Planar millimeter-wave epitaxial silicon Schottky-barrier converter diodes

Solid-State

Electronics

Pergamon

PLANAR

Press

1968.

Vol.

11, pp. 517-52.5.

MILLIMETER-WAVE

SCHOTTKY-BARRIER W. Department

of Electrical

C. A. Bell

Telephone

Laboratories,

Crawford

Hill

in Great

Britain

EPITAXIAL

CONVERTER

V. T.

Engineering,

Printed

SILICON

DIODES

RUSCH* University

of Southern

California,

U.S.A.

BURRUS Laboratory,

Holmdel,

New Jersey,

07733,

U.S.A.

(Received 1 November 1967; in revised form 5 January 1968)

Abstract-Silicon Schottky-barrier junctions useful as down-converter diodes in the 5-6 mm-wavelength region have been fabricated by planar techniques. The rectifying junctions were formed by the interface between a low-temperature, solid-solid, palladiumsilicon reaction product and a very thin (m 0.5 pm) low-resistivity epitaxial silicon layer. The junctions were 4 pm in dia. and were about 0.25 pm below the surface of the epitaxial layer. Zero-bias cutoff frequencies were in the range of 500-800 GHz. When mounted in suitable millimeter-waveguide down-converter circuits, the minimum measured conversion loss (51.7-1.3 GHz) was about 6.5 dB, and the minimum value of the product (conversion loss) times (noise temperature ratio) was about 7.5 dB. These values are at least 1 dB worse than values of the same quantities obtained by the use of millimeter-wave planar gallium arsenide Schottky-barrier diodes in the same circuits. RBspmB-Des jonctions barriere-Schottky en silicium miles comme diodes de conversion dans la region de longueur d’ondes de 5-6 m ont ete fabriquees par des techniques planes. Les jonctions redresseuses ont ete formees par l’interface entre un produit de reaction a basse temperature solide-solide en palladium-silicium et une couche epitaxiee en silicium t&s fine (-0,s pm) a basse resistivite. Les jonctions avaient un diamttre de 4 pm et se trouvaient a environ 0,25 pm au-dessous de la surface de la couche CpitaxiCe. Des frequences de coupure sans polarisation se trouvaient dans la gamme 500-800 GHz. Quand elles etaient montees dam des circuits convertisseurs a guide d’ondes millimetriques approprie, la perte minimum de conversion mesuree (51,7-1,3 GHz) etait de 6,s dB et la valeur minimum du produit (perte de conversion) multipliee par la rapport bruit temperature Ctait de 7,s dB. Ces valcurs sont au moins 1 dB plus mauvaises que les valeurs des mEmes quantites obtenues par l’emploi des diodes planes Schottky-barri&re a ondes millimetriques en arseniure de gallium dans les m&mes circuits. Zusammenfassung-Silizium-Schottky-Dioden als Mischdioden fur den Wellenlangenbereich van 5 bis 6 Millimeter wurden in Planartechnik hergestellt. Die gleichrichtenden Ubergange bestanden aus der Grenzflache zwischen einer bei tiefer Temperatur hergestellten PalladiumSilber-Verbindung und einer .sehr diinnen ( - 0,s pm) niederohmigen epitaxialen Siliziumschicht. Die Ubergange hatten 4 pm Durchmesser und lagen etwa 0,25 pm unter der Oberflache der Epitaxieschicht. Die Grenzfrequenzen ohne Vorspannung lagen zwischen 500 und 800 GHz. In geeigneten Mischkonverterkreisen mit Millimeterwellenleitern betrug der kleinste Konversionsverlust (van 51,7 auf 1,3 GHz) etwa 6,s db und der kleinste Wert des Produkts aus konversionsverlust und Rauschtemperaturverkaltnis war ungefPhr 7,5 dB. Diese Werte sind wenigstens urn 1 dB schlechter als jene, welche man mit planaren Schottkydioden aus Galliumarsenid in der gleichen Anordnung erhalt. * This Holmdel,

work was done while the author was on leave at Bell New Jersey, September 1966-September 1967. 517

Telephone

Laboratories,

Crawford

Hill

Laboratory,

518

\V. V. T.

