This month we feature a wafer handling article on the alignment of a device patttern to the crystal plane of the wafer

Alignment of a device pattern to the crystal plane of the wafer

Historically, solid state lasers were produced by cleaving wafers, taking advantage of the material's crystal planes to form the reflective surfaces of the laser cavity. This method relied upon the lithography system’s ability to align and print the device patterns accurately with respect to the cleaved wafer flat. Contact printers had the advantage of being able to directly view the pattern and the cleaved wafer flat simultaneously. Unfortunately, early steppers, using mechanical referencing pins and LED sensors to locate the wafer flat were not accurate or precise enough to meet the cleaving requirement, which is typically within 0.1 degree of the crystal plane.

With the advent of ASML’s 5500 photolithography systems, a new type of wafer edge sensor was introduced. This sensor used an all optical approach which utilized a CCD camera to accurately locate the edge of the wafer. This unique approach delivered performance that met the cleaving requirement. Furthermore, the wafer could be rotated with programmable offsets from 0 to 360 degrees, in 0.01 degree increments.

The advantages of steppers over contact printers are well known, but the alignment of a device pattern to the crystal plane of a wafer was somewhat of a mystery until recently. How does one solve the problem of relating the co-ordinates of a cleaved wafer edge to those of the stepper’s optical edge sensor?

 

Stepper alignment to Crystal Plane, as published here

 

The most workable solution for this application was to use the stepper to print suitable markers, at a known separation, along the cleaved wafer edge so that the difference in the co-ordinate systems could be measured, offsets calculated, and programmed into the stepper recipe. This method has been employed by a number of researchers and is well described in an article published in the July 2007 edition of Laser Focus World magazine, by BinOptics and ASML. This article discusses a new method of Laser Diode singulation and details the cleaving methodology and performance of the ASML edge sensor.

For more discussion, and Simax Lithography engineering support, please contact us at Simaxlithography.com