A blanket layer of silicon can be added to a silicon substrate by a CVD process to achieve changes in the properties, like resistivity, type and defect density. This CVD process is called epi (or epitaxial) deposition. The thickness monitoring of the epi layer is part of the production process of making the epi wafers. This monitoring is executed on a sample basis, or only to confirm that the epi reactor is set up properly.
Semilab can offer non-contact optical solution for Epi thickness monitoring, depending on infrared reflection techniques.
Figure 1. Resistivity measurement flow
Device makers have a critical need to measure sheet resistance. Knowing it as a function of position (mapping) allows for optimization of epi reactors to maximize yield.
The Semilab LEI-1510 family allows for non-destructive measurement of sheet resistance. These instruments are optimized for compound semiconductor applications with ranges of operation that dovetail nicely with customer applications. The LEI technology became the process of record in the RF device area with many people asking to see the “LEI” map.
Figure 1. The “Semilab LEI” Maps - the process of record for sheet resitance
The 1510 Non-destructive Sheet Resistance Measurement System uses eddy current to measure sheet resistance.
Theory of resistivity measurement is based on alternating current in a coil (or pair of coils) inducing alternating Eddy current in a conducting material. The Eddy current measurement is actually the measurement of the electrical loss in the material.
The measured signal depends on:
The Eddy current is higher in good conducting material compared to less conductive ones.
Figure 2. Diagram of Eddy current - upper coil shown only