Patent Application
20090298399
This invention relates to apparatus and methods for polishing semiconductor wafers or similar type materials, and more specifically to such apparatus and methods which facilitate polishing of a semiconductor wafer to have a flat surface.
Polishing an article to produce a surface which is flat, highly reflective and damage free has application in many fields. A particularly good finish is required when polishing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process (hereinafter “lithography”). Flatness of the wafer surface on which circuits are to be printed is critical in order to maintain resolution of the lines, which can be as thin as 0.13 microns (5.1 microinches) or less. The need for a flat wafer surface, and in particular local flatness in discrete areas on the surface, is heightened when stepper lithographic processing is employed.
Flatness of the wafer surface can be quantified in terms of a global flatness variation parameter (for example, total thickness variation (“TTV”)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane, referred to as the “focal” plane. SFQR is a specific type of STIR measurement, as measured from the front side best fit reference plane. A more detailed discussion of the characterization of wafer flatness can be found in F. Shimura, Semiconductor Silicon Crystal Technology 191, 195 (Academic Press 1989). Presently, flatness parameters of the polish surfaces of single side polished wafers are typically acceptable when a new polishing pad is being used, but the flatness parameters become unacceptable as the polishing pad wears, as described below.
The construction and operation of conventional polishing machines contribute to unacceptable flatness measurements. Polishing machines typically include a circular or annular polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on pressure plates above the polishing pad and lowered into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad for greater polishing interaction between the polishing pad and the wafer. This type of polishing operation is typically referred to as chemical-mechanical polishing or simply CMP.
During operation, the pad is rotated and the wafer is brought into contact with the pad using the pressure plate. The pressure plate applies a generally uniform downward force across the wafer pressing the wafer against the pad. As the pad rotates, the wafer is rotated and oscillated back and forth about a portion of the pad that is off-center. As a result, pad wear is most significant in an annular band AB, which is illustrated in
When the pad wears, e.g., after a few hundred wafers, wafer flatness degrades because the pad is no longer flat but instead has an annular depression corresponding to the annular band AB of
“Dishing” of the wafer surface occurs when the front surface of the wafer is caused to have a concave upper surface, which is illustrated in
When the flatness of the wafers becomes unacceptable (e.g., too “domed” or too “dished”), the worn polishing pad has to be replaced with a new one. Frequent pad replacement adds significant costs to the operation of the polishing apparatus not only because of the large number of pads that need to be purchased, stored, and disposed of but also because of the substantial amount of down time required to change the polishing pad.
Accordingly, there is a need for a polishing apparatus that inhibits both doming and dishing of the front surface of wafers during the polishing process and extends the useful life of the polishing pad.
In one aspect, a wafer polishing apparatus generally comprises a base and a turntable having a polishing pad thereon and mounted on the base for rotation of the turntable and polishing pad relative to the base about an axis perpendicular to the turntable and polishing pad. The polishing pad includes a work surface engageable with a front surface of a wafer for polishing the front surface of the wafer. A drive mechanism is mounted on the base for imparting rotational motion about an axis substantially parallel to the axis of the turntable. A polishing head is connected to the drive mechanism for driving rotation of the polishing head. The polishing head has a pressure plate adapted to hold the wafer for engaging the front surface of the wafer with the work surface of the polishing pad. The pressure plate has a generally planar position and is selectively movable from the planar position to a convex position and to a concave position.
In another aspect, a polishing head for holding a wafer in a polishing apparatus generally comprises a pressure plate including a support plate for engaging and holding the wafer during operation of the polishing apparatus. The support plate has a generally planar position and is selectively moveable from the planar position to a convex position and to a concave position.
In yet another aspect, a method of polishing a semiconductor wafer generally comprises the steps of quantifying the flatness of a front surface of the semiconductor wafer. The semiconductor wafer is placed in contact with a polishing head of a wafer polishing apparatus. The polishing head has a pressure plate and the wafer is placed in direct contact with the pressure plate. The wafer is held by the polishing head so that a front surface of the wafer engages a work surface of the polishing pad. The front surface of the wafer is urged against the polishing pad. The pressure plate is deflected from a generally planar position to one of a convex position and a concave position based on the flatness of the front surface of the wafer. A polishing pad is rotated on a turntable of the polishing apparatus about a first axis and the polishing head is rotated generally about a second axis non-coincident with the first axis to thereby polish the front surface of the wafer. The wafer is disengaged from the turntable and removed from the polishing head.
