Highlights of progress in the various technical task areas of the program are listed in this section. Unless otherwise identified the work was performed at the National Bureau of Standards.

Particularly significant accomplishments during this reporting period included (1) completion and analysis of an interlaboratory evaluation of standard reference wafers for resistivity measurement by the four-probe method, (2) analysis of the effect of finite contact size on sheet resistance as measured with a van der Pauw structure, (3) calculation of errors introduced in measuring line width with typical microscope systems, (4) development of procedures for predicting the magnitude of the electron beam induced current in silicon device structures, (5) application of the optical flying-spot scanner to observation of hot spots and nonlinearities in UHF power transistors and of logic patterns in an MOS shift register, and (6) determination of a more accurate electrical method, based on peak junction temperature measurement, to establish safe operating area curves for medium power transistors.

A study of the effect of surface preparation and other measurement parameters on the resistivity of silicon wafers as determined by the spreading resistance technique is nearing completion. Theoretical and experimental development of a simplified algorithm for analysis of spreading resistance measurements on graded structures was begun at Solecon Laboratories. The algorithm is based on a relationship similar to that employed in analyzing differential sheet resistance measurements. Preliminary measurements at RCA Laboratories suggest the feasibility of a high-speed spreading resistance probe which operates without lifting the probe tips from the wafer surface while they are moved across the surface. At present, readings may be taken 4.5 to 7 times faster than with conventional equipment.

As one phase of the work to reevaluate the relationship between resistivity and impurity

density in n- and p-type silicon, several models were used to analyze the ionization state of dopant impurities as a function of dopant density. None of the models studied gave satisfactory results in the intermediate density range around 1018 cm-3.

Surface Analysis Methods - Analysis of Rutherford backscattering measurements on a heavily zinc implanted silicon specimen, made as part of a comparative study of electron, ion, and photon beam surface analysis measurement technologies, illustrated some of the advantages and limitations of this technique for profiling impurity densities. Additional study of the angular dependence of x-ray photoelectron spectra from silicon revealed crystal orientation effects and suggest that the bulk plasmon peak results from an extrinsic process which occurs during the path of the electron out of the solid.

Work was undertaken at Texas Instruments to develop methods for preparation of standard specimens for empirically calibrating an ion microprobe mass analyzer so that quantitative measurements can be made of selected impurities in silicon and silicon dioxide regions of semiconductor devices. A mask was designed and fabricated for use in defining a pattern of small areas in which the impurity of interest can be ion implanted.

A rapid nondestructive infrared reflectance technique to determine the surface quality of sapphire substrates is being investigated at RCA Laboratories. Correlation has been observed between the results of reflectance measurements and other analytical methods such as x-ray diffractometry, x-ray surface and transmission topography, and chemical etch tests.

A study of techniques for determining and controlling the quality of furnace environments used in growing and subsequent annealing of gate oxides for MOS devices was undertaken. Attempts to measure trace amounts of sodium in the furnace gas by means of a flame emission spectroscopy system attached to the exit port of the furnace tube were unsuccessful. Preliminary measurements suggest that resonance fluorescence spectroscopy can be used to detect extremely low levels of free sodium in the furnace tube, and further investigation of this technique is in progress.

Surface analysis for silicon devices was the topic of the fourth ARPA/NBS Workshop

held at NBS Gaithersburg in April. The workshop was attended by 146 representatives from industry, government, and universities. Highlighting the workshop were three approaches to overcoming the problem of sodium migration during distribution profiling measurements and debates about the existence and extent of the non-stoichiometric transition region at the silicon dioxide-silicon interface. Another area of active discussion by speakers and participants involved consideration of the detection limits and capability for making quantitative determinations by beam probe techniques. It was pointed out that no single beam spectroscopy can provide all the answers and that before beam techniques can be used to provide reliable and quantitative determinations, both standard reference materials and standard measurement procedures must be developed.

for application to measurements on metalinsulator-semiconductor structures with thick insulator layers.

Materials and Procedures for Wafer Processing As part of a study being conducted at Hughes Research Laboratories on critical parameters associated with ion implantation, preliminary range and range straggle measurements of boron and phosphorus implants into opposite-type silicon wafers were made by means of an automatic capacitance-voltage method.

In work undertaken at RCA Laboratories to develop techniques for evaluating the integrity of passivation overcoats on metallized integrated circuits, three different techniques are being investigated and compared. Photolithography A theoretical investigation was begun to determine quantitatively the effects of coherence of the illumination on the image of two parallel opaque lines on a transparent background and on the image of two parallel transparent lines on an opaque background. The results of these calculations yield a quantitative measure of the image quality based on root-mean-square differences between the spectra of the image intensity profile and the ideal intensity profile.

Calculations were also made of the effect of the degree of coherence of the illurination at the object plane of a microscope with diffraction limited lenses and its effect on the intensity profile of a single edge. Effects of spherical aberration and defocussing were considered, and the measurement errors which might be encountered in using filar and image shearing eyepieces were estimated.

Because of difficulties in fabricating the originally designed measurement artifact intended for use in establishing standards for line-width measurements in the 1 to 10 um range, a substitute interim artifact was obtained. Comparative measurements were rade on this artifact with filar and image shearing eyepieces. The results of these measurements agreed well with the predictions of the above theoretical calculations.

