Page 367 - Sami Franssila Introduction to Microfabrication

346 Introduction to Microfabrication

efficiency at particle size >0.12 µm. Filter efficiencies valves, regulators, mass flow controllers, etc.), leak rates
can also be classified according to most penetrating par- (static leak test, helium leak test) and gas impurity tests.
ticle size (MPPS). Filter defects (pinholes) are also a Bulk gases (also known as line gases or house gases)
major concern. Air velocity in the cleanroom is usually are gases shared by many tools. These include nitrogen,
ca. 0.35 to 0.45 m/s; and air circulation takes place 50 oxygen, hydrogen, argon and compressed air. Nitrogen
to 500 times/h, depending on cleanliness requirements. is especially widely used, both in processes and as an
Once the air has been processed, it is re-circulated, with inert protective gas. Four purity classes of nitrogen can
only 10% of replacement air introduced in each cycle. be offered for different applications:
Many types of process equipment produce excessive
heat loads, for example, furnaces in the range of 100 kW, – process nitrogen: furnace annealing or reactive
and this heat has to be removed in order to maintain sputtering, 7N purity;
constant temperature in the cleanroom. Most of the – dry nitrogen: venting and flushing of process
excess heat is taken away by cooling water. The design chambers, 5N purity;
of a cleanroom must, therefore, include knowledge of – pistol nitrogen: for drying;
the processes and tools that are going to be employed.
– pump nitrogen: as ballast for pumps.
35.2.3 DI-water Specialty gases are used by dedicated equipment,
and they are supplied from gas bottles in a one-to-
De-ionized water (DI-water), also known as ultra- one distribution topology. These include, for example,
pure water (UPW), is a major sub-system because of SF 6 and Cl 2 for etchers, SiH 2 Cl 2 and NH 3 for nitride
enormous water consumption in modern IC fabrication. LPCVD, SiH 4 and N 2 O for PECVD oxide, PH 3 for
A big fab uses a million cubic metres of ultra-pure water doped polysilicon LPCVD and WF 6 for tungsten CVD.
a year. Ion implanter gas consumption is very small, and AsH 3 ,
Water is treated in many steps as follows: PH 3 and BF 3 mini-bottles are usually located inside the
implanter cabinet. Implanter gases can also be supplied
– sand filter;
from safe delivery system (SDS) sources: the dopant
– active carbon filter;
gases are absorbed in solid absorber material in the
– particle filtering at 3 µm;
bottle, and released by application of temperature or
– softening of water;
underpressure.
– RO: reverse osmosis;
– CEDI: continuous electrical de-ionization;
– UV treatment; 35.3 ENVIRONMENT, SAFETY AND HEALTH
– ion exchangers; (ESH) ASPECTS
– particle filtering at 0.2 µm;
– storage tank; Various gases, chemicals and tools are sources of
– continuous DI-water circulation in the cleanroom potential health hazards to cleanroom personnel. Ion
loop. implanters operate at 200 kV and they are sources of
X-rays (and gamma rays may be emitted in hydrogen
Reverse osmosis is a process in which water implantation); plasma systems may leak microwave
molecules diffuse through a porous membrane, while energy and UV radiation, and wet etch and plating baths
microorganisms, particles and ions are rejected. UV may contain cyanides. These hazards are dealt with in
treatment kills bacteria and reduces total carbon con- different ways.
tent. Both RO and UV treatment can be repeated Strong mineral acids such as H 2 SO 4 , HNO 3 , H 3 PO 4
for improved performance. DI-water quality is mon- and HCl are routinely used. Normal burn hazards are
itored by resistivity measurements: 18 Mohm-cm is associated with them and they must be neutralized after
required. Regular bacteria checks as well as particle tests use. HF is different because its effect is not immediate
are performed. but delayed, and it does not attack skin but bone. Special
care is needed for all HF-containing liquids and separate
disposal of HF is required.
35.2.4 Gas systems
Solvents and organics come from various sources:
Gas system requirements include particle specifications HMDS, which is used as a priming agent before pho-
(which set limits to the choice of materials for piping, toresist coating, is released into cleanroom air (HMDS