In still another aspect, a method of polishing a batch of semiconductor wafer generally comprises the steps of placing one of the semiconductor wafers from the batch in contact with a polishing head of a wafer polishing apparatus. The polishing head has a pressure plate and the wafer is placed in direct contact with the pressure plate. The wafer is held by the polishing head so that a front surface of the wafer engages a work surface of the polishing pad. The work surface has wear. The pressure plate is deflected from a generally planar position to one of a convex position and a concave position based on the amount of wear in the work surface of the polishing pad. The front surface of the wafer is urged against the polishing pad. A polishing pad is rotated on a turntable of the polishing apparatus about a first axis and the polishing head is rotated generally about a second axis non-coincident with the first axis to thereby polish the front surface of the wafer. The wafer is disengaged from the turntable and removed from the polishing head.
Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above-mentioned aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above-described aspects, alone or in any combination.
Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.
Referring now to the figures, and specifically
With reference now to
The rotation of the turntable 27 is controlled by a turntable motor and turntable control device (not shown). The turntable control device controls the rotational speed of the turntable 27 to further adjust the polishing of the wafer 35, as will be discussed in greater detail below. Suitable turntable control devices and motors are well known in the relevant art.
A drive mechanism, generally indicated at 45 in
As illustrated in
With reference again to
The wafer polishing apparatus 21 further comprises a semi-rigid connection, generally indicated at 89, between the drive mechanism 45 and the polishing head 63 for imparting a rotational force from the drive mechanism to the polishing head (
A ring 95 fits over the outer edge of the torque transmittal boot 93 to secure the boot to the polishing head 63. The ring 95 and boot 93 each contain a plurality of matching holes so that a plurality of bolts 103 can pass through the ring and boot to firmly hold the boot to the polishing head 63. The ring 95 reinforces the boot 93 so that the rotational force transmitted through the boot spreads evenly over the circumference of the boot. In one embodiment, the torque transmittal boot 93 is made of an elastomeric material, such as rubber (e.g., urethane), having a stiffness capable of transmitting the rotational energy of the drive mechanism 45 to the polishing head 63 and a resiliency capable of allowing pivoting movement of the polishing head. Other materials capable of transmitting the rotation energy and allowing pivoting motion of the polishing head 63 are also contemplated as within the scope of the present invention.
As illustrated in
The lower body 109 additionally includes an inwardly directed annular flange 111 which projects inward above a portion of the upper bearing member 77 so that when the arm 53 lifts the polishing head 63 upward, the weight of the lower body 109, a pressure plate 115, and the wafer 35 rest upon the rigid upper bearing member, rather than the torque transmittal boot 93. This flange 111 helps preserve the torque transmittal boot 93 by not subjecting it to a repeated vertical tensile load when the arm 53 lifts the drive mechanism 45 and polishing head 63. The lower body 109 further comprises a retaining plate 117 for mounting the pressure plate 115 on the polishing head 63. More specifically, the pressure plate 115 includes a mounting flange 119 mounted beneath the retaining plate 117 for cooperating to create a seat for the pressure plate 115. A plurality of bolts 121 extend through the retaining plate 117 and mounting flange 119 to secure the pressure plate 115 to the polishing head 63.
As illustrated in
A first interior chamber 131 is disposed between and cooperatively defined by the pressure plate 115 and retaining plate 117. The first interior chamber 131 is fluidly connected to a first pressure source 145 via a conduit 143. The first pressure source 145 is operable to apply either a negative (i.e., a vacuum) or a positive pressure to the first interior chamber 131. In one suitable embodiment, the first pressure source 145 is capable of applying a vacuum of up to about 29 inches of mercury (in. Hg) and a positive pressure of up to about 40 pounds per square inch (psi). But it is understood that the first pressure source can apply different ranges of pressures than those provided without departing from the scope of this invention.
As illustrated in
With reference now to
In both the concave position and convex position of the pressure plate 115, the amount of deflection in the support plate 125 is greatest at its center and decreases generally radially outward toward the edge of the support plate. As a result, the support plate 125 is capable of deflecting in a generally smooth curve. In one embodiment, the amount of deflection in the support plate 125 at its center is less than about 100 micrometers, and more suitably less than about 50 micrometers. For example, the support plate 125 is capable of deflecting at its center between about 0 micrometers and about 50 micrometers. It is understood, the support plate 125 can have ranges of deflection at its center without departing from the scope of this invention.
In the illustrated embodiment, the relatively thin annular wall 123 acts as a hinge about which the support plate 125 deflects. In other words, the relatively thin annular wall 123 flexes in relation to the deflection of the support plate 125. When the support plate 125 deflects upward (i.e., the concave position of the pressure plate 115), the annular wall 123 flexes outward away from output shaft 55 of the drive mechanism 45, and when the support plate deflects downward (i.e., the convex position of the pressure plate), the annular wall flexes inward toward the output shaft of the drive mechanism. In another embodiment, the support plate 125 is capable of pivoting upward and downward relative to the annular wall 123 about a corner 151 between the support plate and annular wall. In other words, the corner 151 can act as a hinge. The relative movement of the annular wall 123 and support plate 125 is a function of the type of material used and the thickness of the material.
