Method of grinding a semiconductor wafer

Abstract

A method of grinding a semiconductor wafer, wherein a semiconductor wafer is processed in a material-removing manner by means of a grinding tool comprising grinding teeth having a height h while a cooling medium is supplied to a contact area between the rotating semiconductor wafer and the grinding tool, wherein at each instant of grinding a first coolant flow is applied to a first area of one side of the semiconductor wafer by means of one or more nozzles and at each instant of grinding a second coolant flow is applied to a second area of the one side of the semiconductor wafer by means of one or more nozzles, characterized in that the first area is bounded by the lower right quadrant of the semiconductor wafer and the second area is bounded by the lower left quadrant of the semiconductor wafer, and the ratio of the first coolant flow to the sum of the first coolant flow and the second coolant flow is not greater than 35% and not less than 25%.

Description

Technical Field

[0001] The subject of this invention is a method for grinding semiconductor material wafers. This invention is based on the optimal distribution of fluid around the grinding tool to process the semiconductor wafer in a manner that removes material from both sides simultaneously. Background Technology

[0002] For electronic components, microelectronic components, and microelectromechanical components, semiconductor wafers with extremely high requirements for overall and local flatness, single-sided reference local flatness (nanomorphology), roughness, and cleanliness are needed as starting materials (substrates). Semiconductor wafers are wafers made of semiconductor materials (especially compound semiconductors such as gallium arsenide or elemental semiconductors such as silicon and germanium).

[0003] According to existing technology, semiconductor wafers are produced in multiple successive processing steps. The following production sequence is typically used:

[0004] 1- Production of single-crystal semiconductor rods (crystal growth);

[0005] 2. Divide the semiconductor rod into individual rod segments;

[0006] 3- Separate the rod into individual wafers (using an internal diameter saw or wire saw);

[0007] 4. Perform mechanical processing on the wafer (grinding, grinding);

[0008] 5. Perform chemical treatment on the wafer (alkaline or acidic etching);

[0009] 6. Perform chemical mechanical treatment (polishing) on ​​the wafer;

[0010] 7- Optional further coating steps (e.g., epitaxy, heat treatment).

[0011] Mechanical processing of semiconductor wafers is used to remove ripples caused by sawing, as well as surface layers that have been damaged by rough sawing or contaminated by saw lines. Most importantly, it is used to planarize the semiconductor wafer overall. Furthermore, mechanical processing of semiconductor wafers is also used to produce a uniform thickness distribution, meaning that the semiconductor wafer has a uniform thickness.

[0012] Grinding and surface polishing (single-disc, double-disc) are known methods for mechanically processing semiconductor wafers.

[0013] The technique of simultaneously grinding multiple semiconductor wafers on a dual-disc basis has a long history and is described, for example, in EP 547894A1. In dual-disc grinding, the semiconductor wafer moves under pressure while a suspension containing abrasive material is conveyed between an upper and a lower working disk, which are often made of steel and have channels for better distribution of the suspension, thereby enabling material removal. During grinding, the semiconductor wafer is guided by a carrier with grooves for holding the wafer, and the wafer is held in a geometric path by means of a rotating carrier with drive gears.

[0014] In single-disc grinding, the semiconductor wafer is held on a chuck at the rear, while at the front it is planarized by a cup-shaped grinding wheel as the chuck and grinding wheel rotate and advance slowly axially and radially. Methods and apparatus for single-disc surface grinding of semiconductor wafers are known, for example, from US2008021 40 94A1 or EP 0 955 126 A2.

[0015] KR 2011 006 6282A discloses a grinding wheel equipped with at least one coolant delivery hole to improve cooling efficiency so as to deliver coolant directly to the grinding tool.

[0016] In simultaneous double-disc grinding (sDDG), a semiconductor wafer is processed simultaneously on both sides while floating freely between two grinding wheels mounted on opposing collinear spindles. In this case, the semiconductor wafer is essentially unaffected by constraint forces and is axially guided between water cushions (hydrostatic principle) or air cushions (aerostatic principle) acting on both sides, and radial floating is loosely prevented by surrounding thin guide rings or individual radial spokes. Methods and apparatus for simultaneous double-disc surface grinding of semiconductor wafers are known, for example, from EP 0 755 751 A1, EP 0 971 398 A1, DE 10 2004 011 996 A1 and DE 10 2006032 455A1.

