Page 359 - Sami Franssila Introduction to Microfabrication

338 Introduction to Microfabrication

Pressure regimes:
Reactor Reactor
−8
module 1 module 2 Reactor module 10 torr
Rotation
−7
Central handler 10 torr
Translation
Cool-down Pre-clean Cool-down/pre-clean 10 torr
−3
module module
−3
Cassette ports 10 torr
Cassette input/output ports
Figure 34.2 Multichamber vacuum cluster for PVD. Reproduced from Grannemann, E. (1994), by permission of AIP

Integrated vacuum tools are single-wafer tools for
ease of automation. In the titanium/platinum example, 1 atm Air ambient Cleanroom
the two steps were carried out in one chamber, 4
sometimes called multiprocessing, but most integrated MINI-ENVIR.
processing tools have separate chambers for each 2 Atmospheric 1000 ppm
integrated
process. This enables a much tighter ambient control, processing
and it enables chemically different steps to be integrated. 0 Nitrogen ambient
If Ti/TiN/Al/TiN sputtering would be carried out in a 1 ppm
single chamber, nitrogen carryover from TiN step would LOG partial press imp. (Pa) −2 VACUUM Log conc. in 1 atm. inert ambient
contaminate aluminum films. CLUSTERS
In a mini-environment approach, a small cleanroom −4 Vacuum-based 1 ppb
is built locally around the tools or the wafers. It is integrated
processing
easier to keep a high purity level locally over a small −6
area, than in the whole room. In one extreme, the wafer 1 ppt
box is the cleanroom, filled with high purity nitrogen. −8
Compared to the cleanroom, it has two benefits: nitrogen Ultrahigh vacuum
is inert, so reactive impurities from the atmosphere −10
are eliminated, and the gas is stagnant in the box and 0.01 0.1 1 10 100
particles do not move, as they do in the laminar airflow Particle class
of the cleanroom. Figure 34.3 Environmental control: chemical/reactive
Integrated processing has two major sources of contaminants and particles in vacuum clusters vs.
variation under control: particle cleanliness and ambient mini-environments. Reproduced from Grannemann, E.
chemical environment (Figure 34.3). Elimination of the (1994), by permission of AIP
cleanroom itself has been toyed with: if all tools would
use a standard interface, wafers could be carried in
mini-environment boxes from tool to tool, and they 34.2 DRY CLEANING
would never see the cleanroom air, in which case the
cleanroom would become redundant. Wafer fabs with Because it is easy to integrate process modules with
such standard mechanical interfaces (SMIF) have been similar pressure and temperature regimes, dry cleaning
built, but cleanrooms have not been made redundant methods are attractive in vacuum integrated cluster tools.
because the conversion of all process and measurement Reduced pressure dry cleaning modules could fit into
tools has been elusive. This topic will be touched upon plasma etchers, sputters, PECVD, RTP and single-wafer
again in Chapter 35. epitaxial reactors.