Stäubli six-axis robots benefit plastic molder
News from Staubli Corporation09/20/2007
Six-Axis Robots in Cleanroom Applications: Rick Palmer of Stäubli Unimation explains how the use of six-axis robots in cleanrooms is becoming more popular, especially by, producers of semiconductor process equipment. Six axis robots have been used in a wide range of cleanroom applications since the 1980s. The Unimation PUMA was the first robot capable of operating in a low particulate cleanroom environment, and thousands were sold for use in applications requiring more dexterity than three and four axis robots. For a variety of reasons, the six axis approach to cleanroom applications is currently enjoying a resurgence of interest, especially by the producers of semiconductor process equipment. This article seeks to make the case for the six axis cleanroom robot and offers examples of several successful applications. A six-axis robot mimics the shoulder, arm, waist, and wrist motions of the human. This design is made up of a series of arm segments using rotary joints for articulation. These robots feature a variety of reach, payload, speed and dexterity specifications which vary widely depending on the design of the arm segments and their articulation. Several cleanroom six-axis robots are basically industrial robots that have been modified to contain particulate generation. The six-axis cleanroom robots currently on the market offer a variety of speed, precision, and cleanliness options representing the approach of several manufacturers to meet the needs of their target customers. A wide variety of cleanroom applications benefit from the capabilities of the modern six-axis cleanroom robot. The unique combination of dexterity, precision, and work envelope provide the customer with a powerful tool to meet demanding wafer handling challenges. End user customers are demanding tools featuring high throughput multiple process chamber capability in as small a footprint as possible. Often, these demands are most efficiently met using a six-axis robotic solution. The adoption of 300mm wafers for production has made the use of vertically oriented process chambers more popular. Owing to the slow adoption by industry of the 300mm wafers, several process equipment manufacturers are building "bridge tools". These tools are capable of both 200mm and 300mm operation. Obviously, the tool must be designed for the larger wafer size, so even though 300mm is slow to reach production quantities, the equipment community is creating a tool set to accommodate the larger wafer. Several of the leading process equipment manufacturers have adopted a six-axis solution for their bridge tool offerings, and more are on the way to market. The six-axis robot can offer several advantages over other wafer transfer options. Some of the issues that should be considered are: Work Envelope: There are a number of different designs of arm segments and their articulation. The size and shape of work envelopes offered by different manufacturers varies wildly. The most efficient work envelope is a hemispherical shape as viewed from the side. This arrangement, coupled with large travels of individual axes, yields the most efficient work envelope for most wafer handling tasks. Several of the industrial six-axis robots that have been modified for cleanroom operation feature a “teardrop” shaped worked envelope. These design, while adequate for use in a welding or material handling robot can create limitations when the robot is installed in a small area, such as a wafer handling front end. Often the problem with this arrangement is that the robot does not have the dexterity to maneuver in a small space. This often means a larger footprint than is desirable by the end user customer. It is important to carefully study the configuration of the robot under consideration. It is not enough to look only at total reach. It is critical to understand issues such as total reachable area, minimum swept radius, and other key issues that can have a large effect on a particular application. Several leading semiconductor process equipment manufacturers are using a "vertical cluster tool" approach for their new systems. This approach involves stacking process chambers vertically to provide a high throughput system in as small a footprint as possible. Class 1 cleanroom space currently costs many thousands of dollars per square foot, so keeping the footprint small is obviously important. Since the height of the system does not increase footprint, the advantage of vertically stacking process chambers is clear. One leading process equipment manufacturer using this approach is SVG-Thermal Systems Division. The design of their new tool involves the use of four process chambers arranged in two stacks of two chambers. Given the size of the chambers, a robot with the capability of a Z-axis of at least 1m was required. Other stations that require servicing are four horizontally arranged 200mm cassettes, or 300mm FOUPs. Also, since the process requires wafer orientation, the wafers must be presented to a wafer aligner. Servicing all these points required a robot with a large, efficient work envelope in as small a package as possible. After much research and the creation of several proof of principal mock-up cells, the Stäubli-Unimation . RX-90CR was chosen for the application. Dexterity An important factor in reaching the goal of a large work envelope in a small space is dexterity. The six-axis robot offers the advantage of human like dexterity, which offers two big advantages to the horizontally articulated three or four-axis design. Not only does the robot's) dexterity allow the wafer to be presented at nearly any angle, it also allows the robot to maneuver around obstacles in the work cell. Six degrees of freedom allows the arm to position the wafer very near the base of the robot so radial moves can be made in a very tight space. As an example, the Stäubli-Unimation RX-90CR robot has a maximum reach of 900mm between joint 2 and the centerline of joint 5. The same robot