Selecting UHV-compatible encoders for demanding semiconductor and scientific applications

Many high-end semiconductor manufacturing processes require the processing environment to be completely free of any contaminants that cause imperfections within the semiconductor wafer and transistors being created. As the semiconductor industry moves to smaller feature sizes, larger wafers and more complex components with more processing steps, acceptable levels of wafer contamination are increasingly small.

These critical industrial processes take place in a vacuum because the presence of significant amounts of air or other gases would be detrimental to the finished product. Processes like semiconductor manufacturing that demand precision motion control need vacuum-compatible optical encoders for position feedback. Locating the position encoders within the vacuum chamber ensures that the measurement device is as close to the working environment as possible, ultimately minimising errors.

Optical encoder components, such as the readhead and scale, installed inside a vacuum chamber have several vital features including:

• High temperature resistance (>100 °C) to endure the bake-out procedures required for UHV.
• High cleanliness to eliminate surface fingerprints, oils, and lubricants.
• Low outgassing of contaminants into the process chamber.
• Air vents to ensure full venting of air spaces within the readhead.
• Cable with PTFE insulation and silverplated copper braiding.
  

Low outgassing characteristics and high precision are key requirements in most UHV applications.

Wafer inspection

Wafer inspection is used throughout the semiconductor wafer patterning process. It is important for process control and helps to maintain high yields.

A wafer motion stage has at least two linear (X, Y) axes. Servo axis control loops use encoder feedback to deliver high levels of positioning accuracy.

An optical inspection head features high-resolution cameras with separate dark-field and light-field illumination sources. In automatic inspection, the head inspects the wafer and a comparison between each image capture and a ‘golden' or model reference image is used to locate defects.

Semiconductor inspection stage with RLE laser encoder system

Encoder requirements for wafer inspection:

• Analogue signal outputs enable highest levels of interpolation and resolution.
  
• ≤1 nm jitter for exceptional servo control, static and dynamic stability, which is required to ensure successful image comparison and the stitching together of images.
  
• Non-contact measurement at the point of interest, to eliminate Abbe offset and mechanical errors.
  
• Measurement systems that can operate within ultra-high vacuum environments.
  
• Optical encoders with Functional Safety (FS) and dual analogue and digital output for the wafer scanning motion.

Wafer handling

Semiconductor wafers are fragile and expensive, and they must be handled with care so that they are not damaged during the manufacturing process. Consequently, wafer handling robots are commonly used to move semiconductor wafers inside process equipment.

A wafer-handling robot has multiple degrees of freedom and consists of a body, an arm with multiple joints, and a wafer holding tool (paddle). Handling robots typically move a wafer from one transfer chamber to another or between process steps and their throughput has a direct impact on the overall performance of semiconductor fabs.

The straight line and angular motions of a handling robot must be accurate to avoid any contact that might result in either damage or contamination of the wafer surface.

Robot arm control is achieved with a motorized upper arm pulley and upper-arm spindle that drive a series of endless belts, which link the shoulder joint to the elbow and wrist joints of the arm.

Encoder solutions for wafer handling

RESOLUTE™ UHV encoders bring absolute encoder technology to ultra-high vacuum applications down to 10^-9 Torr, allowing customers to achieve higher throughput, reduce risks of damage and increase dependability.

RESOLUTE encoders offer outstanding metrology performance in UHV applications with resolutions down to 1 nm at a wide range of speeds.

TONiC UHV encoders are suitable for UHV conditions and use filtering optics and dynamic signal processing to give low Sub-Divisional Error which enables highly precise motion control.

Actuators attached to the ends of each mirror also require encoder feedback to precisely control the mirror shape.

Encoder requirements for synchrotron applications:

• Translational accuracies of better than 0.5 µm.
  
• Angular resolutions down to 0.1 µrads.
  
• Robust design resistant to high levels of vibration.
  
• UHV-compatibility and resistance to 120°C.

Encoder solutions for mirror benders

RESOLUTE UHV encoders are suitable for use inside UHV enclosures. Renishaw's range of RESOLUTE UHV encoders are intended for high performance, semiconductor, and scientific applications.

UHV motion stage for synchrotron mirror