Page 140 - Sami Franssila Introduction to Microfabrication

11

Etching

The pattern transfer process consists of two steps: wafer. Gas bubbles formed according to Equation 11.1
lithographic resist patterning and the subsequent etching can protect the surface from further etching.
of the underlying material. The resist pattern can Etch rates are typically 100 to 1000 nm/min, for
always be removed if found faulty on inspection, both wet and plasma processes. The lower limit comes
but once the pattern has been transferred on to solid from manufacturing economics, and the upper limit
material by etching, rework is much more difficult, and from resist degradation, thermal runout and damage
often impossible. considerations. Silicon etching is exceptional: rates up to
Etching is often divided into two classes, wet etching 20 µm/min are available in both wet etching (HF:HNO 3 )
and plasma etching. Wet etching equipment consists and in plasma etching (DRIE) in SF 6 /C 4 F 8 .
of a heated quartz bath ($10 000), and plasma-etch There are materials that cannot be wet etched, for
equipment is a vacuum chamber with an RF-generator example, SiC, GaN, TiC and diamond. These materials,
and a gas system (costing up to millions of dollars). can, however, be plasma etched. Some materials cannot
The basic reactions in etching are as follows: be etched even by plasmas because no suitable source
gas/volatile product combination exists. In that case,
Wet etching purely physical etching, known as ion milling or
ion beam etching (IBE), can be used: argon ion
solid + liquid etchant −→ soluble products
bombardment will erode any material. Many solid-
− state laser garnets and magnetic materials (of the type
Si (s) + 2OH + 2H 2 O −→
Gd 3 Ga 5 O 12 , gadolinium gallium garnet) are etched by
Si(OH) 2 (O ) 2 (aq) + 2H 2 (g) (11.1)
−
ion milling. It is, however, difficult to find suitable non-
Plasma etching eroding masking materials: if anything can be etched by
argon bombardment, this applies to masking materials
solid + gaseous etchant −→ volatile products as well. Typical ion milling rates are 10–100 nm/min,
an order of magnitude less than in plasma etching.
SiO 2 (s) + CF 4 (g) −→ SiF 4 (g) + CO 2 (g)
(11.2)
There are three steps that must take place for etching Note on terminology
to proceed:
The term dry etching, as opposed to wet etching, is often
• transport of etchants to surface; used as a synonym for plasma etching, but there are dry
• surface reaction; methods that do not involve plasma, for example XeF 2
• removal of product species. gas etching. Plasma etching, in the older literature, can
also mean a specific type of etch reactor, the parallel
If etching does not take place, any of the three steps plate plasma reactor, in which the wafer is placed on the
could be causing the problem: transport could be grounded electrode. The opposite of the plasma etcher
prevented or reduced by, for instance, a thick boundary is the RIE reactor (reactive ion etching), with the wafer
layer; a native oxide or residues from the previous steps on the powered electrode. Today, both plasma etching
could retard or prevent etching; or the products may not and RIE are used as general terms and not as reactor
be volatile or soluble enough, and they redeposit on the descriptions.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)