[0017] However, due to kinematic reasons, dual-disc grinding (DDG) of semiconductor wafers inherently leads to the removal of more material at the center of the semiconductor wafer ("grinding navel"). To obtain a semiconductor wafer with the best possible geometry after grinding, the two grinding spindles on which the grinding wheels are mounted must be perfectly collinearly aligned, as radial and / or axial misalignment negatively impacts the shape and nanostructure of the ground wafer. German patent application DE 10 2007 049 810 A1, for example, illustrates a method for correcting the position of the grinding spindles in a dual-disc grinding machine.

[0018] In grinding processes (both single-disc and double-disc grinding methods), cooling the grinding tool and / or the semiconductor wafer being processed is essential. Water or deionized water is often used as the coolant. In double-disc grinding machines, the coolant typically emerges from the center of the grinding tool and is centrifugally transported or ejected onto the circularly arranged grinding teeth on the outer edge of the grinding wheel. The coolant throughput (Kühlmitteldurchsatz) (i.e., the amount of coolant that appears within a given time) can be controlled electronically or mechanically.

[0019] Document DE 10 2007 030 958 A1 describes a method for grinding semiconductor wafers, in which the semiconductor wafer is processed by means of at least one grinding tool to remove material from one or both sides while a coolant is provided. To ensure continuous cooling during grinding, the coolant flow rate is reduced as the height of the grinding teeth decreases, because otherwise maintaining a high and unchanged coolant flow rate would inevitably lead to an aquaplaning effect.

[0020] The invention in document DE 10 2017 215 705 A1 is based on the optimal distribution of fluid in a grinding tool to process semiconductor wafers in a manner that removes material from both sides simultaneously. This is achieved by using an optimized Schleuderplatte. The document explains that non-uniform fluid distribution can negatively impact grinding results.

[0021] Patent document US2019 / 134782 A1 discloses some designs for grinding wheels that can be used in dual-disc grinding. This document also describes the use of water applied internally to the grinding teeth of the grinding wheel via a nozzle. The water, along with grinding water, washes away grinding abrasives and tool abrasives from the processing area.

[0022] All the aforementioned prior art documents share a common drawback: material removal is higher at the center of the semiconductor wafer than at the edges. Consequently, the geometric parameters of the semiconductor wafer deteriorate during this processing step. This distortion cannot be, or cannot be adequately, corrected in subsequent processing steps. The quality of geometry achievable by grinding using methods known in the prior art is insufficient. Summary of the Invention

[0023] The purpose of this invention is to provide a method that does not exhibit the above-mentioned disadvantages.

[0024] This objective is achieved by a method for grinding a semiconductor wafer, wherein the semiconductor wafer is processed in a material-removing manner by means of a grinding tool comprising grinding teeth having a height h, while coolant is supplied to the contact area between the rotating semiconductor wafer and the grinding tool. At each moment of grinding, a first coolant flow rate is applied to a first region on one side of the semiconductor wafer by means of one or more nozzles, and at each moment of grinding, a second coolant flow rate is applied to a second region on the same side of the semiconductor wafer by means of one or more nozzles. The method is characterized in that the first region is bounded by the lower right quarter circle of the semiconductor wafer and the second region is bounded by the lower left quarter circle, and the ratio of the first coolant flow rate to the sum of the first coolant flow rate and the second coolant flow rate is not greater than 35% and not less than 25%.

[0025] Preferably, the semiconductor wafer has a nominal diameter of 300 mm, and the total coolant flow rate is not less than 800 ml / min and not more than 1200 ml / min.

[0026] Preferably, both sides of the semiconductor wafer are processed by removing material simultaneously.

[0027] Similarly, it is preferable that the total coolant flow rate decreases as the height h decreases.

[0028] More preferably, the first region is an annular sector region having an annular width w, a midpoint, and an outer radius, and is bounded by a first straight line passing through the midpoint, which is inclined at an angle α with respect to the vertical axis of symmetry of the semiconductor wafer, and further bounded by a second straight line passing through the midpoint, which is inclined at an angle β with respect to the vertical axis of symmetry of the semiconductor wafer. The second region is derived from the mirror image (Spiegelung) of the first region with respect to the vertical axis of symmetry of the semiconductor wafer, with the midpoint located on the vertical axis of symmetry of the semiconductor wafer and at a distance of not less than 75 mm from the midpoint of the semiconductor wafer, the angle α not less than 25°, the angle β not less than 45°, the annular width w not less than 10 mm and not more than 25 mm, and the outer radius not less than 80 mm and not more than 90 mm. Attached Figure Description

[0029] Figure 1 An example of a device (10) is shown to be used to apply a desired amount of coolant during grinding. In this case, the nozzle (11) is mounted in such a way that coolant can be applied to the side of the semiconductor wafer to be ground. Ideally, in this case, the nozzles can be controlled individually so that each nozzle can operate at a predefined and optionally time-varying throughput of coolant.

