Process for obtaining a layer of single-crystal germanium or...

Details Process for obtaining a layer of single-crystal germanium or... Process for obtaining a layer of single-crystal germanium or...

2000-09-11

2002-08-06

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

C148S033400

Reexamination Certificate

active

06429098

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates in a general way to a process for obtaining a layer of single-crystal germanium on a substrate of single-crystal silicon or, conversely, a layer of single-crystal silicon on a substrate of single-crystal germanium.
2. Description of the Related Art
Silicon (Si) is the basic compound of microelectronics. It is currently available on the market in the form of wafers 200 mm in diameter. The performance limits of integrated circuits are in fact therefore those associated with the intrinsic properties of silicon. Among these properties, mention may be made of the electron mobility.
Germanium (Ge), which belongs to column IV of the Periodic Table of Elements, is a semiconductor. It is potentially more beneficial than Si since (i) it has a higher electron mobility, (ii) it absorbs well in the infrared range and (iii) its lattice parameter is greater than that of Si, thereby allowing heteroepitaxial structures using the semiconductor materials of columns III-V of the Periodic Table.
Unfortunately, germanium does not have a stable oxide and there are no high-diameter germanium wafers on the market, except at prohibitive prices.
Si
1−x
Ge
x
alloys have already been grown on substrates of single-crystal Si. The alloys obtained only rarely have germanium contents exceeding 50% in the alloy.
Moreover, when SiGe alloys are grown on single-crystal Si, the growth of the SiGe alloy is initially single-crystal growth. The greater the thickness of the layer and the higher its germanium content, the more the layer becomes “strained”. Above a certain thickness, the “strain” becomes too high and the layer relaxes, emitting dislocations. These dislocations have a deleterious effect on the future circuits which will be constructed on this layer and the relaxation of the layers causes certain advantages of the strained band structure (offsetting of the conduction and valence bands depending on the strain states: Si/SiGe or SiGe/Si) to be lost. Corresponding to each composition and to each production temperature there is therefore a maximum thickness or strained layer.
In some applications, the concept of “relaxed substrates” has been developed, that is to say Si
1−x
Ge
x
layers are grown on silicon so as to exceed the critical thickness for a given composition, but by adjusting the deposition parameters for the layers so that the dislocations emitted do not propagate vertically but are bent over so as to propagate in the plane of the layer in order subsequently to evaporate at the edges of the wafer. Growth therefore takes place from increasingly germanium-rich layers, it being possible for the germanium gradient to change stepwise or in a continuous fashion.
However, the layers deposited by this “relaxed substrate” process either have a relatively, low (<50%) degree of germanium enrichment or have an unacceptable density of emergent dislocations for applications in microelectronics.
Thus, the article entitled “Stepwise equilibrated graded Ge
x
Si
1−x
buffer with very low threading dislocation density on Si (001), by G. Kissinger, T. Morgenstern, G. Morgenstern and H. Richter, Appl.Phy.Lett. 66(16), Apr. 17, 1995”, describes a process in which the sequence of the following layers is deposited on a substrate:
250 nm Ge
0.05
Si
0.95
+100 nm Ge
0.1
Si
0.9
+100 nm Ge
0.15
Si
0.85
+150 nm Ge
0.2
Si
0.8
.
After each layer has been deposited, it undergoes in situ annealing in hydrogen at 1095 or 1050° C. By way of comparison, similar sequences of layers have been deposited, but without annealing.
A 300 nm layer of Ge
x
Si
1−x
of the same composition as the upper buffer layer is also deposited on the latter.
The specimens which did not undergo intermediate annealing have an emergent-dislocation density of 10
6
cm
−2
, whereas the specimen which underwent annealing has an emergent-dislocation density of 10
3
−10
4
cm
−2
.
The article entitled “Line, point and surface defect morphology of graded, relaxed GeSi alloys on Si substrates”, by E. A. Fitzgerald and S. B. Samavedam, Thin Solid Films, 294, 1997, 3-10, describes the manufacture of relaxed substrates comprising up to 100% germanium. However, the process employed takes a long time (more than about 4 hours per wafer) and is consequently unattractive from an industrial standpoint. Moreover, this process is not reversible, that is to say it does not allow pure silicon to be deposited on a germanium substrate.
Furthermore, during the fabrication of such relaxed substrates, a surface roughness is observed which increases depending on the deposition conditions and which may have negative defects—since they are cumulative—that is to say an onset of roughness can but increase during definition.
SUMMARY OF THE INVENTION
In one embodiment a process for obtaining, on a substrate of single-crystal silicon, a Si
1−x
Ge
x
layer which has a high germanium content and which may be pure germanium, having a low emergent-dislocation density, and vice versa is described.
In one embodiment a process for obtaining a Si
1−x
Ge
x
layer having a high germanium content and a very low surface roughness is described.
In one embodiment a process as defined above which may be implemented in an industrial reactor, for example an industrial single-wafer reactor is described.
According to the invention the process for obtaining a layer of single-crystal germanium or of single-crystal silicon on a substrate of single-crystal silicon or of single-crystal germanium, respectively, includes the chemical vapour deposition of a layer of single-crystal silicon or germanium using a mixture of germanium and silicon precursor gases, the said process being characterized in that:
a) in the case of deposition of the layer of single-crystal germanium, the deposition temperature is gradually reduced in the range of 800° C. to 450° C., preferably 650 to 500° C., while at the same time gradually increasing the Ge/Si weight ratio in the precursor gas mixture from 0 to 100%; and
b) in the case of deposition of the layer of single-crystal silicon, the deposition temperature is gradually increased in the range of 450 to 800° C., preferably 500 to 650° C., while at the same time gradually increasing the Si/Ge weight ratio in the precursor gas mixture from 0 to 100%.
Any Si and Ge precursor gas, such as SiH
4
, Si
2
H
6
, SiH
2
Cl
2
, SiHCl
3
, SiCl
4
, Si(CH
3
)
4
and GeH
4
, may be used in the process.
The preferred precursors are SiH
4
and GeH
4
.
As is well known, the precursor gases are preferably diluted in a carrier gas such as hydrogen. The dilution factors may vary from 10 to 1000.
The chemical vapour deposition preferably takes place at low pressure, typically 8 kPa, but may also be carried out at atmospheric pressure by adapting the gas phases.
It has been determined that a pressure of about 8 kPa (60 torr) gave the best compromise between a high growth rate of the layers and deposition control.
Again preferably, the surface of the substrate is subjected to a preparation step prior to deposition.
This preparation step may conventionally be a surface cleaning step, for example any process in the liquid or gas phase which cleans the silicon surface of the metallic and organic residues, such as the conventional solutions SC1 (NH
4
OH+H
2
O
2
) and SC
2
(HC1+H
2
O
2
) or else H
2
SO
4
+H
2
O
2
. In all cases, the cleaning is completed by a phase of treatment using a dilute HF aqueous solution followed by rinsing in water.
Preferably, the process is carried out in stages of defined duration during which the temperature and the gas fluxes are modified linearly as a function of time. In other words, in the case, for example, of deposition of a layer of pure germanium on a silicon substrate, the temperature is lowered from the maximum deposition temperature to the minimum temperature in stages of defined duration during which the temperature is reduced linearly from a first value to a second value. During this same time interval, the

Affiliated with

Also associated with

Rating

Process for obtaining a layer of single-crystal germanium or... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for obtaining a layer of single-crystal germanium or..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for obtaining a layer of single-crystal germanium or... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2884616