceeds a preset rotational speed; and increasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table is below a preset rotational speed.

The predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy may be expressed as Y=B−A*X, where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

FIGS. 7 to 8 show schematic representations of structures corresponding to steps in a method for mechanical grinding of an SOI wafer, according to an embodiment of the present invention. Specific embodiments of the present invention will be described in detail below with reference to FIGS. 7 to 8 so that the above-described objects, features and benefits thereof will become more apparent.

As shown in FIG. 7, the SOI wafer is provided, which includes a device wafer 20, a carrier wafer 10 and a buried oxide layer between the carrier wafer 10 and the device wafer 20.

In detail, the SOI wafer may be formed using a process including the step of providing the carrier wafer 10 and the device wafer 20. The carrier wafer 10 is provided to provide support for the device wafer 20 during thinning thereof. The carrier wafer 10 may have a thickness of 772 μm to 777 μm, for example. The carrier wafer 10 and the device wafer 20 may both have a total thickness variation (TTV) of less than 0.4 μm and a diameter of 200 mm or 300 mm. The device wafer 20 is provided as an object on which subsequent processes are to be carried out. It may be any substrate known to those skilled in the art for carrying semiconductor integrated circuit components, such as a bare wafer, or a wafer that has experienced epitaxial growth. In particular, the device wafer 20 may be provided as a substrate such as a silicon-on-insulator (SOI) substrate, a bulk silicon substrate, a germanium substrate, a silicon-germanium (SiGe) substrate, an indium phosphide (InP) substrate, a gallium arsenide (GaAs) substrate, or germanium-on-insulator (GeOI) substrate, amongst others. In the present embodiment, the device wafer 20 is provided as a silicon wafer. A thermal oxidation process is then carried out to form a buried oxide layer on the carrier wafer 10 and/or the device wafer 20. Specifically, the thermal oxidation process may be performed on one of the carrier wafer 10 and the device wafer 20 to form a buried oxide layer thereon. Alternatively, the thermal oxidation process may be performed on both of the carrier wafer 10 and the device wafer 20 to form a buried oxide layer on each of these wafers. Those skilled in the art can make an appropriate choice depending on the thickness of the buried oxide layer. In the present embodiment, the thickness of the buried oxide layer may range from 0.2 μm to 2 μm, for example. Accordingly, a buried oxide layer is formed on each of the carrier wafer 10 and the device wafer 20. That is, a first buried oxide layer 11 is formed on the carrier wafer 10, and a second buried oxide layer 21 on the device wafer 20. A bonding process is then performed to bond the carrier wafer 10 and the device wafer 20 to each other so that the buried oxide layers are sandwiched between the carrier wafer 10 and the device wafer 20. That is, the first buried oxide layer 11 and the second buried oxide layer 21 are situated between the carrier wafer 10 and the device wafer 20. In other words, the first buried oxide layer 11 and the second buried oxide layer 21 are brought into contact with each other. After the SOI wafer is formed, it may be successively subjected to a heat treatment for strength enhancement, chamfering and edge etching.

As shown in FIG. 8, a mechanical grinding process is performed to thin the device wafer 20 on a side thereof away from the carrier wafer 10 to a predetermined thickness, concurrently with a damaged area proportion on a side of the carrier wafer 10 away from the device wafer 20 being maintained lower than a predetermined threshold. That is, after the mechanical grinding process is completed, no significant damage is caused to the side of the carrier wafer 10 away from the device wafer 20. Here, by “no significant damage”, it is intended to mean that the damaged area proportion is lower than the predetermined threshold. The predetermined threshold is 5%, meaning that the damaged area proportion is lower than 5%.

The mechanical grinding process includes a coarse grinding process and a fine grinding process. In the coarse grinding process, a coarse grinding wheel with a mesh size of 50 to 800 may be driven by a spindle to rotate at a speed of, for example, 500 rpm to 4,000 rpm. In the fine grinding process, a fine grinding wheel with a mesh size of 1,500 to 10,000 may be driven by a spindle to rotate at a speed of, for example, 500 rpm to 4,000 rpm.