Test Patterns Finite difference methods were used to solve Laplace's equation in two dimensions in order to calculate correction factors to account for the finite length and width of contact arms on sheet

An interlaboratory evaluation of the destructive, double-bond pull test is being conducted in cooperation with ASTM Committee F-1 on Electronics. In addition, a twolaboratory comparison of the bond pull test with the bond shear test was conducted to determine their significance for evaluating aluminum-aluminum ultrasonic bonds. The shear test was found not to be sensitive to the weakest part of the connection, the heel of the first bond. The pull test, on the other hand, is particularly sensitive to the weakening of the bond heel.

Hermeticity Several schemes are being investigated for quantitative measurement of gross leaks in hermetic packages. The rapid cycle dry gas gross leak test was analyzed further to include the case of repeated testing without repressurization in helium in order to pick out quickly leakers from a batch of parts.

The effect of helium absorption on the test specimens being used in an interlaboratory evaluation of the back-pressurization technique for leak detection with a helium mass

spectrometer, being conducted in cooperation with ASTM Committee F-1 on Electronics, was further studied. It was decided that the interlaboratory test should be deferred until test specimens which are less susceptible to helium absorption can be obtained.

A study has been initiated at MartinMarietta Aerospace to derive a quantitative relationship between leak size in hermetic packages and moisture infusion. The initial phase of the work involved the calibration of an in-situ moisture sensor, development of microventing procedures, and selection of a moisture sensitive integrated circuit.

Device Inspection and Test Work on the automated scanning low energy electron probe at the Naval Research Laboratory continued with assembly of the system and demonstration of its ability to observe variations in dielectric uniformity on an oxidized silicon wafer.

To facilitate application of the scanning electron microscope operating in the electron beam induced current (EBIC) mode for the diagnostic examination of silicon devices, a procedure was developed for predicting the magnitude of the EBIC signal. The procedure is based on published depth-dose curves and an analysis of the value for the fraction of energy backscattered which was shown to be essentially constant over the energy range of interest. Despite the fact that several simplifying assumptions were made, reasonable agreement was achieved with an experimental result.

3.1. Standard Reference Materials

The multipass interlaboratory test (NBS Spec. Publ. 400-17, pp. 8-9) of SRM 1520 [1] was completed. The purpose of the test was to establish both the stability of this SRM over a reasonable period of time and the level of agreement to be expected by any two laboratories using the SRM for referee purposes.

Each of the six participating laboratories, including NBS, was required to measure the room temperature resistivity of both wafers of its resident SRM set once a month for a twelve-month period. The measurements were made according to the standard four-probe method [2], except that only two repetitive measurements on each wafer were required inIstead of the normal ten. Raw data were submitted directly to NBS for calculation and reduction. This part of the experiment served both as a data base from which to estimate the time stability of the SRM sets, and also as a screen against sudden changes in any laboratory's measurement response which would influence the second part of the study.

In the second part of the test, two complete SRM sets, two silicon wafers at each of the two resistivity levels, were circulated to each of the participating laboratories with a frequency of one laboratory per month. Two complete cycles of the laboratories were made during the study. The estimate of precision to be expected in referee use of the SRM's was generated from these data. The resistivity was again measured according to the standard four-probe method [2] but only five repetitions were required per wafer. The use of two complete sets in this part served both to enlarge the data base and to provide allowance for possible specimen breakage during the study.

The data reported by one participant showed a statistically significant high-side bias for all measurements. Since it was determined that this participant failed to follow the measurement procedure with respect to probe specifications and correction for temperature, these data were excluded from the final analysis of the results of the experi

ment.

The stability was evaluated by analyzing the drift of the resistivity values obtained on the ten resident specimens during the course

of the experiment. The results are summarized in table 1. The data from each resident specimen were fitted to individual linear regressions [3] as a function of month number. The slopes of the individual regressions, if statistically significant above measurement scatter, indicate the extent of drift for that specimen-laboratory combination over the course of twelve months.

Individual estimates of slope and intercept with their estimated standard deviation are shown in the table. While seven of the ten estimated slopes were negative, indicating slight decrease of resistivity with time on balance, only one of the calculated slopes was larger than its estimated standard error. In addition, the small values of correlation coefficient suggest that the resistivity changes are not linear with time.

With separate analyses for high and low resistivity levels, pooled estimates indicate that only slopes greater than 0.018 and 0.00018 ohm centimeters per month, respectively, would have been statistically significant at the 95 percent confidence level. The point of interest is the typical drift, if any, of all the specimens at either the low resistivity or the high resistivity level, taken as a whole for each level. Since there was only one slope in either category which exceeded its respective significance limit, this is well short of the 95 percent yield expected from the analyses performed if the hypothesis of detectable drift for material of either resistivity level were true. Hence, as a whole, this hypothesis must be rejected.

A statement of the precision to be expected in various referee measurement uses was derived from analysis of the measurements on the circulating sets of wafers using the calculated values of within laboratory and between laboratory variation for each specimen used [4]. Table 2 summarizes the results of the measurements and gives estimates from these results of the repeatability for replicate measurements within a lab and the reproducibility of measurements between two labs which could be expected with 95 percent confidence if the same specimens were used for referee measurement. The estimates of repeatability and reproducibility [4] are broken into three situations, namely those where the two laboratories involved would measure the specimen one, five, or ten