Tilt mechanism for wafer cassette

Details Tilt mechanism for wafer cassette Tilt mechanism for wafer cassette

2000-01-07

2001-02-20

Bratlie, Steven A. (Department: 3652)

Material or article handling

Process

Of emptying portable receptacle

C414S937000

Reexamination Certificate

active

06190118

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to cassettes used for receiving and holding a plurality of silicon wafers or the like, and more particularly, relates to a novel tilt mechanism used in conjunction with such a cassette.
BACKGROUND OF THE INVENTION
In the semiconductor manufacturing industry, silicon workpieces are used in the manufacture of integrated circuit components and the like. The workpieces are known in the industry as “wafers” and typically have a flat, circular disk-like shape. During processing of silicon wafers, they are subjected to a number of operations performed by one or more machines. In the course of these operations, the wafers often need to be transported from one processing position to another position within the same machine, or from a processing position within one machine to a position within another machine. Plastic or metal wafer cassettes are typically used to carry and transport the wafers.
A typical wafer cassette comprises a lightweight, hand-carryable housing having an open, front-facing portion that is configured with a plurality of parallel, spaced apart horizontal slots. The slots can receive and hold a plurality of wafers and permit convenient loading and unloading of the held wafers. Loading and unloading may be performed manually or, more typically, through the use of precision robotics. The bottom of the cassette usually includes a flat portion so that the cassette remains stable when placed on a flat surface. The top of the cassette typically includes a handle for convenient manual grasping of the cassette during transportation from one processing position to the next.
Silicon wafers are often processed by Chemical Mechanical Planarization (CMP) machines, which polish or planarize the wafer surfaces to a flat condition. Because of the precision required in the production of integrated circuits, at least one side of a semiconductor wafer must have an extremely flat surface to ensure proper accuracy and performance of the microelectronic structures being created on that surface. CMP processes and machines are known and are described in detail in U.S. Pat. Nos. 4,805,348, 5,099,614, 5,329,732, 5,498,196 and 5,498,199.
The loading and unloading of wafers to and from cassettes is often carried out robotically. Typically, a fully loaded cassette, holding in the range of 25 wafers, is placed in the unload compartment of a CMP machine, cleaning machine, or the like. An unloading robotic arm sequentially retrieves the wafers one at a time from the cassette and transports them to a processing station. The wafers may thereafter be moved through a number of processing positions within the machine. When processing is complete, the wafers are retrieved by an unloading robotic arm from a final processing position and are transported and loaded into the slots of an empty cassette in the unload compartment of the machine. Once a cassette is fully loaded with processed wafers, it may be grasped by the handle and transported to a subsequent processing station or machine. For economy and interchangeability, the same or similar cassette housings are usually employed both as load cassettes and as unload cassettes and can be used with a broad variety of machines.
The processing operations of CMP machines and the like often create a significant amount of vibration which is transmitted to the wafer cassettes. The vibrations act as a “lubricant” which reduces static friction between the wafers and cassette surfaces which, in turn, permits the wafers to be heaved or fed forward bit by bit. Eventually, this vibratory motion may cause the wafers to become mispositioned within the cassette or even completely dislodged from the cassette. The cost of breakage should a wafer become mispositioned or dislodged from a cassette, particularly a silicon wafer in the latter stages of semiconductor fabrication, can be quite high. To combat this problem, tilt mechanisms have been developed for tilting wafer cassettes upwardly and rearwardly. When tilted in this fashion, the wafers inside the cassette become gravity loaded against the rear portion of the cassette housing and are protected from mispositioning or dislodgement that might be occasioned by machine or workplace vibrations.
Known tilt mechanisms are relatively large and cumbersome. The space above the machine platforms or tables available for mounting a tilt mechanism or other devices, however, is extremely limited. For this reason, large portions of known tilt mechanisms have been mounted below the CMP machine platform or table. While this arrangement has been workable, it has not been ideal because the volumetric space underneath the table must also house control systems and electronics associated with the CMP machine. In addition, mounting much of the tilt mechanism in such a limited and sensitive space underneath the table creates obvious access and servicing problems. This problem is exacerbated by the complexity and relatively large number of custom parts which characterize the design of known tilting mechanisms.
As CMP machine designs have continued to evolve, the space available underneath the platform has become even more limited. With respect to the most current machine designs, the control systems fill virtually all of the space underneath the table and there is no longer enough space available to accommodate portions of a cassette tilt mechanism. A tilt mechanism is thus needed that is possessed of a design sufficiently simple and compact to permit placement of the entire mechanism in the limited space above the CMP machine platform.
SUMMARY OF THE INVENTION
The present invention provides a simple and compact tilt mechanism which addresses and resolves the shortcomings of the prior art set forth above.
In accordance with a preferred embodiment of the present invention, a tilt mechanism is provided for periodically tilting a cassette configured to hold a plurality of wafers or workpieces such that the wafers or workpieces become gravity-loaded against a rear portion of the cassette. The tilt mechanism is suitably mounted entirely above a worktable of a CMP or other processing machine.
In a most preferred embodiment of the invention, the tilt mechanism comprises a housing which houses a circular cam having a spiral groove formed therein. A tilt arm is pivotally mounted to the housing and extends vertically between a lower end which is adjacent the cam and an upper end which is fixed to a platform supporting a cassette holding a plurality of workpieces. A cam follower is attached to the lower end of the arm and projects into the spiral groove. Rotary motion of the cam effects pivotal movement of the tilt arm which, in turn, effects tilting of the support platform and the cassette.
A further aspect of the present invention is a method for sequentially unloading wafers from a cassette configured to hold the wafers, wherein the cassette is secured to a tilt mechanism which is mounted entirely above a worktable of a chemical mechanical planarization machine. A control system first verifies that the cassette is in a planar state relative to a robotic arm configured to retrieve wafers from the cassette. Once a planar state is verified, the robotic arm retrieves a wafer from the cassette and moves away from the cassette to a central position. The robotic arm then signals the control system that this task has been completed. Having received the signal from the robotic arm, the control system actuates the tilt mechanism to tilt the cassette back to a position wherein the wafers held by the cassette are gravity-loaded against a rear portion of the cassette. Most preferably, the cassette is tilted back twenty-five degrees from a planar state. After this task is completed, the control system directs the robotic arm to transfer its wafer to a subsequent processing station. The robotic arm signals the control system when complete, and the control system actuates the tilt mechanism to move the cassette back to a planar state. This task should have a cycle long enough to prevent the wafe

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