In order to improve throughput in semiconductor process tools, one must first consider the stepwise nature of the specific processes involved. Often the semiconductor wafer must be conveyed from process chamber to process chamber with branching capabilities. Thus, lengthy process times are supported by additional process chambers.
Breaking Down Photolithography Equipment & Processes
Let’s consider as an example photolithographic processing. A typical process for coating a wafer with photoresist involves the following steps:
- An initial heating step (HMDS Prime, hereafter referred to as Prime)
- A cooling step
- A spin coating step
- A second heating step
- A final cooling step
Now the wafer is coated.
For this specific process, it is required that the wafer proceed:
- from a supply source (i.e. cassettes)
- to a heated chamber (often a hot plate)
- to a cooled chamber (a chill plate)
- to a spinner (where resist is applied to the now cooled and temperature-controlled wafer)
- to a second hot plate
- to a chill plate (where the wafer returns to room temperature for further processing (i.e. exposure)
The initial Prime process takes about 60 seconds, the chill 30 seconds, the spin 60 seconds, the second heating between 90 and 110 seconds, and the final chill 30 seconds.
Bottlenecks in the Photolithographic Process
The second heating step has the longest process time, which creates a bottleneck. To support the tach (or rhythm) of the system, a single chamber for each of steps 1, 2, 3, and 5 will suffice. However, two hot plate chambers are required to support step 4 so that the overall system has a tach of approximately one wafer per minute.
Further, the contemplated process will not support the reuse of a rather lightly used resource, in this case it is the chill plate, because of difficulties that will arise from the its reuse. This effectively eliminates its “go back” capability.
Therefore, to optimize the throughput of this array of process modules, the formation of two chill plates, three hot plates and one spinner will constitute a balanced production line.
Robotic Equipment in Photolithography
We now consider the robotics needed to move the wafer from place to place. The wafer must proceed from cassette, to Prime, to chill, to spin, to heat, to chill, back to cassette; this constitutes at least six moves. If the system is provided with a single robot to move the wafers through the process, and the single robot has dual end effectors so that it can swap wafers at each sequential step, we have 6 moves total. If the tach of the process is one wafer every 60 seconds and the robot must move 60 wafers per hour, aka 6 moves/wafer x 60 wafers/hour = 360 moves/hour, or one move every 10 seconds. That is a lot of moving.
The robot must first interact with the source cassette and perform the initial swap (about 5 seconds), then move in the Z and Theta directions (another 4 seconds), then swap again (about 5 seconds); roughly 14 seconds in total, so that the system is more or less balanced and the throughput will be 43 wafers per hour. But what if we need higher throughput? We must add resources. This means the addition of more process chambers AND more robots that can move a lot faster and reduce the effective number of moves.
The quick thought analysis above demonstrates that when adding process resources, it becomes necessary to increase the number of robots in the system and effectively the total number of end effectors.
How to Advance Efficiency in Semiconductor Wafer Processing
The most advanced wafer processing tools will utilize the combination of very simple high-speed process robots and more traditional robots to unload and reload cassettes: The front-end robot handles wafers from the cassette to a buffer, where the process robot can acquire and return it. The X axis move can be in performed by the buffer so that the process robot itself does not need an X or theta axis, just an ultra high-speed Z axis and dual y axes.
The process robot is now a very simple 3 axis robot that is very fast due to its simplicity. Look out for more innovation in this space from S-Cubed.
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