[0030] Figure 2 Two regions on a semiconductor wafer (20) are shown, to which coolant is preferably applied to achieve a geometrically advantageous effect. The figure shows a first region (22) located in the lower right quarter circle of the semiconductor wafer and a second region (21) located in the lower left quarter circle of the semiconductor wafer.

[0031] Figure 3 Two particularly preferred regions (shaded) on the semiconductor wafer (30) are shown, to which coolant may be applied to further improve the geometry. The first region (32) is an annular sector region having an annular width w, a midpoint (33), and an outer radius. It is bounded by a first straight line passing through the midpoint (33), which is inclined at an angle α relative to the vertical axis of symmetry (35) of the semiconductor wafer (30). Furthermore, it is bounded by a second straight line passing through the midpoint (33), which is inclined at an angle β relative to the vertical axis of symmetry (35) of the semiconductor wafer. A second region (31) is also shown, which is derived from a mirror image of the first region (32) relative to the vertical axis of symmetry (35) of the semiconductor wafer. The outer boundaries of the two regions lie on the indicated circle (34).

[0032] The midpoint (33) is located on the vertical axis of symmetry (35) of the semiconductor wafer (30) and below the horizontal axis of symmetry of the semiconductor wafer. Detailed Implementation

[0033] There are many standardized measurement procedures and methods for evaluating the geometry of semiconductor wafers. The inventors have limited their evaluation to semiconductor wafers using THA25 and warpage.

[0034] Regarding THA25: To study nanomorphology, an interferometer (such as KLA-Tencor Corp.'s WaferSight) was used. TMA suitable instrument is available. This interferometer is suitable for measuring the top morphology of a semiconductor wafer. The instrument generates a height map of the top side of the semiconductor wafer, which is then filtered, and an analysis window with a defined analysis area is moved across it. The evaluation of the height difference within the analysis window is performed by THA (“Threshold Height Analysis”) according to the method specifications of SEMI M43-0418 and SEMI M78-0618 standards.

[0035] For example, warpage measurements can be performed according to SEMI MF 1390-0218.

[0036] Although optimally tuned DDG machines enable the production of ground semiconductor wafers with improved shape, bowing, warping, and nano-morphology, the quality of these semiconductor wafers is still found to be insufficient.

[0037] The inventors have discovered that improved semiconductor wafer geometry can be achieved by controlling the delivery of coolant to a defined region of the semiconductor wafer during grinding.

[0038] A silicon crystal with a nominal diameter of 300 mm, obtained from a crystal rod drawn using the Czochralski method, is cut into semiconductor wafers using a wire saw.

[0039] On the Koyo DSGX320 grinding system, semiconductor wafers are ground under different conditions with varying coolant flow rates. In this case, the grinding system is equipped with commercially available ALMT 3000-OVH grinding tools (grinding wheels).

[0040] In a dual-disc grinding machine, the machining coolant typically emerges from the center of the grinding tool and is delivered to the grinding teeth by centrifugal force. The coolant throughput can be adjusted in a way that keeps the coolant flow rate at a set value.

[0041] According to existing technology, the amount of grinding water supplied to the machining process is adjusted as a function of tooth height (according to DE 102007 030958A1). This ensures that during machining, when too much coolant is discharged from inside the tool, there is no tool floating on the wafer to be processed (equivalent to hydroplaning); and when too little coolant is available during machining, there is no overheating of the wafer to be processed (equivalent to grinding burn) and failure of the grinding wheel.

[0042] The inventors discovered that the distribution of coolant on a semiconductor wafer has a significant impact on the results. For example, in conventional grinding systems, the coolant is distributed on the semiconductor wafer to be ground by centrifugal force, which is obviously not always sufficient to achieve the desired surface quality (flatness).

[0043] Therefore, the inventors developed a device by means of which coolant is delivered through multiple nozzles in a manner with time and position resolution (zeit-als auch). This approach can be applied to both the front and back sides of a semiconductor wafer.

[0044] Figure 1 An apparatus for applying coolant to a semiconductor wafer during grinding is illustrated schematically.

[0045] The inventors have successfully improved the method for grinding semiconductor wafers by using a grinding tool containing grinding teeth with a height h to remove material while delivering coolant to the contact area between the rotating semiconductor wafer and the grinding tool.

[0046] At each moment of grinding, a first flow of coolant is applied to a first region on one side of the semiconductor wafer via one or more nozzles.

[0047] At the same time, a second coolant flow rate is applied to a second region on one side of the semiconductor wafer using one or more nozzles.

