US20090148256A1

US20090148256A1 - Support Pin with Dome Shaped Upper Surface

Info

Publication number: US20090148256A1
Application number: US11/953,768
Authority: US
Inventors: Jaime Poris
Original assignee: Nanometrics Inc
Current assignee: Nanometrics Inc
Priority date: 2007-12-10
Filing date: 2007-12-10
Publication date: 2009-06-11

Abstract

A chuck, which may hold a substrate during stress measurements, includes a number of pins that support the substrate. Each support pin has a dome shaped upper surface that contacts a bottom surface of a substrate when supporting the substrate. The dome shaped upper surface minimizes contact with the substrate as well as assists in maintaining the same contact location with the substrate regardless of substrate shape. The dome shaped upper surface may be formed of a layer of soft material having a high coefficient of static friction to hold the substrate stationary with respect to the pins when the chuck is accelerated moved during or between stress measurements. Additionally, the layer of soft material may be a thin layer that covers a hard internal dome to reduce creep.

Description

FIELD OF THE INVENTION

The present invention describes a pin used to support a thin substrate, such as a silicon wafer, during stress measurements.

BACKGROUND

Flat substrates, such as semiconductor wafers, are stressed during certain processing steps, e.g., depositing or etching thin films. Stress in deposited layers can warp the substrate, which can adversely affect subsequent process steps, device performance, reliability and line-width control. Thus, it is desirable to measure the radius of curvature of a substrate as well as measure the stress on a substrate that is associated with a processing step. The measurement of stress is executed many times during processing and is used to help ensure that the substrate has been properly processed and that the completed devices will perform as expected.
In general, the basic procedure for measuring stress is to measure the shape of the substrate, process the substrate, measure the shape of the substrate again and then calculate the change in shape of the substrate that is associated with that particular process step. The change in shape is correlated to the stress on the substrate. Processing steps that are generally monitored include an etch process, a thermal process or a thin film deposition process (the process most commonly monitored). Typically, a diameter scan of the substrate is made and the Stoney equation is utilized to calculate the stress:
$\begin{matrix} σ = \frac{{Et}_{s}^{2} (\frac{1}{r})}{6 (1 - v) t_{f}} & eq . 1 \end{matrix}$
where σ is stress, E is Young's modulus, 1) is Poisson's ratio, t_sis the substrate thickness, t_fis the film thickness, and r is the difference in the radius of curvature that is measured during the pre-processing diameter scan curve and the post-processing diameter scan curve. The Stoney equation (eq. 1) is used for round substrates. It should be understood that rectangular or square substrates may also be measured for stress using a different equation or other computational techniques.
For a thermal process where no film is added or removed, the calculated change in the radius of curvature of the substrate may be used to gauge process control since the Stoney equation of eq. 1 is not valid when the film thickness t_fis zero.
Another parameter that can be used to quantify the stress measurement is bow. Bow is the difference in height between the chord and the arc of the substrate for the curve that is generated from the difference between the before and after processing scans. Bow is sometimes reported instead of stress and is used for process control, particularly for substrates that are not round. For purposes of the present disclosure, stress and bow are interchangeable and are therefore collectively referred to herein as stress.
During most processing and metrology steps, the substrate is held to a chuck on a stage using vacuum. The employed vacuum will easily overcome the stress induced deformation of the substrate and will hold the substrate flat against the chuck. Accordingly, a conventional vacuum chuck cannot be used to hold a substrate for stress measurements. To make a stress measurement, the substrate is supported at a limited number of locations so that the stress can freely deform the substrate.
Generally, the substrate is supported at three locations on pins so that the substrate is supported at the identical locations for both the pre and post measurement. If four or more pins were used, the substrate might be supported at different locations for the pre and post measurements due to the change in shape of the substrate after processing compromising the quality of the stress measurement.
A stress measurement generally requires the collection of a multitude of points along a diameter of the substrate when the Stoney equation (eq. 1) is employed. In conventional stress metrology tools, the measurement hardware is held stationary and the substrate is moved to the desired measurement locations using some form of a stage. For example, a stage may move the substrate in X,Y and Z coordinates or to reduce the stage footprint, in R, θ and Z coordinates. Alternatively, the measurement hardware may move relative to the substrate, or both may move. In general, holding the substrate stationary and moving the measurement hardware is less desirable for most of the commonly employed measurement techniques, particularly when prealignment and loading of the substrate is considered.
Of course, it is important that the substrate does not move relative to the support pins when the stage moves the substrate to a new measurement location. To prevent movement of the substrate relative to the pins, the acceleration of the stage may have to be reduced compared to its value when the substrate is being held by vacuum so as to not disturb the substrate's position with respect to the pins. Other techniques that are sometimes used to prevent the substrate from moving relative to the pins are vacuum gripping the substrate before stage motion or a contact edge gripper. These techniques, however, complicate the hardware and degrades the throughput, as well as potentially creating particulate problems.
Another consideration for supporting the substrate during the stress measurements is gravity. For very small stress values, the substrate deflection due to gravity may overwhelm the deflection due to stress. Gravity, substrate orientation, measurement precision and other factors limit the smallest stress values that can practically be measured. To minimize the gravitational contribution to substrate deflection, the three support locations are typically chosen to be on a circle that has a radius that is ⅔ of the radius of the substrate. If the support locations were on a smaller radius, the outer part of the substrate would sag down while the center would bulge upward. If the support locations were on a larger radius, the outer part of the substrate would be relatively flat, but the center of the substrate would sag down. Generally, the chosen support locations should minimize the total height range of a flat substrate due to gravity.
In a stress metrology tool, it is desirable to have the highest throughput possible while maintaining the best precision. Thus, improvements of support pins used to support a substrate during stress measurements are desired.

