Metrology F
E
A
T
U
R
E
S
Replacing C-V Monitoring with Non-Contact COS Charge Analysis by Kelvin Catmull, Richard Cosway, Motorola; Brian Letherer, Greg Horner, KLA-Tencor
Monitoring contamination levels in diffusion furnaces is necessary to ensure that a consistent environment is maintained for the production of semiconductor devices. Due to the large load sizes of diffusion furnaces, there is a potential for significant amounts of scrap if adequate contamination monitoring is not maintained. In addition, a significant amount of product remains at-risk if contamination monitoring is not performed in a timely manner. Clearly, the value of monitor data is greatest immediately after a product run and this value decreases with time. Poly MOSCAP process vs. in-line
Electrical testing is often used after thermal oxidation as a means of detecting oxide contaminants introduced or activated during processing. It is important, however, to recognize that the degree and type of processing prior to test will influence the type of information received. For instance, the sample preparation necessary to get poly metal oxide silicon capacitors (MOSCAP) wafers ready for capacitance voltage (C-V) testing results in a significant exposure of the test structure to high temperatures. This process mimics the thermal exposure to full-flow devices, so the C-V electrical test parameters should ostensibly detect oxide problems that will ultimately result in end-of-line test failure. The natural annealing and cleaning action of the process, however, tends to mask true variations in the as-grown oxide quality. From a manufacturing viewpoint, it would be preferable to have an early warning system that flags impending problems before they have reached a critical stage. The standard C-V parameters are still desired, but without the cleaning action inherent in the poly MOSCAP deposition process. A preferred method would be an in-line technique analogous to C-V that does not require MOSCAP processing. This paper describes one of the first production implementations of such a system,
based on the corona-oxide-semiconductor (COS) technique. To provide a well-known reference for this work, we will concentrate on the sensitivity differences between poly MOSCAP test structures and the COS technology. COS technology
COS is similar to quasi-static (low frequency) C-V testing. The principal difference is that COS is a non-contact method, whereas C-V requires MOSCAP processing. As in C-V technology, COS analysis requires applying an electrical bias to the sample to measure the oxide’s electrical properties. For C-V, this bias is a voltage applied to the MOSCAP through an electrical prober and the response is the measured capacitance. With COS, the bias is applied by charging the oxide surface. The bias, in charge/area, is measured by a coulombmeter attached in series with the chuck. A typical sweep may bias the surface to create an electric field of ¹1MV/cm2 (the same bias range used in conventional C-V testing). The full sweep is composed of approximately 40 small charge depositions. Two techniques are used to measure the response of the semiconductor after each charge deposition: 1. Surface voltage (Vs) is measured by a noncontact vibrating Kelvin probe. Vs is controlled by the capacitance of the series-connected oxide and silicon. The oxide capacitance is a constant, while the silicon capacitance has an inherent bias dependence due to the semiconducting nature of the silicon. 2. Surface photovoltage (SPV) is the temporary voltage created when free carriers are photo-injected into the Spring 1999
Yield Management Solutions
21 1