Page 290 - Sami Franssila Introduction to Microfabrication

26

Bipolar Technology

Both transistors and integrated circuits were initially is formed on the wafer surface and this can cause pat-
made by bipolar technologies. The MOS transistor was tern shift and distortion in the growing epitaxial layers
conceived of and patented in the 1920s, well before (it can also cause growth defects if oxide removal is
the bipolar transistor (1947), but it was not realized incomplete or if implant damage is not fully annealed).
until 1960. Bipolar transistors today are used in many When the epitaxial-film growth from edges of a pat-
specialty applications in which high speed, low noise or tern is in the same direction, the pattern shifts later-
high current carrying capability is needed. ally (Figure 26.2). If the pattern edges are not identical
Bipolar transistors are traditionally fabricated on (recall <111> symmetries in Figure 21.19 to understand
<111> because of epitaxial film growth reasons but why rectangular structures on <111> must have differ-
there is no fundamental reason why they cannot be ent crystal planes at edges), structures can experience
fabricated on <100> as well. In fact, BiCMOS circuits, a shift in one direction and distortion in the direction
which have both bipolar and MOS transistors, are orthogonal to the shift. In the extreme case, the epi-
fabricated on <100> wafers because the quality of taxial layer ‘planarizes’ patterns in what is known as a
thin oxide, the MOS gate oxide, is better on <100> wash-out. Alignment problems will be encountered in
orientation silicon. This has to do with the atom all cases.
arrangement on the silicon surface and the resulting Buried layers are sources of dopants, and autodoping
Si–O bonds and their spatial restrictions. Oxide is not from buried layers must be considered. An isolated
a part of the active bipolar device; it has the role heavily doped region can dope areas many millimetres
of sacrificial and passivation layer. Bipolar transistors away in the downstream direction of the epi gas flow.
are vertical devices, that is, currents are transported When buried layers are tightly and uniformly spaced,
perpendicular to the wafer surface, whereas MOS autodoping non-uniformity is reduced, but the doping
transistors are lateral devices with currents parallel to level change must be accounted for. Buried layers
the wafer surface. The standard buried collector (SBC) are heavily doped because their role is to minimize
bipolar transistor is shown in Figure 26.1. It exemplifies collector resistance, but heavy doping will change the
the importance of epitaxy and diffusions in bipolar lattice constant slightly, and there is a danger of misfit
fabrication. dislocations (as shown in Figure 6.2). Different epitaxial
Bipolar transistor fabrication was already touched growth conditions (temperature, gases, pressure, reactor
upon in Chapter 14, in which the UV photodiode process design) will result in different shifts, distortions and
was described (Figure 14.3). A more detailed outline levels of autodoping.
of the SBC process is given below. Before that, a
short excursion to epitaxy on processed wafers is under-
26.1 FABRICATION PROCESS OF SBC BIPOLAR
taken.
TRANSISTOR
Buried layers are formed either by ion implantation or
thermal diffusion. The oxide acts as a mask for thermal There are many bipolar technologies but we will discuss
diffusion, but it is involved in the implanted process as a technology known as standard buried collector (SBC)
well: during annealing, a thin thermal oxide is grown bipolar technology, which has been widely used for
to prevent dopant outdiffusion. Before epitaxy, these decades. Even though current bipolars do not resemble
oxides have to be removed. As a consequence, a step it, they share many basic features with SBC.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)