Technique for Trim Etching Diffused Resistors in SiliconEdgar J. Evans Citation: Review of Scientific Instruments 36, 1248 (1965); doi: 10.1063/1.1719854 View online: http://dx.doi.org/10.1063/1.1719854 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/36/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Application of ion beam etching technique to the direct fabrication of silicon microtip arrays J. Vac. Sci. Technol. B 22, 2853 (2004); 10.1116/1.1826061 Modeling the impact of photoresist trim etch process on photoresist surface roughness J. Vac. Sci. Technol. B 21, 655 (2003); 10.1116/1.1545735 Importance of fluorine surface diffusion for plasma etching of silicon J. Vac. Sci. Technol. B 20, 791 (2002); 10.1116/1.1469015 Automatic trimming technique for multipolar magnets J. Appl. Phys. 71, 3053 (1992); 10.1063/1.350994 A new trench fabrication technique for silicon substrate utilizing undercutting and selective etching J. Vac. Sci. Technol. B 3, 905 (1985); 10.1116/1.583079

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1248 NOTES

Technique for Trim Etching Diffused Resistors in Silicon

EDGAR J. EVANS

Engineering Sciences Laboratory, Feltman Research Laboratories, Picatinny Arsenal, DO'IJer, New Jersey

(Received 21 December 1964; and in final form, 15 January 1965)

WE have recently been faced with the problem of pre-paring accurate resistance ratios on simple inte­

grated circuits. Four resistors had to be balanced to better than 0.3% and track within 0.5% over a temperature range of -54 to +74;°C. There was no requirement for passification, however techniques were to be considered which might later be applied.

The resistors were p-type diffused strips on 10 Q-cm n-type silicon wafers. The diffusion produced sheet resis­tivities of about 40 Q/square ±20% and the sensor geom­etry was controlled by photoresist oxide masking to about 1 %. The junction depth was approximately 3 Jl..

After diffusion and contacting, differences in resistance were generally less than 10%. Since the diffusion time and temperature were the same for each diffused strip on a given wafer, the variations in resistance were due to variations in the surface concentration. It was felt that the surface concentration was essentially uniform over each resistor but that it varied across the wafer, since the length of the resistors was small compared to the distance between them. (Figure 1 is not to scale.) A uniform etching of the unbal­anced resistors decreased their surface concentrations by removing material, until at balance, all surface concentra­tions were equal. Since the diffusion profiles were then nearly the same each one could be considered to be the same material. This produced the desired temperature tracking.

Since p-n junctions isolate the various resistors, selective electrolytic etching can be used to trim the resistors as long as the voltage used is below the junction breakdown voltage. Subsequent passivation can be performed by anodic oxidation.!

TEFLON

/ L -SILICON WAFER

DIFFUSED RESISTORS

':""'GOLD WIRES

FIG. 1. Sample holder -the electrolyte flows from the nozzle across the sample and cathode during etching.

L----O...--- CATHODE

FIG. 2. The circuit used for trim etching. During etching, elec­trolyte flows across the sample and cathode. At all other times distilled water washes the sample and permits the resist­ance to be measured.

The diffused wafers were photoresist etched to remove the silicon dioxide over the unbalanced resistors. Small aluminum pads were evaporated and alloyed at the ends of each resistor. Gold wires (0.05 mm) were thermocom­pression bonded to the aluminum pads. The wafer was then mounted on a Teflon sample holder with Apiezon wax and the gold wires attached to external leads as indicated in Fig. 1. The electrolyte was allowed to flow down the wafer and connect to the gold plated cathode during etching. Immediately thereafter, distilled water washed away the electrolyte and permitted the resistance to be measured. The etching electrolyte is 0.5 wt% HF in distilled water. The process, described by Turner,2 is allowed to proceed at constant current density of 10-2 A/cm2• It is desirable to have the etching voltage between 5 and 30 V since this is the electropolishing region. (The electrolyte is adjusted to meet this condition.) The resistance change under these conditions is in the order of 10 Q/min. A diagram of the etching apparatus is shown in Fig. 2. When the mode switch connects the sample to the etching supply, elec­trolyte flows across the sample. After a brief etching period the switch is reversed connecting the sample to the resistance meter and washing it with distilled water. After trim etching, the wafers are washed with water and dried with dry nitrogen.

