IMPROVEMENT OF ADHESION, LINE DEFINITION, CONTACT RESISTANCE AND SEMICONDUCTOR PROPERTIES BY SPUTTER-ETCHING

In an RF-sputtering unit, with suspended substrates mounted in lieu of a target, sputter-etching of the substrates and consecutive vapor deposition of Au is performed in order to achieve good adhesion of the Au-film without an adhesion layer. The method has been applied for surface-acoustic-wave filters on piezo ceramics and for noise measurements in Au-films on sapphire. The sputter-cleaning as described also reduces the disturbing contact resistance between the conductive Cr-Au-layers and the previously sputtered Ta-film. Considerable progress has also been made in the manufacture of thin-film transistors by photolithography, chemical etching, and sputter-etching instead of using evaporation masks.


INTRODUCTION
Gold layers vapor deposited onto glass or ceramic substrates or onto other films such as Ta20s or Ta usually require an adhesion layer of Cr, NiCr or Ti. In some applications we have to consider the contact resistance between the conductive films as well as the line definition whenever small structures are manufactured. Moreover, cleanness of surfaces is essential in the successful evaporating of semiconductive materials because of the sensitivity of semiconductors to all contaminations. This paper describes how these problems can be solved by sputter-etching of the surfaces prior to the vapor deposition within the same vacuum system. The experiments can be divided into three groups: a) Vapor deposition of Au on piezo ceramic for surface-acoustic-wave filters and deposition of Aulayers on A12 O3-ceramic and sapphire, both without using adhesion layers. b) Vapor deposition of Cr-(NiCr)-Au-layers for the manufacture of integrated R-C-circuits with low contact resistances. c) Vapor deposition of CdSe for thin-film transistors. 215

DESCRIPTION OF THE EQUIPMENT AND OF THE SPUTTER-ETCH PROCESS
In a modified RF-sputtering unit as shown in Figure   the substrates are attached to a suspended target which is water-cooled from the back. The substrate station formerly used below the shutter has been replaced by the vapor sources. The substrates are cleaned by sputter-etching using the closed shutter as anode and later on they are coated with material from the vapor sources after the shutter has been opened.
In detail the process is as follows. After initial pump down to 10 -s torr the RF-plasma is fired between the target and the closed shutter at an argon pressure of about 10 -3 torr. The etching depth within the range of 10 Angstroms depends on the RF.power, value for the depth is an estimated one. It was found to be too small to be measured because of the low sputter-rate of Ta2 as, the low power applied of only 0.5 W/cm 2 to 2.5 W/cm 2 and because of the short sputter time of only 4 min to 20 min. After the sputter-cleaning, the plasma is shut off and the second pump down to < 10-s torr is done in less than [2 min. To reduce the time between cleaning and coating, pre-evaporation towards the lower side of the shutter is performed simultaneously to the second pump down. Finally the shutter is opened and the substrates are coated. The procedure, as described, is also applicable in large evaporation units with substrates mounted on rotating planetary wheels.
For that the whole substrate holder has to be isolated and a suitable RF-power applied.

VAPOR DEPOSITION OF Au WITHOUT
ADHESION LAYER of the Cr attacked the ceramic. Therefore, the lack of an adhesion layer improves the line definition after etching of the 12/am wide fingers of the electroacoustic transducer of the filter. Due to the better line definition the reproducability and the performance of the filter is considerably improved.

Gold Conductors
Au-layers were evaporated in order to measure current-noise effects in pure Au-films. These effects are so small that they can be observed only at high current densities and at temperatures above 900C. Therefore, a good adhesion of the 1/am thick Aulayer without any conductive under-layer was required. For substrate material mostly sapphire but also A12 O3-ceramic was used. Good adhesion was achieved by etching the sapphire surface with 2.5 W/cm z for 20 min, whereas AI O3-ceramic could be sufficiently cleaned in 10 min.

Surface-acoustic-wavetFilter Applications
The polished surface of the piezo-ceramic substrates was etched using a sputtering power of 800 W which corresponds to 2.5 W/cm: in the case of a 8 in target diameter. The sputtering time was 10 min. The thickness of the evaporated Au was about 0.5/am. The bonding pads were additionally electroplated up to 2/am to 5/am.
In earlier experiments an adhesion layer of Cr was used. However, the Au-lines lost their accuracy and line definition during the time needed to remove the Cr totally from the piezo-ceramic because the etchant

Tests
The two methods used for testing were a pull rod test and a wire pull test. The pull rod as shown in Figure 2 was attached normal to the surface of the substrate using a glue.
With the etching parameters given in 3.1. and 3.2.
for polished piezo-ceramic, sapphire and AI 03substrates, but also for glass substrates the tests revealed adhesion forces > 2 kp/mm 2 where the area enlarged by the glue has been considered. At  reported by Kallfass and Lueder. 11 larger forces these tests became unreliable because the glue partially broke. The wire pull test, as shown in Figure 3, was performed with Au-wires bonded by thermocompression to the Au-film.
In Table the  The investigations were made in a modified diode sputtering unit with suspended substrates mounted in place of the target, and with evaporation sources built in underneath a movable shutter. The same process is also applicable in large evaporation units.

REFERENCES
During the manufacturing process of integrated R's and C's using Ta-films doped with N2 and O2,2-s the Ta is coated with a thin oxide film when exposed to air. 6 This is especially undesirable in the areas where conductors are to be attached as it may generate contact resistances in the range of 5 ohm to 20 ohm. This value belongs to a Ta-resistor of about 100 gm line width and conducting pads of mm 2 to 2 mm 2 Additionally the adhesion usually decreases because of the contaminations originating from the production steps. By sputter-etching with a power of 0.5 W/cm to 1.0 W/cm during 2 min to 4 min the contact resistance can be lowered down to 0.2 ohm or less and the adhesioh is increased again to the convenient values of 2 kp/mm to 5 kp/mm Using a larger power may damage the dielectrics on the substrate, which can be easily recognized by the decay of the break through voltages of the C's.