RUSCH

1. I~ODUCTION

FOR MANY years diodes with sufficiently small resistance-capacitance products for effective use at millimeter wavelengths could be made only by placing a pointed metal wire in contact with a semiconductor surface. The most successful example of such millimeter-wave ‘point-contact’ diodes was made by contacting the polished surface of p-type silicon with a pointed tungsten wire.(l) Additionally, if an impurity suitable for the formation of a p--tr junction could be incorporated from the metal point into a tiny area on the semiconductor surface by ‘forming’ or welding the point contact, useful millimeter-wave diodes having the properties of alloyed junctions could be made, such as, for example, millimeter-wave varactors and tunnel diodes.(2) However, the fabrication of all such diodes has remained essentially a ‘one-at-a-time’ operation with obvious resulting difficulties in manufacture and quality control. The photolithographic techniques widely used for the fabrication of many semiconductor devices gradually have been refined to the point that extremely small areas can be defined. Thus it is now possible, using these techniques, to make semiconductor junctions which have areas and capacitances comparable to those of millimeterwave point-contact diodes. Specifically, planar diodes developed for use at the lower microwave frequencies now can be adapted to the millimeterwave region of the spectrum while retaining many of the manufacturing and reproducibility advantages inherent in the process. Such millimeter-wave adaptations of planar gallium arsenide Schottkybarrier diodesc3) and planar gallium arsenide diffused-junction varactors(4) have been reported. The Schottky-barrier diode is of particular interest because of its useful properties when employed as a down-converter in a superheterodyne receiver. Ideal diodes of this type have somewhat more abrupt nonlinearities than do conventional point-contact diodes orp-n junctions, and the junction noise is very low. The practical result (even with imperfect junctions) is that very simple low-loss, low-noise receiver front-ends can be constructed from diodes of this type in combination with low-noise transistor amplifiers. Because of their simplicity, such receivers can compete with parametric amplifiers in many applications even

and C. A. BURRUS

though the parametric devices can have lower noise figures. It is the purpose of this paper to describe an adaptation of a particular planar silicon Schottkybarrier diode, first reported by KAIING and LEPSELTER in 1965,c5) to millimeter-wave use, and to compare the performance of this diode as a millimeter-wave down converter with that of planar millimeter-wave gallium arsenide Schottkybarrier diodes. 2. DIODE

FABRICATION

hIany Schottky-barrier diodes utilize a simple metal contact on the semiconductor surface to form the rectifying junction. However, in the silicon diodes described by Kahng and Lepselter the metal side of the barrier was replaced by a lowtemperature solid-solid reaction product produced by heating the metal-semiconductor interface. Temperatures involved were below that normally considered to be the temperature at which the particular metal-silicon eutectic alloy would form, and the heating time was of the order of 1 hr. Although the exact chemistry of the reaction product is not well understood, it was labeled; for convenience, a metal ‘silicide’, and was described as a metal in which an appreciable percentage of silicon was dissolved. Thus the interface between the ‘silicide’ and the silicon formed the junction and this junction lay a small distance below the surface on the silicon side of the original metalsilicon interface. This process seemed to offer two primary advantages over the simple metal-semiconductor silicon Schottky barrier:@) (a) most of the difficulties due to oxide formation between the junction-forming metal and the silicon surfaces were circumvented, and (b) the junction edges were effectively passivated by a dense metal overlay covering the ‘silicide’ and the immediately surrounding oxide. It was not anticipated that this second advantage could be incorporated into the millimeter-wave version of the diode at this time. However, the first advantage seemed to overcome a basic problem associated with metal-silicon diodes and this, alone, appeared to be sufficient reason to try it for the millimeter-wave application. Consequently, this process was chosen for USC‘ in the fabrication of the silicon millimeter-wave diodes.

