Patent GB-1520961-A (GB)
Title: Measuring surface states in semiconductor structures
IPC: H01L21/822
Filed: 1976-03-18
Abstract: 1520961 Measuring parameters of semiconductor material INTERNATIONAL BUSINESS MACHINES CORP 18 March 1976 [28 April 1975] 10950/76 Heading G1U The surface state charge density in a metalinsulator semi-conductor structure is measured by injecting charge into the insulator from the semiconductor, measuring the change in surface potential of the semi-conductor and the net charge injected into the insulator and determining therefrom the number of surface state charges per unit energy band. The initial charge state in the insulator 10 is determined by a measure of the flat-band voltage, by means of a voltage-capacitance plot. For this, a ramp voltage source 21 is connected across the structure 10-12 via.summer 20 where a small A.C. (10 KHz) signal is superimposed. This voltage causes charge to be injected from the semiconductor 12 and this is monitored by integrator 22, 23. The capacitance is measured by a phase and frequency sensitive amplifier 27. Switch 37 is closed, thus although pulse switch 29 is also closed, no feedback via integrator 31, 34 takes place. As the ramp voltage increases, the structure capacitance changes Fig. 3 (not shown) and the voltage at which the maximum rate of change occurs, indicates the flatband voltage. Capacitor 23 is set to the value of the structure capacitance at the flat-band voltage and negative voltage pulses are applied to the structure to shift the flat-band voltage to the theoretical value (i.e. with no trapped charge) of -0À2 volts for a silicon/nitride/ oxide device. At this point, switches S2, 37 are opened and switch S set to connect the pulsed D.C. voltage source 14 to the device. At time T o , switch 29 is opened, thus retaining the feedback on line 38 at the initial flat-band voltage level. The A.C. . component detected by amplifier 27 after the voltage pulse at time T 1  now indicates a new value of flat-band voltage VFB. The amount of charge injected is monitored by integrator 22, 23 and passed via buffer 26 to summing integrator 49, 50 where leakage compensation from integrator 42, 43 is applied. At time T 3 , pulses applied at 40a and 53a to sample the injected charge and flat-band voltage. At time T4 switch 29 is closed, completing the feedback loop. This forces the voltage across the structure 10-12 to return to its initial state, at which point the new value of flat-band voltage appears on line 38. At time T 5 , pulses at 52a, 39a sample the injected charge and flat-band voltage again. The sample pairs are differenced 54, 41 to provide measures of the sum of surface state charge and charge in semi-conductor and change in flatband voltage. When these values are differenced 19a the surface potential induced by the injected charge is determined, from which by means of exponential amplifier 19b, 19c the net charge in the semi-conductor is obtained. The outputs of amplifiers 54 (Y) and 19b (Q s (# s )) may then be used to determine the surface state charge density N ss  by means of the integral:- where A is the area of the metal contact 11, C o  is the geometric capacitance, of the structure and q is the electron charge. The integral value is plotted for values of # s  Fig. 6 (not shown) and the slope indicates the actual charge density.