Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
1999-12-13
2002-09-24
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S103000, C117S108000, C117S935000
Reexamination Certificate
active
06454855
ABSTRACT:
The present invention relates to a method for producing coated workpieces according to the preamble of Claim 1, to uses therefor according to Claims 28 to 35, to an installation for implementing the above-mentioned method according to the preamble of Claim 36 and to uses therefor according to Claims 51 to 54.
The present invention is based on problems which occur during the manufacturing of thin layers by means of CVD and PECVD methods. The findings made in this case, according to the invention, can be applied particularly to the production of semiconductor layers, for example, when producing solar cells or modulation doped FETs or hetero-bipolar transistors.
Thin semiconductor films are deposited either in a monocrystalline form, that is, epitaxially, on an also monocrystalline substrate, such as a silicon substrate, or are deposited in a polycrystalline form or amorphous form on polycrystalline or amorphous substrates, such as glass. Although in the following the invention will be described mainly with respect to the production of silicon-coated and/or germanium-coated substrates, it may, as mentioned above, also be used for the production of other workpieces and workpieces coated with other materials.
Known methods for depositing epitaxial semiconductor films are:
Molecular beam epitaxy (MBE),
chemical vapor deposition (CVD),
remote plasma enhanced CVD with DC or HF discharge,
electron cyclotron resonance plasma-assisted CVD. (ECRCVD).
“CVD method” is a collective term for a large number of thermal deposition methods which differ either in the construction of the assigned apparatuses or in their operating mode. Thus, for example, a CVD method can be carried out at a normal atmospheric pressure or at much lower pressures down into the range of the ultra high vacuum. Reference can be made in this respect to (1) as well as to (2).
In the commercial production of epitaxial Si layers, only CVD is normally used. In this case, the applied reactive gases are silicon-containing gases, such as silane chlorides, SiCl
4
, Si,HCl and SiH
2
Cl
2
as well as silanes, such as SiH
4
, or Si
2
H
4
. Characteristics of the standard CVD methods are the high deposition temperatures in the order of 1,000° and more, as well as pressures of typically 20 mbar to 1,000 mbar, that is, to normal atmospheric pressure.
Translator's note: The subscripts on this page are only guesses since most are illegible in the German.
According to the process conditions, coating rates of several &mgr;m per minute can be achieved in this manner. corresponding to several 100 Å/sec., with respect to which reference is again made to (1).
In contrast, low pressure chemical vapor deposition (LPCVD), which is synonymous with low pressure vapor phase epitaxy (LPVPE), takes place at pressures below 1 mbar and permits lower process temperatures to typically 700° C. In this respect, reference is made, in addition to (1), also to (3) and (6).
With respect to the LPCVD and with reference to (6), at a deposition temperature of 650° C., a growth rate of
GR=
50 Å/min
is indicated. This takes place at a reactive gas flow for silane of
F=
14 sccm.
This results in a characteristic number which is relevant to the gas yield, specifically the growth rate per reactive gas flow unit GR
F
at
GR
F
=3.6 Å/(sccm·min)
On 5″ wafers, corresponding to a surface
A
S
=123 cm
2
,
converted from the actual surface A
2
for 2″ wafers, a deposition quantity (growth amount) GA is obtained at
GA=
5.2·10
14
Si atoms/sec.
Again, with respect to a reactive gas flow unit, the characteristic number “deposition quantity per reactive gas flow unit”, in the following called “gas utilization number”, GA
F
is obtained at
GA
F
=8.4·10
−3
,
corresponding to 8.4 o/oo.
At 650°, an epitaxial layer is formed.
If the deposition temperature is reduced to 600° C., a polycrystalline layer is formed. In this case, the following applies:
GR=3 Å/min
F=28 sccm silane
GR
F
=0.11 Å/sccm/min)
GA=3.1·10
15
is Si atoms/sec on A
R
GA
F
=2.5·10
−4
, corresponding to 0.25 o/oo.
Basically, the following criteria are required for a defect-free epitaxial layer growth:
In the case of transmission electron microscopy on cross-sectional preparations, the proof of epitaxy is established by electron diffraction and high resolution.
In the area of 10 to 15 &mgr;m, which in this case can typically be penetrated by radiation, along the boundary surface to the substrate, no defects must be visible. Typical enlargements in the analysis of defects are 110,000 to 220,000.
Another development is the ultra high vacuum chemical vapor deposition (UHV-CVD) with working pressures in the range of 10
−4
to 10
−2
mbar, typically in the range of 10
−3
mbar, with respect to which reference is made to (4) as well as to (5), (7). It permits very low workpiece temperatures; however, the growth rates or coating rates being extremely low; thus, for example, approximately 3 Å/min for pure silicon at 550° C. according to (5).
The reason for the low growth rates is the fact that the absorption rate and decomposition rate of the reactive molecules, thus, for example, of SiH
4
, decreases with an increasing hydrogen coating of the workpiece surface. The layer growth is therefore limited by the desorption rate of H
2
, which, however, rises exponentially with the temperature. In this respect, reference is made to (8). Because of the lower bonding energy of the Ge—H bonding in comparison to the Si—H bonding, the hydrogen desorption of an Si—Ge alloy surface is higher, so that, while the substrate temperature is the same, a higher growth rate is obtained than in the case of pure Si; for example, at a content of 10% Ge by a factor 25 at 550° C. (5).
Another possibility of achieving high deposition rates of an epitaxy quality at low substrate temperatures consists of (9) decomposing the reactive gases by means of a u-wave plasma (ECRCVD).
By the use of plasma sources, which are based on the principle of electron cyclotron resonance, the incidence of high-energy ions onto the substrate is to be avoided.
As a rule, such sources operate in the pressure range of 10
−3
to 10
−4
mbar, which, however, results in larger free path lengths than in the case of capacitively coupled-in high-frequency Hf plasmas. This, in turn, can lead to an undesirable ion bombardment of the substrate and thus to the generating of defects, as indicated in (10). The energy of the ions impacting on the substrate, however, can be limited by an external control of the substrate potential, whereby ion-related damage can largely be avoided. Also by means of the ECRCVD method, the growth rates for pure silicon, as a rule, amount only to a few 10 Å/min, at low deposition temperatures≦600° C.
Summarizing, this results in the following:
Layers which are deposited with a quality which is suitable also for the depositing of epitaxial layers can be deposited at deposition temperatures≦ up to now:
by UHV-CVD with growth rates GR of approximately 3 Å/min or
ECRCVD with a growth rate GR higher by approximately 1 order (30 Å/min).
PECVD methods, whose plasmas are produced by DC discharges, could be used for the manufacturing of layers of epitaxy quality—that is, a correspondingly lower fault density (see above)—neither for the construction of epitaxial nor for the construction of amorphous or polycrystalline layers; at least not with a growth rate GR, reliability and efficiency to be ensured for industrial manufacturing.
On the other hand, the use of capacitively coupled-in high-frequency fields for generating HF plasmas for PECVD methods was reported very early, with respect to which reference is made to (11). The difficulty of this approach is the fact that not only the reactive gases are decomposed in such Hf plasmas. Simultaneously, the substrate surface is exposed to an intensive bombardment of highly energetic ions, as utilized specifically also in the case of re
Ramm Jurgen
Rosenblad Carsten
Von Känel Hans
Crowell & Moring LLP
Kunemund Robert
Unaxis Trading AG
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