Active solid-state devices (e.g. – transistors – solid-state diode – With means to increase breakdown voltage threshold – Reverse-biased pn junction guard region
Reexamination Certificate
2002-01-22
2003-12-09
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
With means to increase breakdown voltage threshold
Reverse-biased pn junction guard region
C257S197000
Reexamination Certificate
active
06661075
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to forming a pn junction in a semiconductor structure and, in particular, to forming a hyper-abrupt pn junction by low-temperature growth of a doped film on a surface of an oppositely doped substrate wherein the metallurgical junction of the pn junction could coincide with the surface of the substrate.
BACKGROUND OF THE INVENTION
The pn junction is the basic building block of most semiconductor devices. It is possible to fabricate these structures by a number of processes that are familiar to those working in the semiconductor industry. Briefly, such techniques used to produce pn junctions include thermal diffusion, ion implantation and epitaxial growth. The present invention is directed to pn junctions formed by growth of epitaxial layers taking the form of any one of silicon (Si), germanium (Ge), silicon germanium (Si
x
Ge
1−x
), or silicon germanium carbide (Si
x
Ge
y
C
1−x−y
), which are used extensively in the production of silicon-based microelectronic devices.
A Si epitaxial layer is typically deposited, or grown, on a Si substrate by high temperature pyrolysis of a chlorosilane precursor, such as dichlorosilane, or at a low temperature if silane (SiH
4
) is the precursor. Another approach is to deposit this layer by molecular beam epitaxy (MBE). MBE produces well behaved, i.e., abrupt, pn junctions, but the practicality of MBE is limited due to its extreme vacuum requirements and very low growth rates.
A commercially viable alternative to MBE is Plasma-Enhanced Chemical Vapor Deposition (PECVD), which has the potential to reduce the substrate temperature required for growing an epitaxial layer for forming a pn junction. To date, however, it is believed PECVD has not produced a properly operating pn junction in which the initial growth surface is contained in the depletion layer or located at the metallurgical junction of the oppositely-doped materials and the doping concentration of the least doped side is greater than 10
16
cm
−3
.
In general, the gaseous discharge associated with PECVD assists in creating the precursors to film growth, such as SiH, SiH
2
and SiH
3
, and also assists in removing impediments to film growth, e.g., hydrogen on the substrate surface. The movement of chemical species on the growth surface is enhanced by the energetic particle flux to the surface, which in general assists in the overall growth process. It is known that growing epitaxial layers at lower substrate temperatures reduces unwanted autodoping, dopant diffusion and the creation of crystal defects. The ability to epitaxially deposit Si-based and Ge-based films at reduced substrate temperatures would improve the quality of semiconductor devices, would simplify the process used to fabricate such devices and would make possible the fabrication of new device structures. Implicit in this capability to fabricate devices is the ability to chemically alter or dope the material with impurity atoms of n or p type. The ability to spatially alternate the placement of these dopant atoms in adjacent regions is required for the construction of a pn junction. There is considerable benefit to the ability to incorporate these dopants at low substrate temperatures that reduce unwanted autodoping and dopant diffusion that degrade the abruptness of a pn junction.
Currently there is considerable interest in growing Si
x
Ge
1−x
and Si
x
GeyC
1−x−y
alloys for producing heterojunction devices. Successful epitaxial growth of these heteroepitaxial alloys on Si substrates requires that low substrate temperatures (i.e., below 650° C.) be used to avoid relaxation of the pseudomorphic crystal structure. Unfortunately, the growth rate at such low substrate temperatures is presently quite small, on the order of 50 Å/minute.
The ability to deposit doped epitaxial semiconductor films is a critical technology required in fabricating integrated circuits. It has long been recognized that the ability to deposit semiconductor epitaxial films at reduced substrate temperatures will be required for the fabrication of subsequent generations of integrated circuit devices. Low substrate temperature epitaxial growth of semiconductor films is expected to eliminate or reduce the problems associated with autodoping, dopant diffusion and microcrystalline defect generation in addition to permitting the deposition process to be performed outside of thermal equilibrium. These advantages would result in a reduction in linewidth and junction depth of present device designs, and would permit the fabrication of new device structures currently limited by high processing temperatures.
The ability to grow epitaxial semiconductor films at reduced substrate temperatures using plasma enhanced processes has been demonstrated in numerous investigations. The exact role of the plasma in the deposition process has been attributed to a number of factors, as described in the article by W. J. Varhue, J. L. Rogers, P. S. Andry, E. Adams, M. Lavoie and R. Kontra, entitled “Low temperature deposition of epitaxial Si,” Solid State Technology, 163 June (1996). These factors include: production of reactive species which are the precursor to film growth, the removal of adsorbed hydrogen from the growth surface which prevents the adsorption of growth species on the surface, and the increase in adatom surface mobility to lower the required deposition temperature.
Despite the anticipated ability of plasma-enhanced processes to potentially produce hyper-abrupt (10
24
atoms/cm
4
) pn junction structures at lower substrate temperatures, such processes have failed to be successful. Problems associated with the electrical performance of pn junction diodes fabricated by this process have caused concern. The problems are a consequence of interfacial defects that occur at the original wafer surface where the deposition was first initiated.
The defects in the original surface of the substrate are believed to be both chemical and physical in nature and are an artifact of the fabrication process. In the case of the PECVD growth process, the surface has historically been prepared by a combination of ex-situ and in-situ processes. The in-situ process typically involves a plasma-enhanced mechanism which generates either an energetic ion or free radical hydrogen flux onto the substrate surface. It is believed that both of these particle fluxes damage or activate interfacial electrical defects on the wafer surface that then compromise the electrical properties of the diode.
One solution has been to grow a thick epitaxial layer of the same doping composition as the substrate. This effectively removes the substrate surface from the depletion layer of the device.
FIG. 1
shows a conventional pn junction diode
320
formed by PECVD and comprising a p-doped substrate
322
and a thick p-doped epitaxial layer
324
deposited onto original surface
326
of substrate
322
. After epitaxial layer
324
is deposited, the dopant is changed from p-type to n-type, and thereafter an n-doped layer
328
is deposited. Epitaxial layer
324
allows the metallurgical junction
330
between the oppositely-doped layers
324
,
328
to be formed remote from defects present on original surface
326
that interfere with the normal electrical function of diode
320
. After the epitaxial layer
324
has been deposited, metal contacts
332
,
334
are formed, respectively, on the upper surface of epitaxial layer
324
and the lower surface of substrate
322
to complete diode
320
.
Until the present invention, a properly functioning diode could be made using PECVD only by either restricting the doping concentration of the least doped side to 10
16
cm
−3
or by first depositing a film with a doping concentration similar to the substrate. The effect of either of these conditions is to eliminate the possibility of trap-assisted tunneling. In the first case, the depletion region is widened to the point that this leakage mechanism is rendered negligible. In the second case, the effect is to move the meta
Reidy Sean G.
Varhue Walter J.
Downs Rachlin & Martin PLLC
Nelms David
The University of Vermont and State Agricultural College
Vu David
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