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SEMICONDUCTOR MEASUREMENT TECHNOLOGY:

Suppression of

Premature Dielectric Breakdown for High-Voltage

Abstract:

Capacitance Measurements

Alvin M. Goodman

RCA Laboratories

Princeton, N.J. 08540

Surface-initiated premature dielectric breakdown is encountered in extended-range MIS C(V)* measurements at applied-bias voltages above some sample-dependent threshold value, e.g., 3 to 5 kV across a 150-um-thick wafer of sapphire. It is necessary to suppress this premature breakdown in order that a much larger applied-bias voltage may be used without damaging the sample. This may be accomplished by eliminating the air space adjacent to the sample surface at the junction of the dielectric and the electrode edge. A simple, easy-to-use apparatus (sample holder and probe assembly) which allows this to be done conveniently and quickly by using a silicone rubber washer to cover the edge of the electrode and the adjacent area is described. Construction details of the apparatus and a test chamber which have been tested to 30 kV are provided in an appendix.

Key Words:

Capacitance-voltage measurements; dielectric breakdown suppression; discharge suppression; electronics; extended-range MIS C(V) measurements; high-voltage C (V) measurements; MIS capacitor; premature dielectric breakdown; semiconductor devices.

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A recently developed modification of the MIS C (V)* technique [1] has extended its useful range allowing it to be used for the measurement of samples with insulating layers more than two orders of magnitude greater in thickness than was previously possible. This use, however, requires the application of large bias voltage to the capacitor sample to be measured. When the bias voltage exceeds some threshold value (e.g., 3 to 5 kV across a 150-um-thick wafer of sapphire [2]) there exists the danger of breakdown in the critical region along the insulator surface or in air at an electrode edge as shown schematically in figure 1(a). The breakdown at the surface or in air usually leads to breakdown of the dielectric under test (sapphire in the present work). This problem is similar to the one of "premature breakdown" encountered in the measurement of "breakdown strength" in dielectric media [3]. The present problem is different, however, in three respects: for C(V) measurements (a) the high-field region should have a well-defined area; (b) the field within this region should be as uniform as possible; and (c) there is no need to raise the applied field to the breakdown level.

*MIS C (V) is a commonly used abbreviation for metal-insulator-semiconductor capacitance as a function of voltage.

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The known methods of preventing premature breakdown [3] can be roughly categorized as follows:

1.

Immersion of the sample or its critical region in

(a)

suitable gas at high pressure (e.g., N2 at 100 atmospheres) (b) liquid, either insulating (e.g., oil) or partially conducting (e.g., aniline)

(c) solid (e.g., silicone grease or elastomer)

2. Shaping of sample geometry to reduce the electric field at the electrode edges (e.g., "recessed specimen" or "McKeown specimen" technique).

Each of these methods in its conventional embodiments is undesirable in the present application for one or more reasons: 1(a) would be difficult and time-consuming to implement; 1(b) and 1 (c) are messy and inconvenient to use; and 2 is inconsistent with our requirement of a uniform field over a well-defined area. Method 1(c) was used [2] with partial success up to about 8 kV. The solid used to immerse the critical region was a silicone grease (Dow Corning "4-Compound").* This is shown in schematic cross section in figure 1(b). The application of the silicone grease to the sample and its removal after the measurement were difficult, messy, and time-consuming. In addition, the technique did not consistently prevent breakdown above about 6 kV. In one set of experiments half of the samples were destroyed by dielectric breakdown when voltages up to 8 kV were applied across 150-um sapphire wafers during measurements. These and other equally catastrophic results made it clear that a better method for breakdown prevention was needed to make the measurement technique a practical one for everyday use rather than a research laboratory curiosity.

The new breakdown suppression technique and the associated apparatus and test chamber described in the following sections of this report were developed as part of a measurement system, to be used for routinely carrying out high-voltage extended-range MIS C(V) measurements. The first part of that system (a capacitance meter bias-isolation unit) has been previously described [4].

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It has been found that if an insulating silicone rubber toroidal washer having appropriate properties is pressed over the sample surface including the electrode edge, premature breakdown can be effectively prevented. The surface of the rubber in contact with the sample must be smooth and the rubber itself must be very resilient. It is also important that the rubber contain no voids in the vicinity of the critical region and that it have both a very high resistivity and a low dissipation factor. technique includes a means for pressing the washer uniformly and controllably over the surface of the sample. Washers molded from two silicone compounds have been used successfully; these are Dow Corning Sylgard #182 and #186.* It has been found that a thin coating of silicone grease on the surface of the washer in contact with the sample gives even more reliable protection against breakdown than using just the washer alone. Photographs of one version of the apparatus used for implementing the technique are shown in figures 2, 3, and 4.

*See Disclaimer on p.iv.

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Figure 2.

Oblique view of apparatus.

The washer frame is raised, the baseplate is moved to the side, and the probe-carrier box is partially raised on its hinge for visual clarity.

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Figure 3. Overhead view of apparatus with sample in place and probe-carrier box in extreme upward position.

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