WAFER GRIPPING FINGERS

Abstract: Wafers (12) are gripped by edge holders (14, 16, 18) which have curved, and optionally, Teflon coated grooves (34) with a radius of curvature greater than that of the edge of the wafer being held. This minimizes stress and torque applied to the wafer that might distort its shape and impair the measurement accuracy of wafer parameters.

Description: TITLE OF THE INVENTION WAFER GRIPPING FINGERS CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 U.S.C. §119 (e) to Provisional Application No. 60/124,647, filed March 16, 1999; the disclosure of which is incorporated herein by reference. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT N/A FIELD AND BACKGROUND OF THE INVENTION In the measurement of semiconductor or silicon wafers various parameters related to wafer shape, such as flatness, bow and warp, are made in industry today. The wafers being measured are transported to measuring stations using edge grippers and are held in place during measurement by edge grippers as well. Typically three or more grippers are used. The typical gripper contains a "V" shaped groove and the wafer which has a generally rounded edge shape is held such that two surfaces in each groove contact the wafer at each of the grippers. Typically the grooves are made of PEEK (Polyetheretherketone) . This over constrains the wafer support points resulting in an indeterminate loading at each of the contact surfaces as well as forces aimed out of the plane of the wafer. As a result, stress and torque of an unpredictable nature can be randomly applied to the wafer in holding it in these "V" grooves. The unpredictable forces thus generated can affect the shape of the wafer and lead to erroneous measurements of dimensional parameters. BRIEF SUMMARY OF THE INVENTION These deficiencies of present wafer gripping grooves are overcome with grooves of the present invention which have a curved shape with a radius of curvature great enough, typically greater than that of the wafer edge, so that contact occurs only are one point, patch or region of the edge. That point or region of contact is also centrally located so that there is a minimum of stress build up that can deform the wafer shape. Optionally the wafer groove is coated or made of Teflon to further minimize the deforming forces thereon. DESCRIPTION OF THE DRAWING The features of the invention are described below in the detailed description and accompanying drawing of which: Fig. 1 is a view of a wafer held in position by a series of grippers; Figs. 2a - 2c are views of a prior art gripper; Figs. 3a - 3c are views of a gripper according to the invention; Figs. 4 and 5 illustrate considerations in the design of a gripper groove according to the invention; and Fig. 6 illustrates the gripper of the invention in use in wafer testing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows a wafer 12, typically of a semiconductor material such as silicon. The wafer 12 is supported by three fingers or grippers 14, 16, and 18 which may be part of a wafer transport handle, not shown, or a support fixture for holding the wafer at a wafer test station, vertically or horizontally. Prior art versions of the fingers or grippers 14, 16, and 18 are illustrated in Figs. 2a - ac. As shown there the fingers have a body 20 with an end section 22, typically of PEEK, having a groove 24 at the end where the wafer 12 is to be held. As seen in Fig. 2b the groove 24 makes contact with the wafer 12 at two points 26 and 28 which are physically separated. With three fingers there are six contact points to hold the wafer and there is a situation of over constrain for the wafer such that the force, torque or stress at any one point cannot be accurately controlled. The forces are additionally directed out of the plane of the wafer. The result is that the wafer will in some or all cases experience a deforming set of forces and torques that can result in erroneous dimensional measurements. Figs. 3a - 3c illustrate a finger 32 and groove 34 of a curved shape where the radius of curvature is greater than that of the edge of wafer 12. The wafer makes contact at only one central point 36, which in reality may be a patch or region of finite size rather than an infinitely small point. The limited single point or region of contact minimizes the deforming forces on the wafer and leads to greater accuracy in their measurement. The fingers 32 may have the grooves coated with Teflon or other material of like low friction properties to facilitate the relaxation of friction induced forces, particularly those out of the plane of the wafer. The finger itself may also be made of such a material. Fig. 4 shows in greater detail the contact patch 36 of the wafer and the groove 34. As shown there it is clear that the forces on the wafer all lie in the plane of the wafer and thus generate little or no forces capable of deforming the wafer. With a "V" shaped groove such as in figs. 2a - 2c, the points of contact apply forces out of the plane of the wafer causing forces capable of deforming the wafer. Fig. 5 graphs the effects on shear and in planer forces of the radius of the groove varying over a range from the same as the edge, re, to twice or more. The shear force clearly drops off substantially with expanding radius of curvature of the groove. Fig. 6 illustrates the use of the edge holding groove of the invention in a typical test fixture. As shown there a wafer 12 is placed in grippers 32 which are part of a test fixture and hold the wafer horizontally or vertically as the instrument warrants. Vertical orientation enhances the benefits of the invention by holding the wafer in a manner where the gravitational forces are also in the plane of the wafer 12. The wafer is held for measurement by one or more probes 40 that measure a desired dimensionally sensitive parameter including warp, bow, flatness among others.

Claims: CLAIMS 1. A wafer holding finger for use in test or transport of semiconductor wafers comprising: a gripper finger having a groove shaped to hold an edge of said wafer; the groove shape being of an arcuate nature that contacts the edge of the wafer in only one patch. 2. The finger of claim 1 wherein said groove shape is curved with a radius of curvature greater than a radius of curvature of said wafer edge to thereby contact said wafer in only one patch. 3. The finger of claim 1 wherein said contact patch generates forces aligned into the plane of the wafer. 4. The finger of claim 1 wherein said groove has a low friction material on a surface of said contact patch. 5. The finger of claim 4 wherein said low friction material is Teflon. 6. A test fixture having three or more fingers of claim 1 holding a wafer for test thereof. 7. A wafer holding finger for use in test or transport of semiconductor wafers comprising: a gripper finger having a groove shaped to hold an edge of said wafer; The groove shape in holding said wafer generating forces resulting from contact with said wafer only in the plane of the wafer. 8. The finger of claim 7 wherein said groove is curved with a radius of curvature greater than a radius of curvature of said wafer edge to thereby contact said wafer in only one patch. 9. The finger of claim 7 wherein said contact patch generates forces aligned into the plane of the wafer. 10. The finger of claim 7 wherein said groove has a low friction material on a surface of said contact patch. 11. The finger of claim 10 wherein said low friction material is Teflon. 12. A test fixture having three or more fingers of claim 7 holding a wafer for test thereof.

Inventors: PODUJE NOEL S, SINHA JAYDEEP KUMAR, PODUJE, NOEL, S., SINHA, JAYDEEP, KUMAR
Applicants: ADE CORP, PODUJE NOEL S, SINHA JAYDEEP KUMAR, ADE CORPORATION, PODUJE, NOEL, S., SINHA, JAYDEEP, KUMAR
Application year: 2000
Main classification: B25J15
IPC codes: B25J15, H01L21