The thickness of the annular wall 123 is one variable that directly influences the amount of deflection the support plate 125 is capable of achieving. (Other variables that influence the deflection of the support plate 125, for example, include the material that the pressure plate 115 is made from, the thickness of the pressure plate 115, and the height of the annular wall 123). The thinner the annular wall 123 is formed, the more readily and more uniformly the support plate 125 will deflect. However, the annular wall 123 needs to be sufficiently robust to withstand the polishing operation. In one suitable embodiment, as mentioned above, the thickness of the annular wall can be between about 2 millimeters (0.079 inches) and about 3 millimeters (0.118 inches). It is understood, however, that the annular wall can have different thicknesses without departing from the scope of this invention. In one suitable embodiment, the pressure plate 115 is made from stainless steel, 10 millimeters thick, but it is understood that the pressure plate can be made from other types of material. For example, the pressure plate 115 can be made from polyetheretherketone (PEEK) or other suitable plastics.
With reference to
Referring again to
In use, one or more semiconductor wafers 35 are delivered to the wafer polishing apparatus 21 for polishing. The wafers 35 are preferably formed from monocrystalline silicon, although the polishing apparatus and method of polishing described herein are readily adaptable to polishing other materials. The semiconductor wafers 35 can be delivered to the wafer polishing apparatus using any suitable manner. In one arrangement, a plurality of wafers 35 are delivered to the polishing apparatus 21 in a cassette (not shown), which are conveniently used, for storage and transfer of a plurality of wafers. These cassettes can be of various sizes for holding any number of wafers, such as 25, 20, 15, 13, or 10 wafers per cassette.
In one embodiment, a single wafer 35 is removed from the cassette and the surface flatness of the front surface 39 of the wafer 35 is quantified using any conventional method. As mentioned previously, flatness of the front surface 39 of the wafer 35 can be quantified in terms of a global flatness variation parameter (for example, total thickness variation (“TTV”)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). In another embodiment, the flatness of the wafer 39 is not quantified before the polishing operation. Instead, the flatness is determined only after the wafer 39 has been polished.
After the surface flatness of the front surface 39 of the wafer is quantified, the wafer 35 is moved to a location suitable for being received in the polishing head 63 of the polishing apparatus 21. More specifically, the back surface 155 of the wafer 35 is contacted by the support plate 125 of the pressure plate 115. A vacuum generated by the second pressure source 147 is applied to the back surface 155 of the wafer 35 via the passages 127 in the support plate to hold the wafer in contact with the polishing head 63. The retaining ring 153 mounted on the support plate 125 inhibits lateral movement of the wafer 35 with respect to the support plate. Using the arm 53, the wafer 35 is lifted, moved, and placed into contact with the polishing pad 29 so that the front surface 39 of the wafer is in direct contact with the working surface 37 of the polishing pad. A downward force is applied by the arm 53 of the polishing apparatus 21 to urge the wafer 35 against the polishing pad 29.
The turntable 27 mounted on the base 23 and thereby the polishing pad 29 is rotated conjointly relative to the base 23 about the axis A. With the polishing pad 29 rotating, a continuous supply of polishing slurry is delivered to the pad via a slurry delivery system (not shown). The rotation of the turntable 27 is controllable by a turntable motor and turntable control device (not shown) to selectively set the rotational speed of the polishing pad 29. The slurry delivery is controllable using the slurry delivery system.
The polishing head 63 is rotated using the drive mechanism 45 about an axis B, which is substantially parallel to and spaced from axis A of the turntable (
With the wafer 35 urged into contact with the polishing pad 29, the second pressure source 147 is operated to apply a positive pressure to negate the presence of the passages 127 in the support plate 125. The positive pressure and vacuum applied by the second pressure source 147 are transferred directly to the back surface 155 of the wafer 35. The second pressure source 147 selectively pressurizes or applies a vacuum to the second interior chamber 137, which is defined by the baffle plate 133 and support plate 125, via conduit 141. The pressure/vacuum is applied directly to the back surface 155 of the wafer 35 through the passages 127 in the support plate 125.
