Compliant silicon wafer handling system

Details Compliant silicon wafer handling system Compliant silicon wafer handling system

1997-04-02

2001-09-11

Redman, Jerry (Department: 3634)

Supports: racks

Special article

Platelike

C414S937000, C414S404000, C206S454000

Reexamination Certificate

active

06286688

ABSTRACT:

TECHNICAL FIELD
The present invention is directed to apparatus and systems used to transfer silicon wafers between processing stations during the fabrication of semiconductor devices, and more specifically, to a compliant comb and support structure for holding the wafers. The compliant comb and support structure permits wafers which are misaligned with respect to the comb or another wafer holding device to be transferred from the holding device to the comb or from the comb to the holding device without causing damage to the wafers.
BACKGROUND OF THE INVENTION
The processing of silicon wafers to form semiconductor devices involves a number of processing stages which are performed at different stations. The wafers are typically held in a wafer cassette or other type of wafer holding device during the processing stage. After completion of the processing stage, the wafers are transferred to a wafer comb in which they are transported to the next processing station, where they are transferred from the comb to another wafer cassette or holding device. Automated wafer handling mechanisms such as robots are often used to load the wafers into a comb, transport the comb to a processing station, and unload the wafers from the comb into a wafer cassette, processing rack, or other wafer holding device. The loading of wafers into a comb from a wafer holding device such as a cassette or process rack is accomplished by arranging the comb slots so that the wafers in the cassette engage the slots, and then moving the wafers into the comb. The loading of wafers into a cassette or holding device from a comb is accomplished in a similar manner. Such wafer transfers can consist of batches of variable size, including single wafer transfers.
In order for the wafers to be processed into properly functioning devices, it is necessary that they not be damaged during the wafer transport and (un)loading processes. One source of potential damage to the wafers is misalignment between a wafer in a wafer cassette or processing rack and the slots of a wafer comb. If a misalignment exists between the wafer positions(s) and the comb slots, forces large enough to cause chipping, scratching, or breakage of the wafer can be produced. Damage to the transport or loading equipment may also result from improper positioning of a robotic arm carrying a wafer or wafers relative to a slot or slots in a comb, cassette, processing rack, or other wafer holding device. Misalignments are even more likely to cause damage to one or more wafers in the situation in which a comb is used to transfer a batch of wafers to another wafer holding device. This is because not all wafers in a batch will necessarily have the same type or degree of misalignment.
When extracting wafers from a process cassette or other wafer holder by engaging the wafers with the slots of a comb, the resulting forces can lead to frictional binding, causing the entire process cassette or holding device to be lifted with the wafers. Both the wafers and cassette or holding device can be damaged in such an event. Other undesirable effects of misalignment include damage to the robotics and loss of mechanism calibration. In addition to wafer or equipment damage, wafer cross-slotting (wafers having one side placed in a slot which doesn't correspond to the slot in which the other side is placed) or misplacement into an incorrect cassette slot are common problems caused by misalignment of wafer positions relative to a cassette or wafer holder.
The potential sources of wafer misalignment go beyond variations in the dimensions of wafers and wafer handling tool parts, assembly tolerances, and robotic “learn” points. There are also dynamic errors which add to the misalignment problem. High process temperatures can lead to thermal expansion of process cassettes (especially those made of plastic resins), producing an additional source of misalignment between a wafer handling tool, a wafer, and the slots in a comb or cassette. Wafer cassettes are also prone to warping over time due to swelling, fatigue, and creep. In addition, human errors during manual operation of robots can lead to collisions between wafer handling tools and cause misalignments. These sources produce misalignment problems which may be imperceptible to the human eye, but can be significant in the wafer transfer process.
Current robotic technology utilizes precise control of position and motion to permit interaction between robot mechanisms and a wafer comb, cassette, or processing rack. This precision can be difficult to attain if what is being positioned with the robot is delicate, yet like wafers, not consistent in size from one unit to the next. The dimensional variability of the wafers, separately or in addition to wafer handling tool control limitations, can produce misalignment of the wafers in the cassette slots or processing rack with respect to a wafer comb or other transfer device.
One method of reducing some of these sources of misalignment is to attempt to guarantee extremely precise robotic movements through parts tolerancing, use of alignment fixtures and measurement devices during setup, and other quality control measures to screen out “bad” parts. However, this approach leads to unnecessarily high material and labor costs. In addition, the risk of frictional binding between a wafer and a cassette is not greatly reduced by these measures. As noted, frictional binding results from misalignments which cause cassette lifting during wafer extraction, especially when a high Coulomb friction coefficient exists (e.g. quartz cassette and silicon wafer). This problem is exacerbated during the batch transfer of wafers from a cassette or other holding device to a comb where binding of one wafer in a slot during engagement with the comb is all that is required to potentially damage the entire batch.
The sources of misalignment can also be compensated for in a number of different ways. One way is to use a multitude of sensors to detect the position of the wafer(s) and then control the motion of the robotics to compensate for any misalignment. This is done by adjusting the position of the robotic arm which moves the wafer(s) or comb with respect to the cassette, usually by altering the coordinates of the destination position of the wafer. The robotic arm will then transfer the wafer to a corrected destination position which accounts for the misalignment. However, one disadvantage of such systems is that they require a feedback loop between the sensor(s) and the robotic arm and hence can significantly slow down the wafer transport process, since each wafer in a batch may be misaligned in a different manner. Another disadvantage is that when extracting more than one wafer at a time, dynamic (re)positioning of the robot will not compensate for slot-to-slot dimensional variations in the process cassette or wafer holder relative to the comb.
Another method of dealing with the problems caused by wafer misalignments is to have a compliant system for handling the wafers. Such a system is tolerant of a degree of misalignment and compensates for the misalignment(s) by passively adjusting the position(s) of the wafers. A compliant system can cope with misalignments created by dimensional variabilities in the objects being positioned, as well as misalignments of the wafer handling tools.
However, a disadvantage of some existing compliant systems is that motion of the wafer holder or comb which is part of the system along one axis in order to compensate for a misalignment produces motion along a second axis. This prevents independent control of motion in one direction, which may be what is required in order to correctly position the wafers without causing damage. For example, if lateral compliance of the wafer comb or holder to a wafer misalignment is associated with a vertical motion, then a wafer may end up being positioned high enough above a wafer slot to be damaged when it is released from the wafer comb into the slot. Another disadvantage of some compliant systems is that because all of the wafers in a batch be

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