Radiant energy – Inspection of solids or liquids by charged particles – Analyte supports
Reexamination Certificate
2000-03-10
2004-01-13
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Analyte supports
C250S440110, C250S441110, C250S443100, C250S458100, C250S492300, C250S492210, C414S217000, C414S415000
Reexamination Certificate
active
06677594
ABSTRACT:
The present invention relates to improvements in scanning wheels used in ion implantation process chambers.
Ion implantation is used for doping semi-conductor materials to achieve desired conductivity states. A beam of ions of a desired species for implantation is formed and directed at the surface of a semi-conductor wafer for implantation therein. In batch processing ion implanters, it is usual practice to arrange for a batch of semi-conductor wafers to be processed simultaneously, and for these wafers to be repeatedly scanned throughout the ion beam to ensure a homogenous distribution of ions over the surfaces of the wafers.
U.S. Pat. No. 5389793 discloses a scan wheel used for scanning a batch of wafers in the process chamber of an ion implanter. This scan wheel has a plurality of wafer support elements circumferentially disposed about the axis about which the scan wheel rotates. A respective heat sink insert is mounted to each wafer support element and an elastomer layer is provided on the surface of the heat sink insert which will receive the wafer to provide good thermal, contact between the wafer and the heat sink insert.
When the wheel is loaded with wafers, a gripper arm presents a wafer to the heat sink insert and presses it against the elastomer layer. A pair of spring loaded clips then push the wafer across the elastomer layer into contact with a fixed restraint forming part of the wafer support element. Once the wafer is in place, the gripper arm releases the wafer which is then held between the spring loaded clips and the fixed restraint.
One problem with this arrangement is that the wafer tends to stick to the elastomer layer and is therefore not properly pushed into the correct location by the clips. However, the operating mechanism detects that a cylinder driving the clips has been moved and therefore assumes that the wafer is in the correct position. When the gripper arm is released, the clips then push the wafer across the elastomeric layer in an uncontrolled manner such that the wafer is not properly engaged with the fixed restraint. This may cause damage to the wafer, or may mean that the wafer is insecurely held such that it is thrown from the wafer support element when the wheel is spun up. Any attempt to increase the force provided by the clips to prevent the wafer sticking introduces a risk that the wafer will be damaged by being driven too hard into the fixed restraint.
As the scan wheel rotates at high speed, it is not practical to have any electronics on the scan wheel itself to detect the position of the wafer. However, as the water is not always fully moved into the correct position until after the gripper is released, it is also not possible to provide a detector on the gripper to ensure that the wafer is in the correct position. With the prior art, therefore, if a wafer is incorrectly positioned on the wheel, this is impossible to detect.
In one attempt to overcome these problems, a pair of elastomer bumps have been provided on the elastomer layer in order to support the wafer away from the elastomer layer when is it slid into position. The elastomer bumps are compressible so that they are squashed by the wafer when the machine is spun up to ensure a good thermal contact between the wafer and the elastomeric layer. However, these bumps proved to be counter-productive in practice as the wafers tended to pivot around the bumps when they were pushed into position, thereby taking the wafer even further away from its correct position.
According to a first aspect of the present invention there is provided a scan wheel for an ion implanter adapted for carrying a plurality of semi-conductor wafers, the scan wheel being rotatable about an axis for scanning wafers carried by the wheel across an ion beam, the scan wheel comprising a plurality of wafer support elements disposed circumferentially about the axis, each wafer support having a wafer supporting surface covered with an elastomer layer, and a plurality of projections which are resiliently biased such that they protrude beyond the elastomer layer to receive a wafer, wherein the coefficient of friction between the material of the projections and the backside of a wafer is less than that between the material of the elastomer layer and the backside of the wafer.
Thus, when a wafer is loaded onto a wafer support, it is received on the projections and can therefore be readily pushed across the wafer support into the correct position. As the wafer slides across projections having a low coefficient of friction, rather than sliding across an elastomer layer having a relatively high coefficient of friction, the wafer will not tend to stick and will be pushed into place reliably. Once the wheel is spun up, centrifugal forces will push the wafer down onto the elastomer layer against the action of the resiliently biased projections, thereby ensuring good thermal contact between the wafer and the elastomer layer.
A further problem with the elastomer bumps as used in the prior art is that the restoring force that they exert against the back of the wafer is unpredictable as the force required to collapse them is unpredictable. The inconsistency is such that, in certain cases, the elastomer bumps will not collapse even when the scan wheel is spun up.
According to the second aspect of the present invention there is provided a scan wheel for an ion implanter adapted for carrying a plurality of semi-conductor wafers, the scan wheel being rotatable about an axis for scanning wafers carried by the wheel across an ion beam, the scan wheel comprising a plurality of wafer support elements disposed circumferentially about the axis, each wafer support having a wafer supporting surface covered with an elastomer layer, and a plurality of projections which protrude beyond the elastomer layer to receive a wafer, each projection comprising a rigid plunger which receives the wafer, and a resilient element biasing the rigid plunger into the protruding position when unloaded. With this arrangement, the plunger and resilient elements are provided as separate entities, so that the resilient element can be selected entirely on the strength of the biasing force that it provides, thereby allowing a high degree of consistency to be achieved for the restoring force.
By avoiding sliding the wafer across the elastomer layer, particle build up on the back of the wafer is reduced and wear of the layer is reduced thereby prolonging the life of the layer.
The resilient projections also help to push the treated wafer away from the wafer support elements once the wheel has stopped spinning. This reduces the chances of a wafer sticking to the elastomer layers and therefore reduces the damage caused when a wafer is unstuck from the wafer support element.
Further, as the wafer can be reliably slid into the correct position, a gripper arm which brings the wafer to the wafer support element can be provided with a position detecting means so that confirmation can be obtained that the wafer has indeed been correctly positioned. Thus, even if the wafer is not initially correctly positioned, it is possible to make a further attempt to correctly position the wafer, or to alert an operator that the wafer is incorrectly positioned. In the prior art, this was not possible, and the first indication that a wafer was incorrectly positioned was when it flew off the wheel when the wheel was spinning.
Conventionally, a wafer support element will have a heat sink insert on which the elastomer layer is provided. The invention therefore extends to a heat sink insert for a wafer support element, the heat sink insert having a wafer supporting surface covered with an elastomer layer, and a plurality of projections which are resiliently biased such that they protrude beyond the elastomer layer to receive a wafer wherein the coefficient of friction between the material of the projections and the backside of a wafer is less than that between the material of the elastomer layer and the backside of the wafer.
The invention also extends to a heat sink insert for a wafer suppo
Applied Materials Inc.
Boult Wade & Tennant
Lee John R.
Vanore David A.
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