[0048] They found that, particularly preferred, the first region is bounded by the lower right quarter circle of the semiconductor wafer, and the second region is bounded by the lower left quarter circle. Furthermore, the inventors found that it is necessary for the ratio of the first coolant flow rate to the sum of the first and second coolant flow rates to be no greater than 35% and no less than 25%.

[0049] Preferably, the semiconductor wafer is rotated during the grinding process. The rotation occurs in a clockwise direction, which is intended to represent what is seen when observing the contact area on the semiconductor wafer.

[0050] The first and second zones are in Figure 2 The text is presented graphically.

[0051] Particularly preferred is that the semiconductor wafer has a nominal diameter of 300 mm, and the total coolant flow rate is not less than 800 ml / min and not more than 1200 ml / min.

[0052] More preferably, both sides of the semiconductor wafer are processed by removing material simultaneously.

[0053] Preferably, the total coolant flow rate decreases as the height h of the grinding teeth decreases.

[0054] Figure 3The region in which the inventors achieved optimal results is shown. In this case, the first region is an annular sector region having an annular width w, a midpoint, and an outer radius, and is bounded by a first straight line passing through the midpoint, which is inclined at an angle α relative to the vertical axis of symmetry of the semiconductor wafer, and further bounded by a second straight line passing through the midpoint, which is inclined at an angle β relative to the vertical axis of symmetry of the semiconductor wafer.

[0055] In this case, the second region is derived from the reflection of the first region relative to the vertical axis of symmetry of the semiconductor wafer, with the midpoint located on the vertical axis of symmetry of the semiconductor wafer and at least 75 mm from the midpoint of the semiconductor wafer.

[0056] The angle α is preferably not less than 25°, the angle β is preferably not less than 45°, the ring width w is preferably not less than 10mm and not more than 25mm, and the outer radius is preferably not less than 80mm and not more than 90mm.

[0057] Water is preferred as the coolant, although the use of additional additives is also conceivable.

[0058] Adding coolant significantly improved wafer geometry. However, if coolant flow was interrupted during grinding, the semiconductor wafer geometry deteriorated again. Therefore, uninterrupted coolant flow during grinding is crucial.

Claims

1. A method for grinding semiconductor wafers, The semiconductor wafer (20, 30) is processed by means of the grinding tool, which contains grinding teeth with a height h, to remove material while coolant is delivered to the contact area between the rotating semiconductor wafer and the grinding tool. At each instant of grinding, a first coolant flow rate is applied to a first region on one side of the semiconductor wafer via one or more nozzles (11), and At each instant of grinding, a second coolant flow rate is applied to a second region on one side of the semiconductor wafer via one or more nozzles (11). wherein The first region is bounded by the lower right quarter circle of the semiconductor wafer, and the second region is bounded by the lower left quarter circle. The ratio of the first coolant flow rate to the sum of the first coolant flow rate and the second coolant flow rate is not greater than 35% and not less than 25%.

2. The method of claim 1, wherein, The semiconductor wafers (20, 30) have a nominal diameter of 300 mm, and the sum of the coolant flow rates is not less than 800 ml / min and not more than 1200 ml / min.

3. The method according to claim 1, wherein, Both sides of the semiconductor wafer (20, 30) are processed in a manner that removes material simultaneously.

4. The method according to claim 1, wherein, The semiconductor wafer is rotated during the grinding process, and the rotation occurs in a clockwise direction when the contact area is observed.

5. The method according to claim 1, wherein, The total coolant flow rate decreases as the height h decreases.

6. The method according to claim 1, wherein, Semiconductor wafers have a nominal diameter of 300 mm. The first region is a ring-shaped sector. The first region has an annular width w, a midpoint (33), and an outer radius. And the first straight line passing through the midpoint (33) serves as the boundary. The first straight line is tilted at an angle α relative to the vertical axis of symmetry (35) of the semiconductor wafers (20, 30). Furthermore, taking the second straight line passing through the midpoint (33) as the boundary, the second straight line is tilted at an angle β relative to the vertical axis of symmetry (35) of the semiconductor wafers (20, 30), and The second region is derived from the first region as a mirror image of the first region relative to the vertical axis of symmetry of the semiconductor wafers (20, 30). The midpoint is located on the vertical axis of symmetry (35) of the semiconductor wafers (20, 30), and the distance from the midpoint of the semiconductor wafers (20, 30) is not less than 75 mm. The angle α is not less than 25°. The angle β is not less than 45°. The annular width w is not less than 10 mm and not more than 25 mm, and The outer radius is not less than 80 mm and not more than 90 mm.

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