The SEMI International Standards Program is a key service that brings together the global semiconductor equipment makers and the device makers.
JAN OLAF GAUDESTAD
VP Business Development
The SEMI International Standards Program is a key service that brings together the global semiconductor equipment makers and the device makers. This partnership establishes a voluntary technical agreement aimed at improving product quality and reliability ensuring compatibility and inter-operability within the fab. Single crystal silicon wafers are utilized for essentially all integrated circuits and many other semiconductor devices. To permit common processing equipment to be used in multiple device fabrication lines, it is essential for the wafer dimensions to be standardized. SEMI Standards are written documents for specifications, guides, test methods, terminology, or best practices.
One of the 16 standard committees is on metrology, where several guides and test methods have been developed for wafer geometry. Measuring wafer geometry can be divided into two different measurements based on spatial wavelengths. Roughness and nanotopography are features so small that gravity does not play a role and only front side measurement is needed. Shape, flatness, and thickness (total thickness variations) have larger spatial wavelengths where gravity will pull the wafer flat necessitating measurements on both sides.
The SEMI standards have been written for mainly optical techniques to measure the features from nanotopography to shape, however for the smaller features, scanning electron microscopy (SEM) and atomic force microscopy (AFM) have also been added to the manufacturing line. As the resolution requirement increases and more data is collected, the time of acquisition also increases proportionally. For that reason, the standards have been written to allow for only 1 or 5 or 9 sections sections of the wafer to be measured for nanotopography. For flatness and total thickness variations, the standards set a minimum of 2mm × 2mm with a minimum of 10nm amplitude while the standards for shape, such as bow and warp, sets the minimum of 4mm × 4mm and 10nm amplitude.
The most advanced wafer geometry fab equipment collects 700 thousand data points on a 300mm wafer in one minute, which equals a spatial resolution of 317µm with an amplitude in the order of 1 nm. Wooptix wave front phase imaging, which collects height (amplitude) information in every pixel in the image sensor, can collect 51 million data points in a single snapshot (100 ms shutter speed) using an 86 MP imaging sensor that provides a spatial resolution of 37µm with an amplitude of 0.5 nm. The introduction of WFPI into the semiconductor manufacturing facilities is expected to generate large cost savings for the device makers.