Optical lithography has undisputabl

Optical lithography has undisputably been the leading integrat-

ed circuit pattern defining technique for many years. It has essen-

tially two steps. First, the design and fabrication ofthe optical mask,which is both costly and time consuming, and secondly, the expo-

sure of the wafer, covered with a layer of light sensitive photoresist

to ultraviolet light shone through the mask. The method is ideal for

large scale because once the expensive wafers may be
cess has been carried out, unlimited number of
patterned at very low cost to the producer, On the other hand, where

specific or semicustom (mony3aka3Hble) ICs are concerned this pro

cess has proved unacceptable since the cost and time involved in

mask fabrication cannot be justified by the production ofonly a few

devices which may require several interactions for optimum results

For these reasons, electron beam direct-write lithography is prov

ng invaluable in the field of application of specific or semicustom

integrated circuits This technique allows fast turnaround, a high

flexibility and comparatively low cost for very small batches. In ad

dition, the short wavelength of electron-beam offers very high res

olution patterning and so may be essential where sub-micron fea

tures are required. Despite the possibility of low throughout, e-beam

generated patterns allow either simple wafer-scale integration or

devices for several customers, each possibly with a variety of trial

designs to be implemented on a single wafer, The major advantage

of the e-beam's high resolution capability will be nullified if the

resist pattern cannot be very precisely reproduced onto the metalli

zation layer. For this reason, wet-etching of the metal with
its in

herent undercutting is particularly unsuitable and plasma-process-

ing becomes necessary Reactive ion etching
is a type of plasma

hing where the wafer is placed on an electrode which is capaci

tively coupled to an RF generator. A second electrode
larger than

this driven one is grounded and a plasma is generated by
electronic

excitation of a low pressure gas contained between th
The ar

rangement of the system is such that the driven electrode experi

ences a negative bias with respect to the plasma causing positive

ions to be accelerated towards the
wafer. This means that not only

is there chemical reaction causing removal of the metallization but

also ion-enhanced chemical etching and physical sputtering to the

vertical etching essential for preeise replication of the resist pat

tern Dry processing has the added benefits of easily handled pro

ess materials, easy automation an

good reprodueibility.