Page 377 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

DEPOSITION OF WAVEGUIDE LAYER 357

Table 12.2 Main steps involved in the etching process
Step Description
(a) Exposure of photoresist metallised wafer with positive IDT mask plate.
(b) Develop photoresist patterned structures.
(c) Removal of unwanted metallisation layer via chemical wet-etching.
(d) Removal of photoresist layer.

Table 12.3 Main steps involved in the lift-off process
Step Description
(a) Exposure of photoresist bare wafer with negative IDT mask plate.
(b) Develop photoresist and formation of the characteristic lip.
(c) Deposition of metal layer onto the wafer.
(d) Removal of unwanted metallisation via acetone rinse.

Table 12.4 Summary of the main advantages and disadvantages of the etching and lift-off proce-
dures
Etching procedure
Advantages: Disadvantages:
Simple and reproducible Process parameters must be characterised
Good resolution achievable Compatibility of chemical etchants with
substrates
Fast realisation Loss of feature resolution due to overetching,
tendency to undercut
Ideal for small batch processing Susceptible to electrical shorts
Lift- off procedu re
Advantages: Disadvantages:
Capable of higher resolution Extreme care in handling before metallisation
Occurrence of electrical shorts minimised Intimate-contact photolithography required to
achieve vertical sidewalls on patterned
photoresist structures
Photolithography process is independent of
pattern resolution
Etchants not required Poor 'lift-off possible because of incorrect
formation of characteristic 'lip'
Ability to reprocess patterned photoresist
structures before metal layer deposition

At the end of this preclean, the plasma remains and TMS vapour is introduced. It is
metered using a conventional 10 cubic centimeter per second (cc/s) mass-flow controller
(MFC) to the desired volumetric flow rate. The oxide deposition begins at this point and
is continued for a predetermined time to achieve an oxide film of the desired thickness.
Following the deposition, the TMS gas is turned off, but the O 2-He plasma is kept on