Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
1999-01-08
2001-07-10
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C118S503000, C118S728000
Reexamination Certificate
active
06258228
ABSTRACT:
This invention relates to the processing of wafers, particularly semiconductor wafers by the vapor deposition thereon of films under high vacuum. The invention particularly relates to the solution of the problem of the warping of wafer clamping components, particularly wafer holder clamping rings, in the course of the processing of such wafers.
BACKGROUND OF THE INVENTION
In semiconductor manufacturing processes such as, for example, the sputtering of thin films onto substrates such as those formed of silicon it is necessary to hold substrate wafers in place for processing. While held, a sequence of processes is performed on the wafer, many of which result in the application of a thin film or coating layer to the wafer. Wafers held for the application of such coatings may be held in a horizontal orientation, facing upwardly or downwardly, or may be held in a vertical orientation facing in a horizontal direction. In all such coating processes, the wafer to be coated must be retained securely to a holder in a generally stress free state. To hold the wafers in such a state and to move the wafers safely and quickly between various processors and positions, wafer holders or clamps that employ resiliently supported continuous clamping rings to urge the wafer uniformly around its edge against the wafer holder have gained wide acceptance.
Wafer holders that employ wafer clamping rings are exposed to a range of temperatures when holding a wafer in a vacuum chamber for processing. These rings are further exposed to deposition of the same materials that are being deposited onto the wafers. While each wafer is subjected to a single cycle in a processing chamber in which the temperatures may rise and fall, and where a film thickness measured in microns is deposited on the wafer, the holders, and particularly the clamping rings that urge the wafers against the holders, are exposed to a large number of cycles in each of which the temperatures are cycled from maximum to minimum and in the course of which multiple layers of coating material accumulate on the clamping ring surfaces.
Over the course of many cycles, the clamping rings are observed to warp. The warping of a clamping ring is typically a permanent deformation of the clamping ring that alters the way in which the ring contacts the wafer. When the warping becomes excessive, the ring no longer adequately clamps the wafer. As a result, wafers can move in the holder, can be unevenly subjected to clamping forces in the holder causing breakage, or are not held in the proper position. The amount of warping and the number of cycles that it takes to cause excessive warping is observed to vary with different coating materials and coating processes. When the warping becomes excessive, after a number of wafers have been processed, it is necessary to replace the clamping ring with a new ring having its original design shape that will clamp the wafer uniformly around its edge.
For example, in one processing sequence in which the backside of a semiconductor is coated with two deposition layers of tantalum (Ta) and one layer of gold (Au), it is found that after only several hundred wafers are processed, excessive warping of the inner diameter of the clamping ring occurs lifting the inner rim out of its normal plane and toward the deposition chamber. As a result, a chamber overhaul to replace the clamping ring is required after far fewer wafers are processed than the several thousand desired.
Replacement of the clamping ring is necessary to prevent damage to the wafers and results in a loss of expensive production time in the making of semiconductors. The warping problem has not been effectively solved.
Accordingly, there remains a need to prevent or substantially delay the onset of excessive clamp ring warping in semiconductor wafer holders used in film deposition processes.
SUMMARY OF THE INVENTION
A primary objective of the present invention is to provide a clamping ring for clamping a semiconductor wafer to a wafer holder during the deposition of film onto the wafer that will resist warping over the course of processing a large number of wafers. A particular objective of the invention is to provide a wafer clamping ring that can accumulate deposited film of substantial thickness without warping to the point that cleaning or replacement of the ring is required. A further objective of the present invention is to provide a wafer mounting ring that will return to its original geometry after being cleaned of deposits that have built up to the degree that requires the cleaning of the ring.
Another objective of the present invention is to provide a method and apparatus by which coatings of material, for example materials such as tantalum and gold, can be deposited on a large number of wafers sequentially held by a wafer holder that employs a wafer clamping ring in processes where the deposition process subjects the ring and deposits to wide temperature variation cycles. A particular objective of the invention is to provide such a method and apparatus in systems where the clamping ring is supported on minimal mounting points providing limited or local conduction of heat from the ring, particularly where the process is carried out in a vacuum where heat dissipation by convection is substantially absent.
According to the principles of the present invention, a clamping ring is formed of a material having a coefficient of thermal expansion that is approximately equal to that of the material being deposited. By “substantially equal” is meant that the coefficient of thermal expansion of the material of which the clamping ring is made is closer to the coefficient of thermal expansion of the coating material than are the coefficients of thermal expansion of alternative materials, Preferably the coefficient of thermal expansion of the material of which the clamping ring is made also has a high modulus of elasticity so that it distorts less when subjected to a given thermal stress. In addition, it is preferable that the material of which the ring is made also have a high coefficient of thermal conductivity so that temperature gradients, and thus thermal stresses, are lower. It is also preferable that the material of which the ring is made have a high yield stress at the temperatures to which the ring is to be subjected during the deposition processes performed on wafers while held by the ring so that the ring can withstand higher thermal stress before undergoing permanent or plastic deformation.
In accordance with the preferred embodiment of the invention, a clamping ring is provided that is formed essentially of molybdenum metal. The clamping ring is configured as an annular disc with a substantially circular opening at its center that is bounded by a continues edge that engages the entire rim of a semiconductor wafer being held in a holder for the vapor deposition of film onto the wafer. The inner edge of the opening of the annular disc is circular for most of its circumference, that is, for all of the circumference of the disc except for a cord section configured to conform to the flat orientation edge of a conventional semiconductor wafer. The inner diameter of the disc is preferably about five centimeters less than the outside diameter of the wafers being clamped.
Further in accordance with the preferred embodiment of the invention, a clamping ring is provided which has mounting points spaced around the disc that provide for a balanced resilient mounting of the disc to the housing of a wafer holder and presents a substantially limited portion of the disc in contact with cooler structure to which heat could flow. The ring preferably includes six spring mounting points spaced around the disc which connect to springs on which the disc is mounted. The disc preferably also supports a set of latches, preferably three in number, by which a wafer is latched to the clamping ring.
Preferably, a clamping ring for the deposition of a film of tantalum and gold onto wafers is provided that is formed essentially of molybdenum. Clamping rings of other materials having coeff
Cantelmo Gregg
Nguyen Nam
Tokyo Electron Limited
Wood Herron & Evans L.L.P.
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