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Contactless Method for Measuring Collector Isolation P-N Junction Capacitance in LSI Wafers

IP.com Disclosure Number: IPCOM000049904D
Original Publication Date: 1982-Aug-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 3 page(s) / 57K

Publishing Venue

IBM

Related People

Verkuil, RL: AUTHOR

Abstract

A convenient non-contact measure of collector-isolation P-N junction capacitance, in silicon LSI bipolar wafers, is proposed which uses a contactless combination of photovoltaic and photoconductivity sensing. The photovoltaic and photoconductivity signals are used to generate an effective junction capacitance-voltage characteristic whose slope increases monotonically with increasing junction capacitance. The measurement is believed to be sensitive to the sidewall component of capacitance, based on the anticipated eddy current behavior which controls the photoconductivity signal.

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Contactless Method for Measuring Collector Isolation P-N Junction Capacitance in LSI Wafers

A convenient non-contact measure of collector-isolation P-N junction capacitance, in silicon LSI bipolar wafers, is proposed which uses a contactless combination of photovoltaic and photoconductivity sensing. The photovoltaic and photoconductivity signals are used to generate an effective junction capacitance- voltage characteristic whose slope increases monotonically with increasing junction capacitance. The measurement is believed to be sensitive to the sidewall component of capacitance, based on the anticipated eddy current behavior which controls the photoconductivity signal.

Fig. 1 shows a silicon LSI (large-scale integration) wafer, under test, placed on a vacuum chuck and in close proximity to the upper and lower circuits which are used for photovoltaic and photoconductivity sensing, respectively. Pulsed light (0.65 um LED) impinges on the wafer and generates electron-hole pairs, to a depth of several microns. The light-induced carriers within a diffusion length of the collector isolation junction (also collector-P silicon substrate junction) become separated by the built-in field of the junction and thereby generate a surface photovoltage which capacitively couples up to the pick-up plate. A 3 cm diameter pick-up plate is spaced 2 mm above the wafer surface and connected to a high input impedance source follower Q1 and an associated low noise voltage amplifier. The resultant photovoltage output signal, E(v), serves as an analog of variations in collector-isolation junction voltage. The complementary photoconductivity signal is derived from the lower circuit of Fig. 1, by means of a VHF marginal oscillator, detailed elsewhere (1), whose resonant 3 cm diameter tank coil is positioned about 3 mm below the bottom surface of the wafer. The time-varying magnetic field from the coil couples into the wafer and generates electromotive forces which sst up circulating eddy currents within the wafer. The collector-isolation capacitance is a significant impedance path for these eddy currents. Therefore, surface photovoltage-induced variations in collector- isolation capacitance cause corresponding variations in eddy current losses which are reflected back to the tank coil as subsequent variations variations in oscillatory amplitude. These amplitude variations are detected and AC coupled to a low noise amplifier whose output signal, E(c), serves as an analog of surface photovoltage-induced variations in collector-isolation P-N junction capacitance.

Fig. 2 shows a blown-up version of Fig. 1, where V is the typical surface photovoltage across each junction, C is the junction capacitance, E is the oscillator field induced EMF across the subcollector, R is the effective silicon substrate resistance in the resultant eddy c...