Page 376 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

356 IDT MICROSENSOR FABRICATION

Also, each parent TMS molecule (Si(CH3) 4) contains half as much carbon and three-
fifths as much hydrogen as a TEOS molecule (Si(OC2H 5)4), and it is hypothesised
that carbon and hydrogen-free films will be obtainable at lower temperatures from this
precursor. Additionally, the lower molecular weight of TMS might allow for higher surface
mobility than TEOS at any given temperature, thereby resulting in better-quality films at
temperatures lower than those obtainable by PETEOS (~250 °C). Finally, it is thought that
PECVD TMS oxide (PETMS-O x) deposition conditions could mimic very closely those
conditions found to produce high-quality PETEOS and silane oxides in the semiconductor
industry (Campbell 1996; Ghandi 1994). Such deposition is carried out using a cluster
tool that is specifically fabricated for this process, and the four-chamber showerhead
Vactronics PDS-5000 S cluster tool PECVD reactor (Figure 12.6) is used.
The deposition procedures and conditions involve units 3 and 4 as follows: initially,
in the deposition chambers, TMS, 02, and He gas lines are evacuated of residual gas
and then a sample is placed in a load-lock chamber (unit 3), which is evacuated from
—5
atmosphere to a low pressure (typically 10 -10 —6 torr). This preinsertion vacuum time
is held at 30 minutes. The SAW-IDT wafer is then placed on the preheated sample stage
(unit 4) in the deposition chamber, which is maintained at 10 —6 to 10 —7 torr, by the
robotic loading mechanism. A period of 1 hour is allotted for the sample to come to
temperature, after which 02 and He gases are input via the gas-dispersion showerhead
and a period of 5 minutes is allotted for the flows to stabilise. A plasma is struck with
the same pressure, RF power, and gas flow rates. This 10-minute preclean plasma purge
serves three purposes:

1. It removes any residual carbonaceous matter left on the SAW device

2. It helps to form a stable interface oxide
3. It provides a high flow, stable plasma into which a miniscule flow of TMS can be
injected

Figure 12.6 Schematic representation of a PECVD unit