PLANAR

MILLIhIETER-WAVE

EPIT.AXIAL

(a) Semiconductor junctions Diodes were fabricated on n-type epitaxial silicon layers which had been grown on the (1, 1, 1) face of a more heavily doped n-type silicon substrate. The surface of the epitaxial layer was oxidized in moist air to provide a 5000-A silicon dioxide laver into which were etched the windows for defining the junction areas. These windows were produced by standard photolithographic techniques using Kodak thin-iilm photoresist and an etch of buffered hydrotluoric acid (HF). The commercial photographic masks used in the photoresist-exposure process yielded arrays of windows which were two to eight micrometers in diameter and were separated by distances three-ten times their diameter. after the windows had been etched in the oxide layer, the exposed silicon surfaces were cleaned, metal was applied and heated to produce the rectifying ‘silicide’-silicon Schottky-barrier junction, ohmic contacts were applied, and the resulting diodes were suitably mounted for evaluation of their electrical characteristics. (i) Semiconductor preparation Preliminary csperimcnts indicated that useful down-converter diodes were obtained only on silicon of relatively low resistivities, and that the thickness of available cpitaxial layers limited the performance by adding unnecessarily to the diode resistance. Thus it was necessary to use lowrcsistivity epitaxial layers, and to reduce the thickness below that of the grown layer. This was accomplished bv alternately growing an oxide layer on the eiitaxial silicon surface and then removing it with an HF etch.(7) Diodes with minimum conversion loss were made from material with an original epitaxial-layer thickness of about 1.5 pm (as determined by angle-lapping) and an original surface resistivity of about 0.08 Q-cm. This thickness was decreased by successive oxidation-etching cycles in the following way: A part of the slice was oxidized in moist air at 1050°C for 30 min, and the resulting oxide layer thickness was determined interferometrically to be about 5000 A. After removal of this oxide in HF, a rectifying junction was formed by contacting the exposed silicon surface with a sharpened tungsten wire. The I-V curve of this junction was observed, and the reverse voltage at a reverse current of 0.5 mA was determined and plotted on

SILICON

SCHOTTKY-BARRIER

DIODES

519

a curve similar to that of Fig. 1. This procedure was repeated until an I-V curve characteristic of the substrate was obtained (6 cycles for the example of Fig. 1). With the assumption that

FIG. 1. Control plot used in the thinning of silicon epitaxial layers by alternate growth and removal of SiOa.

approximately 50 percent of the oxide layer thickness represented silicon removal from the surface, the original layer thickness for the control piecesof Fig. 1 was deduced to be about 1.5 pm, in good agreement with the value of 1.5 pm determined by angle lapping. In the actual fabrication of diodes the number of such 30-min cycles was adjusted to leave about 0.5 pm of the epitaxial layer under the final oxide needed for masking. (ii) Junction formation After etching an array of windows in the final oxide layer of a thinned piece of epitaxial silicon (Fig. 2), the silicon surfaces exposed in the windows were cleaned by thermal growth of a few hundred Angstroms of silicon dioxide in dry air,

FIG. 2. Artist’s illustration of the near-planar diode structure. A spring-loaded mire point in one of the holes in the oxide provided a connection between a randomlychosen junction and the circuit.

520

W. V. T. RUSCH

followed by removal in dilute HF. The wafer then was rinsed immediately in an iodine-methanol solution to retard oxide growth on the exposed silicon(*) and transferred to a vacuum evaporator. After a suitable vacuum had been achieved, the iodine was sublimed by heating and a layer of palladium several thousand _&ngstoms thick was evaporated onto the surface. The palladium-coated wafer was then placed in an evacuated quartz ampule and Faked for 30 min at selected temperatures between 400 and 450 C to heat the palladium-silicon interface in the windows. The resulting reaction product, fog simplicity called ‘palladium silicide’,‘“’ was insoluble in aqua regia; thus the evaporated film 01’ palladium on the oxide sllrface and tllr csccss palladium in the windows could he removed 1,~. dissolution in aqua rcgia without advcrscly atfccting the junction. Angle lapping of control picccs indicated that the ‘silicide’ penctratcd about 0.25 pm into the epitasial layer for the processing conditions indicated here. The Schottky-barrier rectifying junction was formed by the interface between this ‘palladium silicide’ and the silicon. The cffectivc thickness of the epitaxial layer remaining under the ‘silicidc’ between the junction and the substrate was about 0.25 pm. Thus the junction was formed in a regiorl of steep impurity gradient since, during the deposition of cpitaxial layers, impurities from the more highly doped substrate diffuse into the epitaxial layer for distances of tenths of a micrometer (Fig. I). The effective semiconductor resistivity at the diode junction then WJS considerably less than that of the original cpitasial layer surface (nominally 0.08 <2-cm). This effcctivc rcsistivity, for the particular diodes clcscribcd hcrc. \vas estimated from the diodcl areajcapacitanc~ ratio to be about 0.03-0.04 !!-cm. The rcsistivit\of the substrate was mcasurcd to bc 0.0033 I!-cm Following a brief etch in dilute H 1: to remo\ c’ residual oxides from the windows, ohmic contact to the ‘silicide’ surface was provided by an electroplated gold layer. An ohmic back contact of allo!-cd nickel had hccn formed prior to etching of tilt \vindows. (iii)