SUMMARY

A chuck, which may hold a substrate during stress measurements, includes a number of pins that support the substrate. In accordance with one embodiment, each support pin has a dome shaped upper surface that contacts a bottom surface of a substrate when supporting the substrate. The dome shaped upper surface minimizes contact with the substrate as well as assists in maintaining the same contact location with the substrate regardless of substrate shape. The dome shaped upper surface may be formed of a layer of soft material having a high coefficient of static friction to hold the substrate stationary with respect to the pins when the chuck is accelerated by a stage during stress measurements. Additionally, the layer of soft material may be a thin layer that covers a hard internal dome to reduce creep.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a chuck with a plurality of support pins, in accordance with an embodiment of the present invention.

FIG. 2 shows a perspective view of one support pin with a dome shaped upper surface.

FIG. 3 shows a cross-sectional view of one support pin with a layer of material that covers an internal dome to form the dome shaped upper surface.

FIGS. 4A and 4B illustrate side views of a conventional flat topped pin in contact with a portion of a substrate that has a convex and concave curvature, respectively.

FIGS. 5A and 5B illustrate side views of a support pin with a dome shaped upper surface in contact with a portion of a substrate that has a convex and concave curvature, respectively.

DETAILED DESCRIPTION

FIG. 1 shows a perspective view of a chuck 100 with a plurality of pins 110 in accordance with an embodiment of the present invention. The pins 110 can move up and down from the chuck. Vacuum, air pressure or motors can be the energy source for the movement of the pins 110. The pins 110 may be used to contact and hold a substrate (illustrated by broken lines 102) during stress measurements. FIG. 2 shows a perspective view and FIG. 3 shows a cross-sectional view of one of the pins 110. As can be seen in FIGS. 2 and 3, pin 110 includes a dome shaped upper surface 112, which contacts the back of the substrate 102 (FIG. 1). In one embodiment, the dome shaped upper surface 112 of the pin 110 is formed with a layer 114 of relatively soft material that covers a hard internal dome 116.
The convex dome shape of the upper surface 112 reduces the contact area of the pin 110 relative to a conventional flat topped pin. A reduced contact area reduces the potential for particulate contamination. Moreover, because the contact area of the pin 110 is relatively small compared to the diameter 110 d of the pin 110, the location of the contact between the pin 110 and the substrate 102 is relatively insensitive to changes in the shape (i.e., convex or concave flexing) of the substrate 102 and therefore increases precision in the stress measurement relative to conventional flat topped pins.
To obtain the best precision during stress measurements, the substrate 102 must be supported at identical locations for both the pre and post measurements. If the contact area between the pin and a substrate is large, a potential placement problem can occur. A large contact area of a support pin can cause the contact location on a substrate to change when the shape of the substrate changes. FIGS. 4A, 4B and FIGS. 5A, 5B illustrate a conventional flat topped pin 200 and the pin 110, respectively, supporting a substrate 102 and illustrate the sensitivity of the contact locations to changes in the substrate shape. FIGS. 4A and 4B illustrate side views of a conventional flat topped pin 200 in contact with a portion of a substrate 102 that has a convex curvature in FIG. 4A and a concave curvature in FIG. 4B. It should be understood that in practice, the shape of the substrate is typically complex and is not a simple concave or convex curvature. Nevertheless, the illustrated curvatures are useful to illustrate the sensitivity of the contact position due to the substrate shape.