Since the SOI wafer is brought into contact with a grinding table at a higher pressure in the coarse grinding process, damage is more likely to be caused to a backside of the SOI wafer (i.e., a side of the carrier wafer away from the device wafer) in this process. Accordingly, a feed speed of the coarse grinding wheel and a rotational speed of the grinding table are configured to satisfy a predefined condition in the coarse grinding process to prevent significant backside damage to the grounded SOI wafer.

Specifically, the predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy takes into account inter-dependence of the feed speed of the coarse grinding wheel and the rotational speed of the grinding table. To meet this condition, if the rotational speed of the grinding table exceeds a preset rotational speed, i.e., if the rotational speed of the grinding table is too fast, the feed speed of the coarse grinding wheel may be decreased to avoid causing backside damage to the SOI wafer. If the rotational speed of the grinding table is below a preset rotational speed, i.e., if the rotational speed of the grinding table is too slow, the feed speed of the coarse grinding wheel may be appropriately increased to accelerate material removal on a front side of the SOI wafer without causing backside damage to the SOI wafer. The front side of the SOI wafer is provided by a side of the device wafer away from the carrier wafer.

In greater detail, the predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy may be expressed as:

$Y=B-A*X,$

where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

With continued reference to FIG. 8, after experiencing the mechanical grinding process, the device wafer in the SOI wafer has a thickness of greater than 12 μm and a TTV of less than 1.5 μm.

FIG. 9 shows a schematic diagram of a mechanical grinding apparatus, according to an embodiment of the present invention. Embodiments of the present invention also provide a mechanical grinding apparatus 30 for implementing any of the methods as defined above. The mechanical grinding apparatus 30 includes a coarse grinding wheel 32, a fine grinding wheel (not shown) and a grinding table 31. The coarse grinding wheel 32 may have a mesh size of, for example, 50 to 800, and may be driven by a spindle to rotate at a speed of, for example, 500 rpm to 4,000 rpm. The fine grinding wheel may have a mesh size of 1,500 to 10,000, and may be driven by a spindle to rotate at a speed of, for example, 500 rpm to 4,000 rpm. Since an SOI wafer is brought into contact with the grinding table at a higher pressure in the course of coarse grinding, damage is more likely to be caused to a backside of the SOI wafer (i.e., a side of a carrier wafer away from a device wafer therein) in this process. Accordingly, a feed speed of the coarse grinding wheel 32 and a rotational speed of the grinding table 31 may be configured to satisfy a predefined condition during coarse grinding to prevent significant backside damage to the grounded SOI wafer.

The predefined condition that the feed speed of the coarse grinding wheel 32 and the rotational speed of the grinding table 31 satisfy takes into account inter-dependence of the feed speed of the coarse grinding wheel 32 and the rotational speed of the grinding table 31. To meet this condition, if the rotational speed of the grinding table 31 exceeds a preset rotational speed, i.e., if the rotational speed of the grinding table 31 is too fast, the feed speed of the coarse grinding wheel 32 may be decreased to avoid causing backside damage to the SOI wafer. If the rotational speed of the grinding table 31 is below a preset rotational speed, i.e., if the rotational speed of the grinding table 31 is too slow, the feed speed of the coarse grinding wheel 32 may be appropriately increased to accelerate material removal on a front side of the SOI wafer without causing backside damage to the SOI wafer. The front side of the SOI wafer is provided by a side of the device wafer away from the carrier wafer.

In greater detail, the predefined condition that the feed speed of the coarse grinding wheel 32 and the rotational speed of the grinding table 31 satisfy may be expressed as:

$Y=B-A*X,$

where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

In summary, embodiments of the present invention provide a method for mechanical grinding of an SOI wafer, which includes a coarse grinding process. In this coarse grinding process, a feed speed of a coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition, thereby allowing for a damaged area proportion on a side of a carrier wafer away from a device wafer to be lower than a predetermined threshold during thinning of the device wafer on a side thereof away from the carrier wafer by mechanical grinding to a predetermined thickness. The present invention overcomes the problems that significant backside damage may be caused to an SOI wafer during its mechanical grinding, which may affect the performance and reliability of a device fabricated from the wafer. That is, with the present invention, damage to a carrier wafer in an SOI wafer on a side thereof away from a device wafer in the SOI wafer can be remarkably reduced, promising good performance and reliability of a device fabricated from the wafer.

