Page 348 - Sami Franssila Introduction to Microfabrication

Vacuum and Plasmas 327

32.5 PECVD handle. It is also a greenhouse gas just like fluorinated
hydrocarbons.
PECVD reactors are very much like plasma etchers.
From the hardware point of view, the heated elec-
trode is the main difference. Other aspects, such as 32.6 RESIDENCE TIME
RF generators, reactive gases and pumping systems,
among others, are similar. In etching, high density plas- The effects of pressure and flow can be deduced from
mas (HDP) offer enhanced etch rates; in PECVD, HDP residence time τ (for PECVD and other processes alike):
equals enhanced deposition rate and/or improved film
quality. τ = (p/p 0 )(V/F)(273/T ) (32.15)
Higher deposition temperature leads to denser, more
stable films. This may be useful, but the main advan- where p 0 is a reference pressure of 1 atm.
tage of PECVD is low deposition temperature. Typ- Residence time is the characteristic time that a
◦
ical PECVD temperature is 300 C, but there is no molecule spends in the reactor before being pumped
away. Increasing the pressure leads to increased residence
fundamental lower limit to deposition temperature. Pro- time, which translates to higher deposition rate: the
◦
cesses at 100 C have been demonstrated but film prop- molecules have a higher probability of being incorporated
erties are strongly temperature-dependent. In particu- into the film if they spend more time in the reactor.
lar, hydrogen content of the films increases rapidly
Increasing the flow will sweep the molecules away faster,
as temperature is lowered, and the films become less
leading to smaller τ and lower deposition rate.
dense. The above discussion is about first-order effects
only: when two reactant gases interact, many things can
be different. 32.7 EXERCISES
Increasing RF power initially increases the depo-
sition rate, because more reactant gases are ionized, 1. What is the Knudsen number in
fragmented and available for reaction. Further increase (a) sputtering;
in power leads to decreased rate, however: more and (b) evaporation;
more ion bombardment causes sputtering of the grow- (c) MBE;
ing film. (d) RIE.
Utilization is a measure for reactant usage. It is the 2. What is the maximum theoretical pumping speed
ratio of atoms incorporated into the film to atoms in of a diffusion pump with vacuum flange of diam-
incoming gases. Utilization cannot even approach 100% eter 10 cm?
because flow patterns in a reactor cannot be optimized 3. If the sticking coefficient of a water molecule is 0.01
for such a high efficiency. Some metal–organic precur- and the partial pressure of water is 10 −4 Pa, how long
sor molecules undergo disproportionation reaction, and will it take to form a monolayer?
only 50% of source gas atoms are available for deposi- 4. What must the leak rate be in an MBE system in
tion in the best case. order to achieve a base pressure of 10 −11 torr?
Deposition takes place not only on the wafers but 5. What would the crossover pressure be for film
also on the reactor walls and the electrodes. It is purity to become dependent on target purity when
standard procedure to etch these deposited layers away a 99.9999% pure target (6N) is used?
at regular intervals, for example, after every wafer, after 6. How deep into aluminium sputtering target will
a certain thickness has been deposited, when deposition 500 eV argon ions penetrate?
temperature is changed or when the material to be 7. Pulsed (Bosch) process DRIE chamber volume
deposited is changed. The similarity of PECVD to RIE is 50 L, flow rate is 200 sccm and operating
is evident from the fact that introduction of CF 4 or pressure is 20 mtorr. What is the shortest possible
NF 3 gas into a PECVD reactor chamber turns it into pulsing period?
an etch system. In situ cleaning of the PECVD chamber 8. If 5-kW power is applied to aluminium sputtering
can thus be accomplished easily. NF 3 gas has a nice target of 200 mm diameter, what is the maximum
feature in that it decomposes into gaseous products only, possible deposition rate?
whereas CF 4 or SF 6 are potential sources of carbon and 9. XPS measurement takes 15 min. What is the pressure
sulphur residues. NF 3 is, however, toxic and hard to in a XPS chamber?