Grayscale Masking

Fill-Factor Enhancement of LCDs and Image Sensors

• In LCDs, the TFT transistor circuit blocks 70% of the display area. A microlens array is used to funnel light through each TFT transistor.

• In a detector array of an image sensor, 80% of a detector cell is blocked by the electronic circuit. A microlens array is used to focus light onto each detector cell.

Microlens Array for Image Sensors

• SEM Micrograph of 60-Micron Lenslet Array

• Atomic Force Micrograph of 5.5-Micron Lenslet Array

A Common Process Step in IC Chip Fabrication

Chrome Mask Lithography

A1. Chrome on Glass Mask
A2. A Layer of Photoresist Coated on a Substrate
A3. The Substrate is Chosen to Have Correct Material Properties, e.g., a Si Wafer
B. The Areas Exposed to UV Become Soluble and Are Removed
C. Transfer the Micro-Structure Into Substrate via the RIE Process

IC Patterns Produced in Photoresists Have a Rectangular Cross-Section

• Chrome mask lithography can only produce two-dimensional (2D) structures.

• This is because areas in a chrome mask can only be totally opaque or transparent.

• There cannot be gray areas in a chrome photomask.

Envisioning a Grayscale Photomask

• In Chrome on Glass Masks, Each Pixel Has Only Two Choices: Either Totally Transparent (100% T) or Opaque (0% T)

• The Question Was: Can One Build a 3D Microstructure by Changing %T Continuously From One Pixel to the Next and the Next Pixels

• The Invention of an All-Glass Grayscale Photomask Turned Imagination Into Reality