Due to small leakage currents through the junction there can be electrolytic etching of the other resistors. (During this process the junction is exposed to the elec­trolyte.) In practice the resistors were about 500 1J and trim etching the anode resistor 10 Q resulted in changes in the other resistors of less than 1 Q.

The electrolyte of aqueous HF slowly dissolves the aluminum bonding pads. However, the thermocompression bond of silicon-aluminum-gold is not affected by the acid. There is an initial change in the strip resistance from this process due to changing the strip length. Prior to electro­lytically trimming the resistors the wafers were flooded with 5% HF for 2 min. This dissolved the aluminum pads

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NOTES 1249

and prevented any further resistance change except that from electrolytic etching.

The resistors were trim etched easily to 1% balance and with additional effort 0.2% was considered practical. Electrical evaluation consisted of measuring the value of each resistor as a function of temperature and computing the maximum percent deviations at each temperature. Resistances were read to four significant figures and the temperature was controlled to ±0.05°F during each meas­urement. Approximately 10% of the completed wafers were within tracking specifications (i.e., 0.5%). Almost all were within 1.0% over the complete temperature range.

1 P. F. Schmidt and W. Michel, J. Electrochem. Soc. 104, 230 (1957).

2 D. R. Turner, J. Electrochem. Soc. 105, 404 (1958).

Spectrophotometer Signal Enhancement by Digital Computer

R. E. CUTHRELL AND C. F. SCHROEDER

Sandia Corporation, Albuquerque, New Mexico

(Received 9 April 1965 ; and in final form, 6 May 1965)

AN Enhancetron ND-800 digital computer1 has been coupled to a Perkin-Elmer model 13 spectrophotom­

eter by an electromechanical2 linkage (Fig. 1) for purposes of signal-to-noise ratio and signal-to-background enhance­ment in the ultraviolet through infrared spectral ranges. This type of linkage has the advantage of wide applica­bility to any signal recording system through complete electrical isolation of the computer from the recorder and the disadvantage that recorder servo-system noise is am­plified along with the signal of interest (usually an insigni­ficant source of error).

Figure 2 shows the computer signal summation capa­bility for a noise obscured absorbance peak due to methyl

Pulse Generator

lr"'-ijr=r:;:::;J;== To ND-SOO Trigger

9Vt...,....WO.1fLF

1 M.I 10K

Ramp [: Generator .L,! -.1.l:~---..-..

'l,

c o

2.6 V

.--~...-,,,,,,,-.+ 225K

Adder/ Amplifier

J. ·e. To ND-800

FIG. 1. Spectrophotometer-computer link. Capacitor A is varied to give a ramp signal (polarity switch at P) for flattening single beam background; B is asia ve potentiometer connected to the recorder pen drive shaft for signal input; C and D provide zero and 100% adjust­ments, respectively.

FIG. 2. Absorption spectrum of methyl or­ange indicator in water after one, two, four, and six scans with the ND-800 computer.

400 450 500 550

Wavelength (mu)

orange indicator in aqueous solution. The signal-to-noise ratio and the signal-to-background level were increased by factors of about 600 and 6, respectively. This was accom­plished in six 2 min scans through the visible range at an amplifier gain setting of about 2. On successive summations the random positive and negative noise components tend to cancel while the positive signal of interest steadily in­creases. When the computer zero voltage level can be set near a very flat and stable background signal as shown in curve A of Fig. 3 (the low amplification visible spectrum

FIG. 3. Mercury arc emission spectrum input (A) to the computer, and the result (B) of a single scan.

?: 'Vi c: 2 c: H

A

-------~------

350 400 450 500 550

Wavelength (mu)

from a mercury arc), then the operational amplifiers may be used at relatively high gain (10) and a large signal-to­background enhancement may be obtained with the ND-800 in a single scan (curve B of Fig. 3). Curve A represents a simulation of the signal level expected from multireflec­tance infrared spectra of organic compounds at the mono­layer level (0.002 absorbance units per monolayer for four reflections of polarized light through the film).3,4

1 Purchased from Nuclear Data Inc., Palatine, Illinois. 2 The spectrophotometer-computer link is electromechanical in

that the wiper of potentiometer B (Fig. 1) is mechanically slaved to the recorder pen drive shaft. An electrical signal proportional to recorder pen position can then be generated and fed to the computer as indicated in Fig. 1.

3 S. A. Francis and A. H. Ellison, J. Opt. Soc. Am. 49, 131 (1959). 4 R. W. Hannah, Appl. Spectr. 17, 23 (1963).

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