10.

10.1. Helium Mass Spectrometer Method

Anomalously high values of indicated leak rate were obtained from the second stage of the interlaboratory evaluation of the helium mass spectrometer leak detector method [60] for testing fine capillary leaks in large volume (~1.5 to 2 cm3) containers which is being conducted in cooperation with ASTM Committee F-1 on Electronics (NBS Spec. Publ. 400-4, p. 67). Experimental data obtained with sealed-off capsules of approximately the same dimensions as the test leak specimens indicated that the high, uniform values of measured leak rate were due to initial permeation of helium into the borosilicate glass matrix followed by diffusive outgassing after removal of pressurization (NBS Spec. Publ. 400-12, pp. 33-34). A fixture was devised to allow exposure only of the tip area of the leak to the leak detector which reduced background outgassing by about a factor of ten; however, such manipulation increased the possibility of damaging the fine capillary tip.

At the January meeting of the Hermeticity Section of Committee F-1, it was decided to defer continuation of the interlaboratory test until it could be determined whether a modified pressurization schedule could be used to attenuate outgassing without causing severe degradation of leak testing sensitivity. The possibility of conducting the pressurization step at reduced temperature, to decrease diffusion of helium into the glass, was also discussed but rejected as being too awkward to be practical.

To evaluate the effects of modification of the pressurization schedule, the indicated leak rates were determined for the sealed-off capsules with an exposed surface area of about 16 cm2 following pressurization in helium at 5 atm (absolute) (5 × 105 Pa) for 0.2, 0.5, 1, and 20 h. Equivalent leak, or outgassing, rate, R, was measured as a function of time from a few minutes after pressurization until the rate fell off to about 10-8 atm cm3/s (10-9 Pa⚫m3/s) or less. The results are shown in figure 37. As expected, initial outgassing rates were found to be essentially independent of pressurization time, being a function of helium concentration in the glass surface.

The minimum detectable leak rate is limited by the outgassing rate. The dwell time necessary for the outgassing rate to fall to a

max

d tp P

max

If the leak detector sensitivity, min' is taken as R the smallest and largest detectable leak sizes, L and L for given min values of and t can be determined according to the procedures of ASTM Method F134 [60]. Examples are listed in table 10. It should be noted that about one fifth of the test leaks have leak sizes smaller than 8 × 10-8 atm.cm3/s (8 × 10-9 Pa⚫m3/s) and about two fifths have leak sizes smaller than 2 × 10-7 atm cm3/s (2 x 10-8 Pa⚫m3/s). Only one of the 50 test leaks has a leak size larger than 3 × 10-5 atm cm3/s (3 × 10-6 Pa⚫m3/s).

From these results it was concluded that the interlaboratory evaluation should be deferred until additional test specimens, less susceptible to helium absorption, can be obtained. (S. Ruthberg)

10.2. Correlation of Moisture Infusion, Leak Size, and Device Reliability

A study has been initiated to derive a quantitative relationship between leak size in hermetic packages and moisture infusion along with the consequent effect on device reliability. The approach is to incorporate a dew point moisture sensor and a moisturesusceptible integrated circuit into packages, each with a prefabricated microchannel leak of known size, which would then undergo leak testing and accelerated life test.

The first phase of this effort was undertaken to evaluate the accuracy of the dew point measurement procedure, fabrication procedures for and stability of the microvents, and the moisture sensitivity of a suitable integrated

circuit.

Two types of moisture sensor are being utilized. One is an interdigitated set of thick film electrodes on an alumina substrate. The second is an electrode system formed between the die attach area and the leads of a dualin-line package. In use, the leakage current between the electrodes is measured while the temperature of the package is reduced, by means of a thermoprobe, until the dew point is reached as indicated by an abrupt current increase [61]. A set of 25 specimens was fabricated; each specimen is an open microcircuit package with the two sensors attached. To reduce electrical leakage each specimen was coated with epoxy on all surfaces but the sensor and the area which would be contained within the internal volume of the package if it were sealed. To test the sensors, each package was mounted on a temperature controlled probe and was placed in an environmental chamber which was first stabilized in a dry mode and then set to give the desired relative humidity in the range between 5 and 50 percent. The thermoprobe was stabilized at an initial reference temperature and then brought down in small temperature increments while leakage current was monitored. The relative humidity appropriate to the measured dew points was compared with the relative humidity of the chamber. Initial data indicate close agreement between the two sensors and the psychrometric parameters.