FIGS. 4A and 4B illustrate the center of the substrate 102 with center lines 102c and ⅔ of the radius of the substrate 102, i.e., the desired support location, is illustrated with support lines 102 s. As can be seen, in FIG. 4A, when the substrate 102 has a concave curvature, the flat topped pin 200 contacts the substrate 102 as a location 202 that is less than ⅔ of the radius of the substrate, i.e., between the center line 102 c and the support line 102 s. When the substrate 102 has a convex curvature, as illustrated in FIG. 4B, however, the flat topped pin 200 contacts the substrate 102 at a location 204 that is greater than ⅔ of the radius of the substrate, i.e., past the support line 102 s. The difference between the desired contact location, i.e., at support line 102s and the actual support location 202 or 204 in this case is approximately the radius of the flat topped pin 200. Variation in the location that the substrate 102 is supported changes the gravitational contribution to the substrate shape and thus, can add an error to the Stoney equation (eq. 1) calculation of stress. For example, assume that FIG. 4A illustrates the substrate during a pre measurement and FIG. 4B illustrates the substrate during a post measurement and that three flat topped pins with diameters of 8mm are located on a circle with a diameter of 200 mm and 120° apart (for a 300 mm wafer). The average diameter of the circle of contact will be 192 mm for the pre measurement and 208 mm for the post measurement. This 4% change in contact location will modify the gravitational contribution to the shape a measurable amount and add an error to the stress calculated by the Stoney equation (eq. 1).
FIGS. 5A and 5B are similar to FIGS. 4A and 4B, but illustrate the substrate 102 supported by a pin 110 with a dome shaped upper surface 112. As can be seen in FIGS. 5A and 5B, when the pin 110 supports a substrate 102 with a concave or convex curvature, the actual support locations 122 and 124, respectively, is at approximately the desired support line 102 s. Thus, relative to conventional flat topped pin 200, the location of contact between the pin 110 and the substrate 102 is relatively insensitive to changes in the shape (i.e., convex or concave flexing) of the substrate 102.
The radius of curvature 112R (shown in FIG. 3) of the dome shaped upper surface 112 of the pin 110 effects the sensitivity of the contact location to changes in the shape of the substrate. A small radius of curvature 112R results in a decrease of the sensitivity of the contact location to the shape of the substrate, but also a reduction in the friction that holds the substrate 102 stationary relative to the pins 110 during movement of the chuck. With a radius that is too small, there will not be enough friction to hold the substrate 102 stationary relative to the pins 110. While a small radius of curvature reduces the contact area, it also increases the stress on the layer 114 that forms the dome shaped upper surface 112. The increased stress on the layer 114 may cause increased deformation of the layer 114, and thus, the contact area may not decrease as much as expected. Further, increased stress on the layer 114 may cause the layer 114 to separate from the pin 110 resulting in reliability problems. On the other hand, a large radius of curvature 112R results in an increase in the sensitivity of the contact location to the shape of the substrate, but produces a large contact area thereby decreasing the probability of substrate movement. Thus, a compromise between a large and small radius of curvature 112R is used. For example, for a dome shaped upper surface 112 that has a diameter 112 d of 0.25 inches, a radius of curvature 112R between 0.125 inches to 0.5 inches, and in particular 0.168 inches has been found to be adequate.
As can be seen in FIGS. 2 and 3, a layer 114 of material covers the pin 110 to form the upper surface 112. The layer 114 is formed from a material that possesses a high coefficient of static friction that prevents movement of the substrate 102 relative to the pins 110, even when the substrate 102 is subjected to acceleration. By way of example, the coefficient of static friction may be greater than approximately 3. Accordingly, once the substrate 102 is properly placed on the pins 110, a stage 104 may move the chuck 100 and substrate 102, as indicated by arrows 106 in FIG. 1, to measure multiple locations of the substrate 102 and the substrate 102 will not move relative to the pins 110. If desired, a smaller coefficient of static friction may be used with a reduced acceleration and, thus, reduced throughput. Additionally, the layer 114 is soft enough to avoid scratching the back of the substrate 102, which could generate particles.