features a minimum swept radius of only 289mm. This ability supports the goal of combining a long reach with a small footprint while providing the dexterity to maneuver in a small area. Important areas to be considered when analyzing a six-axis robot for a particular application is the length of the individual arm segments, the design of the wrist assembly (axes 5 and 6), and the travel in degrees of the various axes. It is very difficult to intuitively model the motion of a six-axis robot. Often in wafer handling applications very small motions become important. The only way to insure the functioning of a workcell is to create a 3-dimensional mock-up of the system. This is often accomplished by creating a cardboard and wood workcell representing the critical reach points. The process of creating the most efficient work cell layout usually takes several iterations. The size and geometry of the end effector is critical to creating the most efficient cell layout and can only be determined in its final form through several iterations. Time spent on this process is the only way to create a system that utilizes floor space most efficiently. Payload One of the techniques to meet high throughput goals is to avoid moves that do not support keeping in-process time at a maximum. As an example, handling wafers with a multiple wafer end effector can keep the load/unload time at a minimum. The payload and moment of inertia capabilities of the six-axis robot supports the use of multi-position end effectors, and gives the system engineer another capability to meet his goals. A good example of a cleanroom application t~at requires a high payload is block mounted wafer polishing. This process involves the waxing of 300mm wafers to a large silicon carbide block. The block weighs over 13 pounds and presents a significant moment of inertia given the 300mm diameter of the wafer. The unique requirement of this application was the high payload requirement combined with the dexterity to maneuver in the workcell while placing the wafer on the block with high precision. Precision Since most cleanroom handling applications require the placement of very delicate silicon wafers in process chambers, the precision of the robot is critical. Not only can inaccurate placement of the wafer contribute to wafer breakage, but also the scraping of a wafer on a surface can create a cloud of particulates. Since wafer breakage and particulate generation can negatively impact process yield, the issue of robot precision can be very important. Most six-axis cleanroom robot manufacturers quote the precision of their robots as repeatability. This specification involves the robots ability to reach a point in its work envelope time after time. Most cleanroom six-axis robots specify a repeatability of +/- 0.004". This figure is marginal for wafer handling, but is adequate for less demanding applications. A range of four robots are available with repeatability of less than +/ 0.001 ". One of these, the RX-90CR was used in the application involving the mounting of the wafer on the silicon carbide block. This application called for mounting the wafer within a +/- 0.003" tolerance relative to the centre of rotation of the block. Maintaining this tolerance is critical to the process and was a major issue in the selection of the robot for the task. Cost of Ownership This factor varies widely with the application. The use of a six-axis robot can offer cost of ownership advantages over four-axis robots when the application calls for auxiliary axes. For instance, many applications require a four-axis robot has to be mounted on a track to reach multiple cassettes, or a ' process chamber must be articulated to allow the robot to properly present the wafer. In these instances, the use of a single six-axis robot will often provide a lower cost of ownership model than the four-axis solution. When the cost of auxiliary axes, engineering costs, software costs, and lower mean time between failures are considered, the slightly higher initial cost of the six-axis robot is very attractive compared to the total cost of ownership of the alternative solution. The end user demands for tools that are "better, faster, cheaper" is creating the need for innovative system designs. Requirements for more wafers per hour in the smallest possible footprint, at an attractive cost of ownership is driving the semiconductor equipment community to take a fresh look at the advantages of six-axis wafer handling. Thus the future of the six-axis robot appears very bright as new tool sets to support 300mm wafers and new processes are developed. The benefits of the "vertical cluster tool" concept is becoming well recognized and will create additional demands for high dexterity, high precision six-axis cleanroom robots. About Stäubli Robotics Stäubli Corporation is an international privately held group, founded over 100 years ago in Switzerland, employing over 3,500 worldwide. Robot production is centered south of Geneva in the French Alps, with facilities across Europe, North and South America and the Far and Middle East. In 1980, Stäubli Group made the strategic decision to diversify into robotics, beginning with the affiliation of US Company Unimation (a pioneer in industrial robotics). It launched the large SCARA RS robot range in 1987, acquired Unimation in 1988 and launched its RX range of robots in 1992. Today with the addition of the new RS series due to a recent acquisition of the Bosch Rexroth SCARA robot line, the newly developed TX series, and RX heavy payload robots, Stäubli has the broadest robot line on the market. Thousands of Stäubli robots have now been installed worldwide and the company currently boasts 32 sales and service subsidiaries in 24 countries and agents in over 50 countries. For additional information contact: David ARCENEAUX Business Development & Marketing Manager Stäubli Robotics firstname.lastname@example.org Visit http://www.staubli.com for more information on the company and products.
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