.-llte~nnfe

n2ethotl

Jlillimctcr-wave l)ccn fabricated

c~~~jzrnct~on

ffrmntim

Schottky-barrier diodes ha\,< on ~alllum nrscnicic 13~ sirnpi \

and

C.

A. BURRUS

electroplating a layer of metal on the exposed semiconductor defined by an array of windows in the masking oxide. (3) This electroplating technique offers the advantage of depositing metal only on the exposed semiconductor and not on the masking oxide. Application of this technique to silicon, however, is complicated by the fact that both ordinary electroplated coatings and displacement platings on silicon appear to be separated from the silicon surface by a very thin (and perhaps discontinuous) oxide. That an oxide usually is present under such coatings is evidenced by the csccssivcly high temperature at which the silicon metal cute&c alloy normallv forms. A Inethotl of electroplating gold onto silicon u.itllout the formation of an intervening oxide film has hccn reported.‘“’ Elimination of the oxide filnl TV;N indicated 1~~ the formation of a gol&silicoll plicatioii of an external voltage, plated gold out0 silicon irnmcrsed in the solution. I lowc~ cl-, !)I a~~plic,ition of sufficient re\‘erst: voltage to the silicon in this solution (silicon = anode), dcposition of gold I)!- the displacement proccL:+s could b< prevclltcd ; instead, the silicon surfa.:!. w;Is corroded. 13~ application of a rc\ cric %i)ltap sutfici~ntl\large to retard, but not prcvynt, the. displaccnlcnt plating, a successfrll irltirrlatc contact between the gold plating and the silicon surf;lcc was produced.‘!”

PLANAR

MILLIMETER-WAVE

EPITAXIAL

Diodes made by this alternate procedure exhibited electrical characteristics similar to those units made by the evaporation process. Thus it appears that this type of electroplating can be a satisfactory substitute for evaporation in the preparation of small-area planar silicon diodes. (6) Diode packaging Dice of the prepared silicon containing an array of many junctions were mounted in flat waveguide holders, or ‘wafers’,(l) for millimeter-wave evaluation. Electrical contact was made at random to an individual junction by moving a pointed spring-wire across the smooth oxide surface until the point locked into a diode-containing window in the oxide, making a contact between the junction and the circuit.c3) Thus the mounted units had the geometry of a conventional point-contact diode. However, the point-contact was a metal-metal connection which did not affect the rectifying properties of the junctions, since these properties had already been determined by the previous processing. So far, diodes made in this way have been used under laboratory conditions without protection from ambient atmospheric conditions; no noticeable deterioration in electrical characteristics have occurred over a three-month period. Critical aging tests have not been undertaken.

SILICON

SCHO’M’KY-BARRIER 3. DIODE

DIODES

CHARACTERIZATION

indicated previously, diodes were made from various thicknesses and resistivities of epitaxial silicon. However, the electrical characteristics of only the diodes processed from 0.08 Q-cm epitaxial silicon that had been thinned to 0.5 pm (Section 2 a(i)) will be discussed, since these diodes exhibited the lowest conversion losses when used as millimeter-wave down converters. Three processing batches were involved, all with junctions four micrometers in diameter. The zero-bias capacitance of these diodes, in the vicinity of 0.03 pF, was low enough for the SO-55 GHz signal frequency used in the measurements, but probably would be excessive for significantly higher frequencies (75 100 GHz, for example). As

(a) Resistance For purposes of computing a diode zero-bias cutoff frequency, the resistance of several silicon Schottky-barrier diodes was measured, using a transmission resonance technique,(lO) at frequencies near 55 GHz. The resulting values, listed as RI in Table 1, ranged from 6.7 to 9.5 Q for the nine diodes measured. For comparison, the similarly-measured resistance of a particular gallium arsenide Schottky-barrier diode (selected for low millimeter-wave conversion loss) is listed. For easy initial comparison and screening of the

Table 1. Summary of electrical characteristics of millimeter-wave silicon Schottky-barrier _ ‘Silicide’-silicon Electrical property

No. of meas.