The hardness of the layer 114 that forms the dome shaped upper surface 112 is another consideration. If the layer 114 used to form the dome shaped upper surface 112 is too soft, the stress from the weight of the substrate 102 may increase the contact area too much. However, a layer 114 that is hard may have a lower coefficient of static friction. Using silicone or another similar material, with a hardness of 20 shore A to 50 shore A has been found to be adequate.
As illustrated in the cross-sectional view of FIG. 3, the pin 110 may be produced with a hard internal dome 116. The internal dome 116 is on the top of the body 118 of the pin 110, e.g., on a top surface 117 of the body 118, and in one embodiment is integrally formed with the body 118 of the pin 110. In some embodiments, the body 118 may have no top surface 117 but has a rounded top surface to form the internal dome 116. The internal dome 116 may be, e.g., aluminum or other appropriate material. Alternatively, the internal dome 116 may be separately manufactured and mounted to the top surface 117 of the body 118 of the pin 110 mechanically, e.g., by screwing the internal dome into the pin 110, or through a chemical fastener, such as epoxy. The internal dome 116 is covered with a thin layer of the layer 114 to form the upper surface 112. Using an internal dome 116 with a large surface area and an adhesion promoter between the internal dome 116 and the layer 114 will minimize the potential for damage to the upper dome surface 112 of the pin 110 from improper handling or any type of shear force. For example, sandblasting the surface of the internal dome 116 followed by thorough cleaning can improve adhesion significantly. Alternatively or additionally, machining small features such as grooves or dovetails into the internal dome 116 during the manufacturing process will also be an effective adhesion promoting procedure. Moreover, Dow Corning produces primers that are designed to improve the adhesion of silicone compounds to a range of materials including aluminum or anodized aluminum surfaces, which may be used to promote adhesion between the layer 114 and the internal dome 116. For example, Dow Corning P5200 clear adhesion promoter may be used to improve the adhesion of a range of silicone compounds to anodized aluminum.
The use of a hard internal dome 116 with a thin layer 114, which has a high coefficient of static friction, has been found to significantly reduce creep (compared to the use of a thick layer 114). Creep is the deformation of the layer 114 due to an applied stress over time. Creep can add a significant error to a stress measurement as it can cause the substrate to move in the Z direction over the time that it takes to collect data for a diameter scan of the substrate 102. For example, it has been observed that if the dome 112 is made entirely of a relatively soft dome material, i.e., without an internal dome 116, the material can creep significantly, e.g., 20 microns over 10-15 minutes after not having a substrate on the pins for the previous 60 minutes. Creep can add a non-linear shape to the measured substrate shape which will modify the calculated radius of curvature and add an error to the stress calculation.
With the use of an internal dome 116 with the layer 114, creep is significantly reduced. It has been found that creep is reduced in proportion to the reduction in the thickness of the layer 114. For example, creep was reduced from 20 microns in 15 minutes for a cast silicone dome (approximately 6 mm at its thickest) to 2 microns in 15 minutes when a 0.5 mm thick coating of the same silicone material was applied to an internal dome 116. Such a reduction in creep is particularly significant when measuring a change in shape of the substrate as small as 10 microns. Thus, it is believed that a layer 114 with a thickness between 0.25 mm to 2 mm, and specifically, 0.5 mm, is adequate.
Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.