Further, it will be recognized that while the invention has been described above with respect to preferred embodiments, it is not intended to be limited to these embodiments. In light of the above teachings, any person familiar with the art may make many possible modifications and variations to the disclosed embodiments or adapt them into equivalent embodiments, without departing from the scope of the invention. Accordingly, it is intended that any and all simple variations, equivalent changes and modifications made to the foregoing embodiments based on the substantive disclosure of the invention without departing from the scope thereof fall within this scope.

Claims

1. A method for mechanical grinding of a silicon-on-insulator (SOI) wafer, comprising:

providing the SOI wafer, wherein the SOI wafer comprises a carrier wafer, a device wafer and a buried oxide layer between the carrier wafer and the device wafer; and

performing a mechanical grinding process to thin the device wafer on a side thereof away from the carrier wafer to a predetermined thickness and ensure that a damaged area proportion on a side of the carrier wafer away from the device wafer is maintained lower than a predetermined threshold,

wherein the mechanical grinding process comprises a coarse grinding process, in which a feed speed of a coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition.

2. The method according to claim 1, wherein the predefined condition satisfied by the feed speed of the coarse grinding wheel and the rotational speed of the grinding table comprises: decreasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table exceeds a preset rotational speed; and increasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table is below a preset rotational speed.

3. The method according to claim 1, wherein the predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy is expressed as: Y = B - A * X,

where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.

4. The method according to claim 1, wherein the coarse grinding wheel has a mesh size of 50 to 800 and is driven by a spindle to rotate at a speed of 500 rpm to 4,000 rpm.

5. The method according to claim 1, wherein the mechanical grinding process further comprises a fine grinding process, in which a fine grinding wheel with a mesh size of 1,500 to 10,000 is driven by a spindle to rotate at a speed of 500 rpm to 4,000 rpm.

6. The method according to claim 1, wherein after experiencing the mechanical grinding process, the device wafer in the SOI wafer has a thickness of greater than 12 μm and a total thickness variation of less than 1.5 μm.

7. The method according to claim 1, wherein the carrier wafer in the SOI wafer has a thickness of 772 μm to 777 μm and a total thickness variation of less than 0.4 μm.

8. A mechanical grinding apparatus for implementing a method for mechanical grinding of a silicon-on-insulator (SOI) wafer as defined in claim 1, comprising a coarse grinding wheel and a grinding table, wherein a feed speed of the coarse grinding wheel and a rotational speed of a grinding table satisfy a predefined condition, which enables a device wafer to be thinned on a side thereof away from a carrier wafer to a predetermined thickness and ensures that a damaged area proportion on a side of the carrier wafer away from the device wafer is maintained below a predetermined threshold.

9. The mechanical grinding apparatus according to claim 8, wherein the predefined condition satisfied by the feed speed of the coarse grinding wheel and the rotational speed of the grinding table comprises decreasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table exceeds a preset rotational speed; and increasing the feed speed of the coarse grinding wheel if the rotational speed of the grinding table is below a preset rotational speed.

10. The mechanical grinding apparatus according to claim 8, wherein the predefined condition that the feed speed of the coarse grinding wheel and the rotational speed of the grinding table satisfy is expressed as: Y = B - A * X,

where X is the feed speed of the coarse grinding wheel, Y is the rotational speed of the grinding table, A is an amount of change in the rotational speed of the grinding table that occurs in response to a unit amount of change in the feed speed of the coarse grinding wheel, and B is a safe range of the rotational speed of the grinding table, 5≤A≤10, 100≤B≤200, and 3 μm/s≤X≤10 μm/s.