Based on the flatness of the front surface 39 of the wafer 35, the proper or optimum position of the support plate 125 of the pressure plate 115 is determined. As mentioned above, the support plate 125 can be in a generally planar position (
The support plate 125 of the pressure plate 115 is moved from its generally planar position to its convex position by pressurizing the first interior chamber 131, which is defined by the pressure plate 115 and retaining plate 117. Applying a positive pressure to the interior chamber 131 causes the support plate 125 to deflect downward toward the wafer 35 resulting in the support plate having a generally convex shaped. The amount of downward deflection in the support plate 125 is directly proportional to the amount of positive pressure applied to the interior chamber 131. That is, the greater the positive pressure, the greater the downward deflection. The amount the support plate 125 is deflected is based on the degree of doming of the front surface 39 of the wafer 35. The support plate 125 will be deflected a greater amount for a wafer having more doming than for a wafer having less.
The convex position of the support plate 125 results in the center of the front surface 39 of the wafer 35 being urged into contact with the polishing pad 29 under a greater pressure than the edge of the wafer. As a result, more wafer 35 material is removed from the center of the wafer than from its edges. In other words, the center of the wafer 35 is polished more than its edges. This discrepancy in material removal from the front surface 39 of the wafer 35 results in a wafer having a domed front surface being polished into a wafer having a generally flat front surface.
The support plate 125 of the pressure plate 115 is moved from its generally planar position to its concave position by applying a vacuum to the first interior chamber 131. Applying a vacuum to the interior chamber 131 causes the support plate 125 to deflect upward away from the wafer 35 resulting in the support plate having a generally concave shape. The amount of upward deflection in the support plate 125 is directly proportional to the amount of vacuum applied to the interior chamber 131. That is, the greater the vacuum, the greater the upward deflection. The amount the support plate 125 is deflected is based on the degree of dishing of the front surface 39 of the wafer 35. The support plate 125 will be deflected a greater amount for a wafer having more dishing than for a wafer having less.
The concave position of the support plate 125 results in the edge of the front surface 39 of the wafer 35 being urged into contact with the polishing pad 29 under a greater pressure than the center of the wafer. As a result, more wafer 35 material is removed from adjacent the edge of the wafer than from its center. In other words, the edge of the wafer 35 is polished more than its center. This discrepancy in material removal from the front surface 39 of the wafer 35 results in a wafer having a generally dish shaped front surface being polished into a wafer having a generally flat front surface.
In both the concave position and convex position of the pressure plate 115, the amount of deflection in the support plate 125 is greatest at its center and decreases radially outward toward the edge of the support plate. As mentioned above, the support plate 125 is hingely connected to the annular wall 123. As a result, the support plate 125 is capable of pivoting with respect to the annular wall 123.
In another embodiment, the position and amount of deflection (if any) of the support plate 125 is determined based on the wear of the polishing pad 29. As mentioned above and illustrated in
When the pad wears, the pad is no longer flat but instead has an annular depression corresponding to the annular band AB of
In one embodiment, to compensate for pad wear resulting in an increase in material being removed from the center of the front surface 39 of the wafer 35, the support plate 125 is moved from its generally planar position to its concave position by applying a vacuum to the first interior chamber 131. This causes the support plate to deflect upward away from the wafer 35 as described above. The concave position of the support plate 125 results in the edge of the front surface 39 of the wafer 35 being urged into contact with the polishing pad 29 under a greater pressure than the center of the wafer.
The front surface 39 of the wafer 35 is actively polished by the polishing apparatus 21 for a selected period of time. During the polishing operation, the front surface 39 of the wafer 35 is polished to a finish polish, while the back surface 155 of the wafer is not polished to a finish polish. When the polishing operation is complete, the wafer is removed from the polishing head 63 and the polishing apparatus 21. Removal of the wafer 35 is facilitated by applying air pressure to chamber 137, with the air blowing out the holes 127, causing the wafer to release from the polishing head 63.
After the wafer 35 is removed from the polishing apparatus 21, the surface flatness of the front surface 39 of the wafer 35 is quantified using any conventional method. As mentioned previously, flatness of the wafer 35 can be quantified in terms of a global flatness variation parameter (for example, total thickness variation (“TTV”)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). Based on the surface flatness of the wafer 35, the position of the support plate 125 (i.e., planar, convex, and concave) can be altered for polishing subsequent wafers. Thus, adjustments in the support plate 125 can be made over time as the polishing pad 29 wears to compensate for the changes in the polishing characterization of the pad. That way the flatness of subsequently polished wafers is not adversely affected by pad wear. It is understood that if the wafer's surface flatness is unacceptable, the wafer 35 can be re-polished.
Accordingly, the polishing head 63 and, more specifically, the pressure plate 115 disclosed herein compensates for wear of the polishing pad 29 thereby improving the TTV of the wafers being polished with a worn polished pad and extending the useful life of the polishing pad. This reduces the number of polishing pads 29 that need to be purchased and reduces the number of times the pad needs to be changed.
With reference now to
When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
