Page 93 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

74 STANDARD MICROELECTRONIC TECHNOLOGIES

Pipette

Liquid

Spin
Wafer

Motor

Vacuum line

Figure 4.10 Apparatus used to cast a resist onto a substrate in preparation for optical lithography

resist and final spin speed v approximately according to

^R = -?= x /sc (4.7)

where f sc is the percentage solid content in solution. The resist-coated wafer is then cured
by a soft-bake at a low temperature (80 to 100°C for 10 to 20 minutes) and a process
3
mask applied for shadow or projection printing . The mask is generally a reticule mask
plate and comprises a glass plate coated with a light-blocking material, such as a thin
chromium film, that has itself been patterned using a wet-etching process and a second
resist, but in this case the resist has been written on directly using a high-resolution
electron-beam writer. Then the mask and substrate are exposed to a radiation source,
usually ultraviolet (UV) light, and the radiation is transmitted through the clear parts of
the mask but blocked by the chromium coating. The effect of the radiation depends on the
type of resist - positive or negative. When a positive resist is exposed to the radiation,
it becomes soluble in the resist developer and dissolves leaving a resist pattern of the
same shape as that of the chromium film. Conversely, the negative resist becomes less
soluble when exposed to the radiation and so leaves the negative of the chromium coating.
Negative resists are used more commonly as they yield better results. Table 4.3 shows
4
some commercially available resists for both optical and electron-beam lithography .
Figure 4.11 shows the lithographic patterning of a substrate with a negative resist and
chrome mask plate (mask 1), and subsequent soft-baking and developing in chemicals,
to leave the resist over predefined parts of the oxide layer. The resist is then hard-baked

3
Optical and other lithographic techniques are described later in Section 5.3.
4
These terms are defined later on under bipolar transistor characteristics.