Page 343 - Sami Franssila Introduction to Microfabrication

322 Introduction to Microfabrication

10 4

10 3 0.01 ML 0.01 ML 0.01 ML 0.01 ML
S = 1 S = 0.01 S = 0.0001 S = E-6
Time (s) 10 2 0.01 ML S = 0.001
0.01 ML
S = 0.1
Surface passivation
10 1 1 ML 1 ML
S = 1 S = 0.01

10 0
10 −9 10 −8 10 −7 10 −6 10 −5 10 −4 10 −3 10 −2
Background impurity pressure (Pa)
Figure 32.1 Monolayer (ML) and 0.01 ML formation times as a function of pressure and sticking coefficient (S). Surface
can be passivated by, for example, HF-treatment. Reproduced from Grannemann, E. (1994), by permission of AIP

The order of magnitude for the frequency factor ν is impurity in the film. Purities of typical starting materials
13 −1
10 s , which describes a simple harmonic oscillator for PVD are 99.999%. Poor vacuum can therefore
contribute many orders of magnitude more impurities
with frequency kT/h. Chemisorbed species have an E a
of ca. 1 eV and physisorbed species, an E a of 0.4 eV, into film than the target materials. Of course, not all
which translate roughly, at room temperature, to hours impurities are equal: some manifest themselves much
and microseconds, respectively. more strikingly than others. Unity sticking coefficient
Impurities in the vacuum chamber will be incorpo- presents the worst case. At base pressures of 10 −9 torr,
rated into the growing film. Partial pressure of the impu- target purity starts becoming a limiting factor.
rities must be considered together with the deposition Deposition rates in batch systems are usually much
rate in order to determine the concentration of impurities slower than in single-wafer systems: an order of
in the film. Table 32.1 shows how gas-phase impuri- magnitude difference is not unusual, and therefore
ties are incorporated into growing films as a function of throughput rather than deposition rate is often mentioned
residual gas pressure. for batch systems. But as shown in Table 32.1, film
At 10 −6 torr, impurities deposit approximately at a quality is related to deposition rate, not to throughput.
rate of one monolayer per second (∼0.1 nm/s). Even
the very high rate of 100 nm/s, which corresponds to 32.2 VACUUM PRODUCTION
ca. 1000 atomic layers per second, will result in 0.1%
Starting from the ideal gas law

Table 32.1 Fraction of foreign atoms incorporated into p = NkT/V (32.6)
growing film (unity sticking coefficient; worst case
estimates) we can get a feeling for vacuum production. Vacuum
production means a change (decrease) in the number
Partial pressure Deposition rate (nm/s) of atoms N over time, dN/dt. We use the following
(torr) definitions:
0.1 1 10 100
10 −9 10 −3 10 −4 10 −5 10 −6 Particle density: n ≡ N/V in units atoms/m 3
10 −8 10 −2 10 −3 10 −4 10 −5 Flux: J ≡ dN/dt in units atoms/s
3
10 −7 10 −1 10 −2 10 −3 10 −4 Pumping speed: S ≡ −J/n in units m /s, a.k.a.
10 −6 1 10 −1 10 −2 10 −3 volumetric flow
10 −5 10 1 0.1 0.01 rate