The microvents are produced by laser drilling the case and back plating until a suitable leak size has been obtained. A matrix experiment has been performed to determine

The 741 operational amplifier was selected for trial as a suitable integrated circuit test vehicle. This is a monolithic bipolar device with an MOS capacitor. Two groups of 25 devices each were obtained. One group was unpassivated; the second was passivated with silane glass. All devices were pretested thermally and electrically, mounted for direct exposure, and then submitted to a humidity step-stress test under dc bias at 85°C. The devices were operated for 168-h incremental periods at 20, 50, 70, 85, and 95 percent relative humidity until failure, which was defined as drift of one or more electrical parameters outside specified limits. Two of the unpassivated devices failed in the first step, 15 more in the second step, 7 in the third step, and the last in the fourth step. Of the glassivated devices, one failed in the second step, one in the third step, one in the fourth step, and 4 in the fifth step. Each of the failed circuits was examined microscopically to establish the cause of failure. All failures were determined to be moisture related as evidenced by the presence of corrosion on the aluminum metallization runs. Based on these results the unpassivated 741 operational amplifier was selected for use in the moisture sensitivity life test.

(S. Zatz* and S Ruthberg)

1.1 Automated Scanning Low-Energy Electron Probe

The initial electron gun structure (NBS Spec. Publ. 400-12, p. 28) for the ASLEEP system was completed and mounted in the vacuum system for testing and alignment. The gun was placed in the circuit shown in figure 39 for bean current tests. The target was an aluminum grid comprising 60 um wide lines on 600 centers over silicon dioxide (NBS Spec. Publ. 400-17, p. 46) mounted on the specimen manipulator. The 6-V battery was inserted into the circuit during initial testing to overcome patch effects at the target. The focus coil was adjusted to a magnetic induction of 70 gauss (7 mT) and the target current maximized by adjusting the various potentials. The maximum current collected by the target was 1.7 nA, obtained with the beater current at 9.3 A, grid 1 at 30 V and grid 3 at 500 V.

The first electron gun developed a short beween the high voltage and the cathode necessitating a modification of the design. The lass insulators used to shield the high voltage leads were fractured where they crossed the low voltage elements of the gun. Notchng the grid and cathode holders to allow Clearance for stronger tubular ceramic insuLators corrected this problem. The original en used three sapphire rods to hold the gun elements in place. The rods were notched and metal tabs were inserted in the notches and spotwelded to the gun elements. This scheme sade alteration and repair of the gun diffitult. The elements of the gun are now held their proper relationship by hollow ceric spacers through which a bolt can be run to hold the assembly under compression. The modified gun has been tried and found to be tore successful.

The computer control system rasters the electron beam over the test grid. If the electrometer in the beam current test circuit fig. 39) is replaced by a low noise amplifer, an image may be formed on a cathode ray tube (CRT) by using the amplified target rrent signal to control the CRT beam intensity. Figure 40 is a picture of one such Sage taken of the aluminum grid during preinary system checkout. The narrow line at about one half the grid spacing near the edge of the picture is a scratch in the silicon dioxide surface. Figure 41 is an image of an array of MOS capacitors made of 30-mil 0.75-mm) diameter dots of aluminum on sili

con dioxide 100 nm thick. The capacitors were scanned with a beam of fixed energy (approximately 3.5 eV). The four white capacitor dots in the center of the picture are shorted to the substrate due to defects in the oxide. Similarly, two rows of shorted capacitors are located where the wafer was scribed and broken.

Additional software has been generated for presenting surface potential mapping information. The electron beam is scanned in the x-y plane, the retarding voltage is ramped to a preselected threshold current level, the surface potential is scanned over any selected area of the target, and the results are presented in a three dimensional display all under computer program control. In the presentation, the y-axis is somewhat elongated and the z-axis deflection of the scan line is proportional to the relative surface potential. Figure 42 shows a surface potential scan of an array of capacitors on silicon dioxide. Variations in height of the profiles are a direct measure of the dielectric uniformity assuming the metal work function to be constant. A high dot indicates a high capacitance. This result qualitatively demonstrates the ASLEEP as a non-destructive computerized surface diagnostic technique.

(W. C. Jenkins* and G. P. Nelson*)