Claims

1. An apparatus comprising:

a chuck having a plurality of support pins for supporting a substrate, the support pins having a dome shaped upper surface that contacts a bottom surface of a substrate when supporting the substrate.

2. The apparatus of claim 1, wherein the dome shaped upper surface has a radius of curvature between 0.125 inches to 0.5 inches.

3. The apparatus of claim 1, wherein the support pins have a body formed from a first material, and the dome shaped upper surface is formed from a layer of a second material, the second material being softer than the first material.

4. The apparatus of claim 3, wherein the second material is softer than the substrate to be held by the plurality of support pins.

5. The apparatus of claim 3, wherein the layer of a second material has a hardness of 20 shore A to 50 shore A.

6. The apparatus of claim 3, wherein the layer of a second material has a coefficient of static friction that is greater than 3.

7. The apparatus of claim 6, the apparatus further comprising a stage coupled to the chuck, the stage is configured to move the chuck.

8. The apparatus of claim 3, wherein the second material comprises silicone.

9. The apparatus of claim 3, wherein the layer of a second material overlies an internal dome that is on a top surface of the body of each support pin, wherein the layer of a second material is softer than the internal dome.

10. The apparatus of claim 9, wherein the internal dome is integrally formed with the body of each support pin.

11. The apparatus of claim 9, wherein the layer of a second material is approximately 0.25 mm to 2 mm thick.

12. An apparatus for supporting a substrate, the apparatus comprising:

a chuck having a plurality of support pins for supporting a substrate, each support pin has a layer of material that forms a dome shaped upper surface that contacts a bottom surface of a substrate when supporting the substrate, wherein the layer of material that forms the dome shaped upper surface is softer than a remainder of the support pin; and

a movable stage coupled to the chuck to move the chuck while a substrate is supported by the plurality of support pins.

13. The apparatus of claim 12, wherein the layer of a material that forms the dome shaped upper surface is softer than the substrate to be held by the plurality of support pins.

14. The apparatus of claim 12, wherein the dome shaped upper surface has a radius of curvature between 0.125 inches to 0.5 inches.

15. The apparatus of claim 12, wherein the layer of a material that forms the dome shaped upper surface has a coefficient of static friction that is greater than 3.

16. The apparatus of claim 12, wherein the layer of material that forms the dome shaped upper surface comprises silicone.

17. The apparatus of claim 12, wherein the layer of material that forms the dome shaped upper surface has a hardness of 20 shore A to 50 shore A.

18. The apparatus of claim 12, wherein each support pin comprises an internal dome covered by the layer of material that forms the dome shaped upper surface, wherein the layer of material that forms the dome shaped upper surface is softer than the internal dome.

19. The apparatus of claim 18, wherein the internal dome is integrally formed with a body of each support pin.

20. The apparatus of claim 18, wherein the layer of material that forms the dome shaped upper surface is approximately 0.25 mm to 2 mm thick.

21. An apparatus for supporting a substrate during stress measurements, the apparatus comprising:

a chuck having a plurality of support pins for supporting a substrate, each support pin comprising a body with an integrally formed dome at a top of the body, and a layer of material that covers the integrally formed dome to produce a dome shaped upper surface that contacts a bottom surface of a substrate when supporting the substrate, wherein the layer of material that forms the dome shaped upper surface is softer than the integrally formed dome; and

22. The apparatus of claim 21, wherein the dome shaped upper surface has a radius of curvature between 0.125 inches to 0.5 inches.