Page 325 - Sami Franssila Introduction to Microfabrication

304 Introduction to Microfabrication

Cr
+
(n ) a-Si:H 100 nm
~50 nm
Undoped
a-Si:H ~200 nm
~400 nm

SiN x
Polyimide foil 500 nm

◦
Figure 29.3 TFT on polyimide; the maximum processing temperature is 150 C. From Gleskova, H. et al. (2001), by
permission of The Electrochemical Society

29.2.2 TFTs on other substrates 2. Calculate row and column address electrode resis-
tances on a 15 in. TFT display. Compare ITO and
Limitations that hold true for glass plates are also true real metals.
for TFTs made on steel foils, even though there are 3. Design a fabrication process for the top gate TFT
some differences. Higher processing temperatures can shown below. The maximum process temperature is
be used from a mechanical strength point of view, but 350 C. From Wu, M. et al. (1999), by permission of
◦
iron contamination is a concern. Steel is a conduct- AIP.
ing material and an electrical insulator layer must be
deposited on it before any electrical devices. Iron con- AI 200 nm
tamination concern replaces the sodium-contamination
danger, so an ion barrier is needed. If the same film can SiO 2 200 nm
act both as electrical insulation, ion barrier and smooth- µc-Si n + 75 nm
ing layer, it is better. Steel surface smoothness is inferior Polysilicon 160 nm
to glass, and planarization may be needed. Spin-on-glass 480 nm
can fulfill all these disparate requirements and is clearly Insulaton layer:
a strong candidate. Spin-on glass+SiO 2
Processing TFTs on polymer substrates sets even Steel substrate 200 µm
stricter limits on the thermal budget. Shown in
Figure 29.3 is a TFT on polyimide substrate. Maxi-
◦
mum processing temperature has been limited to 150 C
(polyimide thin films on silicon wafers can tolerate 4. TFT itself takes up very little area compared with
◦
much higher temperatures, up to 400 C because con- pixel, and transistor packing density increase offered
duction to the substrate effectively spreads excess heat). by self-alignment is not important. What are the
Plasma nitride serves two important functions: it pas- benefits of self-alignment in TFT fabrication?
sivates the device from the substrate and it acts as the 5. What are the integration issues when the RCL passive
gate dielectric. chip in Figure 24.13 and TFBAR in Figure 7.9 are
The mechanical strength of polyimide substrates made on:
is inferior to both glass and steel, but fortunately (a) Si
low process temperatures are helpful, and due to low (b) glass
temperatures stresses are also minimized. (c) fused silica.

29.3 EXERCISES REFERENCES AND RELATED READINGS
1. If flat-panel lithography is done with a 50 µm Becker, H. et al: Planar quartz chips with submicron channels
proximity gap, what is the smallest possible linewidth for two-dimensional capillary electrophoresis applciations, J.
on an FPD? Micromech. Microeng. 8 (1998), 24.