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Dynamic duo

Dynamic duo

Boise, Idaho -"Faster, cheaper, better" is perhaps becoming an overworked phrase, but it sums up the effect of competition in the semiconductor industry. In the drive for ever more efficiency, semiconductor manufacturers are seeking leaner production and smarter automation. With wafer sizes growing from 150 mm to 200 and now 300 mm, safe, reliable, but faster wafer handling is of paramount importance. The competitive pressure also filters through to the companies supplying automation equipment to the semiconductor industry-like SCP Global Technologies who teamed up with control specialists at Richmond, CA-based Berkeley Process Control (BPC) to get the best performance from its robot handling system.

SCP's special expertise is supplying robot wafer handling facilities for "wet processing" tools. This is the overall term used to describe the various washing and etching stages through which a wafer passes during production. Speed is of the essence, says Ralph Spearow, SCP's engineering manager. "With up to eighty separate batch processes, even small savings in throughput time per handling sequence soon add up."

With the design of the company's new Stellaris 3.18 Wet Processing Station for 300-mm wafers, Spearow's objective was to increase handling speeds by a factor of four. This would take the throughput up to 600 wafers per hour.

Teaching robots new tricks. In the wet process, wafers move between the different processing stages in cassettes called FOUPs (Front Opening Uniform Pods). Two robots transfer the wafers from the FOUP to a process container or "boat," which is specific to the stage of processing. The first robot takes an individual wafer out of a cassette and sets it on a turntable platform used to align a notch in the wafer so that it is correctly oriented for future process steps. The second robot picks up the wafer from the turntable and places it into the process container.

Engineers at SCP initially hoped to achieve the desired throughput by simply enhancing the existing control system. The main shortcoming was that this distributed system issued commands to the robots and turntable in a serial manner. Since overlapping commands were considered too risky in view of potential collisions, the robots stopped between operations. Since these built-in delay times meant that throughput could not be appreciably improved, a more integrated solution was required.

Consequently SCP approached Berkeley Process Controls, a supplier of machine control systems, with whom they had worked closely on previous projects. SCP shipped the robots to BPC, who stripped them down and removed the dedicated controllers, replacing them with an integrated control system that permits unrelated movements to take place simultaneously.

One robot has three axes with a linear slide and the other has four axes, including a wrist movement, accomplished with a BPC-designed robot wrist. The robots handle the wafers using vacuum wands for pick-up and placement, and pressure sensors check for the presence of the vacuum.

Two, eight-axis S-64 machine controllers control all 12 axes of the front-end wafer handling system, communicating to the supervisory Windows NT host computer. This host computer provides scheduling and recipe control for the process side of the tool.

The controllers are linked in an Ethernet-based MachineNet OpenLink TM network. OpenLink allows several controllers to share a distributed connection using the same network backbone. The controllers themselves run MachineWorks(R), multi-tasking software developed by BPC. The controllers have a touch screen interface for the setting up of inputs and outputs, auto-tuning of servo axes, and programming. No knowledge of conventional software language or syntax is required.

With the new control system the robots can handle up to 600 wafers per hour, a figure that exceeds the target specification. For Jason Jones, BPC's project engineer, attaining the desire speed was not so much of a problem as was managing the challenge of error handling. He says, "Error recovery was one of our primary goals. Apart from the actual error detection, the control system must stop in a controlled manner and be able to restart again as soon as the error has been rectified. You can't afford to have wafers breaking!" It sounds simple, but a wide variation of errors has to be taken into account.

To ensure correct positioning and minimum wafer breakage, SCP has carried out a lot of development in the field of compliant cassettes for wafers. As Ralph Spearow explains, "You don't try to put a brittle, but rigid object into a rigid container. Instead you make the container compliant in the appropriate manners so that it cushions the movement." When the inevitable wafer positioning errors do occur, the new system detects them and waits for the error to be rectified. Laser scanning of the wafer positions in the FOUP detects empty slots. The system can detect cross-slotted wafers where the wafer is lying at an angle to the slot and also whether or not there is more than one wafer in any given slot.

Paul Sagues, President of BPC, puts his faith in the integrated approach. "Berkeley's system-centric control philosophy provides the tool builder with unprecedented integration flexibility. We want to relieve OEMs of the time and cost burdens of front and back-end tool integration." By leaving control integration to specialists, he believes that OEMs can focus on their expertise in wafer handling and tool configuration. He adds that BPC now has new products that offer even better performance. These include the BXi/o OpenLink Ethernet system for the in- tegration of sensors and actuators on an open 100 Mbit network. BPC finds that its customers are selecting Ethernet over standard proprietary networks because they can use their present IT and tool investments.

BPC is also continuing with developments specific to wafer handling. For notch alignment, as an example, the company now has the new BXi/pa integrated pre-aligner available, specially designed for 200-mm and 300-mm wafer handling. It is used in conjunction with the BXi controller, to locate the center of the wafer and orient it for the next process. The control hardware and software resides in the BXi controller forming a plug- & -play feature with the BXi/pa. Since no external data transfer is involved, there are no communications delays and operation is therefore faster.

Controls retrofit boosts performance
BEFORE AFTER

Number of robots

2

2

Wafer size

300 mm

300 mm

Throughput

150 wafers/hour

600 wafers/hour

Command structure

Serial

Parallel

Control architecture

Distributed

System-central (integrated)
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