Page 137 - Sami Franssila Introduction to Microfabrication

116 Introduction to Microfabrication

Wet
etching

Plasma
etching

Electro-
plating

Ion implantation Lift-off
Figure 10.13 Processing after lithography puts varying demands on resists

Lift-off Table 10.1 Photoresist stripping

• thickness of the film needs to be less than resist Techniques Mechanism
thickness; Oxygen plasma Oxidation in vacuum
• resist sidewall profile preferably retrograde; Ozone discharge Oxidation under atmospheric pressure
◦
• deposition process T < 120 C because of resist
Acetone Dissolution in liquid
thermal limitation. Ozonized water Bond breaking and dissolution
Sulphuric acid Oxidation in liquid
10.7 PHOTORESIST STRIPPING/ASHING Organic amines Oxidation and dissolution in liquid
H 2 O 2 Oxidation in liquid
After the photoresist has served its role as a protec-
tive layer, it must be removed. There are a number of
methods to accomplish this (Table 10.1). The choice
depends on the particular process step, the materials The cost structure of photoresist stripping varies with
present on the wafer, resist nature and established labo- the methods: in plasma or ozone ashing, equipment
ratory practice (which may be determined by historical purchase cost is a major issue but oxygen bulk gas
precedence, environmental concerns or other idiosyn- is cheap; in wet stripping (e.g., H 2 SO 4 ) the cost of
cratic factors). Oxygen plasma is a universal method, chemicals is important because large volumes are used
and the liquid phase methods are more or less specific (and disposed of). Some organic amine strippers are very
to certain applications. expensive and can only be used for a few hours; the cost
Sulphuric acid is a strong oxidant, and therefore an is dominated by material cost.
effective resist remover; however, it cannot be used if Ultrapure ozonized water, UPW-O 3 , (in situ genera-
the wafer is metallized because the acid will etch metals tion of 10–100 ppm ozone in DI-water) is potentially a
major cost-reduction invention in stripping. Strip rates of
too. Acetone is a fairly mild remover, and it cannot
be used if the resist has been damaged or transformed 150 nm/min can be achieved, and utilization of ozone is
by plasma or ion bombardment. Oxygen plasma alone very efficient even though the simple chemical reaction
will often suffice, but it is common practice to use two- might suggest otherwise:
step resist stripping: plasma (dry) removal followed by
wet removal. CH 2 + 3O 3 −→ CO 2 + H 2 O + 3O 2 (10.5)