Low

R,, resistance* R2, resistancet C,, zero-bias

9 23

6.7 Q 10.0 R

capacitance

12

0.022

9

560 GHz

fco = 1I257R,C, n, exponent

in diode

equation V,, forward

voltage

at 1 PA V,, reverse

1.36

23

0.27

23

0.4 v

V

8.2 23.0

0.035

790

0.03

650

GaAs S.B. diode B65

3.8 14.0 0.02

2100

1.8

1.5

1.25

0.39

0.33

0.52

3.4

1.7

5.0

voltage

at 1 PA

* RI = resistance t R,

22

Average

9.5 32.0

diodes Au-n-type

S.B. diodes

High

pF

= inverse

measured

at SO-57 GHz by resonant

521

transmission-loss

technique.

slope of 60 Hz I-V curve between 7.5 and 10 mA forward current

points.

W. V. T. RUSCH

522

diodes, the inverse slope of the diode 60-Hz I-V curve between the 7.5- and IO-mA points (arbitrarily selected) was taken as a measure of relative diode resistance. These values are listed as R, in Table 1 for a number of silicon diodes and for the gallium arsenide reference unit. Although correlation between R, and R, was rather poor, diodes with measured values of R, in excess of 30 Q generally had relatively low cutoff frequencies and were poor millimeter-wave down converters. (b) Capacitance The zero-bias junction capacitance of the diodes was measured on a Boonton capacitance bridge at 100 kHz. Values ranged from approx. 0.025 to 0.035 pF for the silicon units, as listed in Table 1. Previous experience has shown that junction capacitances measured on this bridge are within 10 percent or less of the values obtained by the transmission resonance technique. (c) Cutoflfrequency Zero-bias cutoff frequencies, f,, = 1/2?rR,C,, were in excess of 500 GHz for the 4-pm dia. silicon Schottky-barrier diodes described here. The best measured value approached 800 GHz, compared with values of the order of 2000 GHz for the better millimeter-wave &hot&y-barrier diodes on epitaxial gallium arsenide.

If

‘51.7GHr

and C. A. BURRUS

(d) Current-voltage characteristics The forward current of a diode may be expressed as,

I = I,[exp(qV/nkT)-

11

where 4 is the electronic charge, I’ is the applied voltage, R is Boltzmann’s constant, T is the absolute temperature, and pt = 1 for an ideal Schottkybarrier junction. Values of n were determined for a large number of the silicon diodes with junction diameters in the 3-5 pm range. These values seldom were below 1.35 and commonly were 1.4 1.5 (including diodes tabulated in Table 1 as well as other units with poorer millimeter-wave conversion-loss characteristics). However, larger junctions (25 pm dia.) made by the same processes had values of n as low as 1.12. The voltage at which the forward conducting current reached 1 PA ranged from approx. 0.27 to 0.40 V, with typical values near 0.33 V. These values may be compared with 0.5-0.8 V for millimeter-wave Schottky-barrier diodes on gallium arsenide.‘2*11) Useful silicon diodes showed reverse currents of 1 PA at reverse voltages between approx. 0.5 and 3.0 V, with typical values near 1.7 V. The reverse conduction characteristic was quite ‘soft’, as shown in Fig. 3, for both silicon and gallium arsenide units.

A

WAVEMETER

ATTENUATOR

FIG. 4. Block diagram of the conversion-loss and noise-ratio measuring equipment.

-6 FIG. 3. Photographs I-V characteristics barrier diodes: (a) ‘palladium (6) gold-gallium Table 1).

-4 -2 VOLTS

of oscilloscope tracings of millimeter-wave

silicide’silicon arsenide

0

2

of the 60-Hz Schottky-

junction; junction

(reference

unit

of

[faring

p. 522