Remote plasma enhanced CVD method for growing an epitaxial semiconductor layer

A remote plasma enhanced CVD apparatus and method for growing semiconductor layers on a substrate, wherein an intermediate feed gas, which does not itself contain constituent elements to be deposited, is first activated in an activation region to produce plural reactive species of the feed gas. These reactive species are then spatially filtered to remove selected of the reactive species, leaving only other, typically metastable, species which are then mixed with a carrier gas including constituent elements to be deposited on the substrate. During this mixing, the selected spatially filtered reactive species of the feed gas chemically interacts, i.e., partially dissociates and activates, in the gas phase, the carrier gas, with the process variables being selected so that there is no back-diffusion of gases or reactive species into the feed gas activation region. The dissociated and activated carrier gas along with the surviving reactive species of the feed gas then flows to the substrate. At the substrate, the surviving reactive species of the feed gas further dissociate the carrier gas and order the activated carrier gas species on the substrate whereby the desired epitaxial semiconductor layer is grown on the substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a remote plasma enhanced chemical vapor 
deposition (RPECVD) apparatus and method for growing an epitaxial 
semiconductor layer. 
2. Discussion of Background 
Plasma enhanced processes have figured prominently in research efforts to 
lower process temperatures. In conventional plasma enhanced chemical vapor 
deposition (PECVD), the parent gas molecules are dissociated into 
precursor atoms and radicals which can deposit on substrates at lower 
temperatures than in thermal chemical vapor deposition. The deposition 
occurs at lower temperatures than purely pyrolytic processes because the 
plasma supplied energy to break chemical bonds in the parent molecules 
that would only be broken by thermal decomposition if the plasma were not 
present. Parent molecule dissociation is accomplished in the plasma 
through various processes involving collisions with electrons, ions, 
photons, and excited neutral species. Unfortunately, the precursor species 
are also subject to the same active environment which dissociated the 
parent molecules. This can lead to further dissociation or reaction of gas 
phase species to form more complicated radicals before the radicals can 
condense on the substrate. In a low pressure, low power silane (SiH.sub.4) 
immersion plasma, Matsuda et al. Thin Solid Films 92,171 (1982), have 
shown using mass spectroscopy that there are a host of gas phase species. 
These species include H, H.sub.2, Si, SiH, SiH.sub.2, SiH.sub.3, 
SiH.sub.4, Si.sub.2, Si.sub.2 H, Si.sub.2 H.sub.2, Si.sub.2 H.sub.3, 
Si.sub.2 H.sub.4, and Si.sub.2 H.sub.5. The most dominant line in the mass 
spectroscopy is the SiH.sub.2 line, even though it is only 12% taller than 
the SiH.sub.3 line and 125% taller than the Si.sub.2 H.sub.5 line. There 
is a wide spectrum of precursor species incident on the growing film. A 
further complication is that in conventional PECVD the substrate is 
immersed in the plasma region. This results in a large flux of charged 
species incident on the substrate during film deposition. The incident 
energies of these ions may be as high as 160 eV in some immersion systems 
(See Chapman, Glow Discharge Processes, John Willey & Sons, N.Y. 1980, 
Chap. 4). This can lead to ion implantation, energetic neutral embedment, 
sputtering, and associated damage. This residual damage must be annealed 
out during growth if high quality epitaxial layers are to be produced. 
Thus, this damage imposes a minimum growth temperature, based on annealing 
conditions below which high quality material cannot be obtained. Thus, 
there are two major problems associated with conventional PECVD: adequate 
control over incident gas phase species, and ion damage as a result of the 
substrate being immersed in the plasma region. 
RPECVD deposition of silicon nitride Si.sub.3 N.sub.4 and silicon SiO.sub.2 
for gate insulators in (In, Ga) As FET devices has recently been disclosed 
by Richard et al. J. Vac. Sci. Technol. A3(3), May/June 1985 (pages 
867-872). According to this reference, to deposit SiO.sub.2, for example, 
one reactant, O.sub.2, is excited in the plasma tube remote from the 
semiconductor substrate. The other reactant, SiH.sub.4, enters the reactor 
separately, near the substrate and is not excited to a plasma state. An 
important point is that one of the reactants, O.sub.2, bearing one of the 
component atoms of the SiO.sub.2, is introduced through the plasma tube. 
The process is thought to follow the following reaction model. Monosilane 
(SiH.sub.4) molecules interact with the metastable oxygen O.sub.x *(.sup.3 
P.sub.j) flux resulting from the remote plasma. The lifetime of the 
metastable oxygen is quite long, allowing pathlengths of 1-2 meters in the 
RPECVD reactor using the SiO.sub.2 deposition parameters. (In contrast, 
the pathlength of a typical metastable excited noble gas specie, e.g. He*, 
used in the RPECVD epitaxial growth of semiconductor layers, according to 
the present invention, is 5-30 cm.) This interaction leads to disiloxane, 
(SiH.sub.3).sub.2 O, formation in the gas phase. On the heated substrate, 
disiloxane is further oxidized by excess metastable oxygen, O*. This 
oxidation removes H from the silyl groups, SiH.sub.3. Dehydrogenation is 
accompanied by oxygen bridging of silicon atoms originally bound in 
adjacent disiloxane molecules on the heated surface. An excess of the 
plasma excited species is used to drive the dehydrogenation of the silyl 
groups to completion, minimizing Si-H bonding. Silicon-poor films do not 
form; thus the process is stable. For this case, CVD can be thought of as 
a polymerization of disiloxane brought about by oxidation of the SiH bonds 
of the silyl groups. 
Important features of the SiO.sub.2 process described by the above-noted 
Richard et al article are: 
1. In the SiO.sub.2 process, O is activated by the plasma in the plasma 
generation region and becomes incorporated in the deposited layers. 
2. The interaction between the reactive species existing the plasma 
generation region and the injected reactant results in the formation of 
the chemical groups. 
3. The lifetimes and therefore the pathlengths of the reactive species 
exiting the plasma formation is quite long: for metastable oxygen the 
pathlength is 1-2 meters. 
4. The dielectric material formed, SiO.sub.2, is an amorphous material and 
therefore has no long-range or crystalline order. For SiO.sub.2 
deposition, metastable O* promotes the further oxidation of disiloxane 
adsorbed on the substrate surface, which reduces the surface of adatoms 
and enhances the formation of amorphous material. 
Another prior art reference of interest is an article by Toyoshima et al, 
Appl. Phys. Lett. 46(6), Mar. 15, 1985, pp 584-586, which describes a 
PECVD process to deposit hydrogenated amorphous silicon. However, the 
deposited a-Si:H films retain from 5-30 atomic percent hydrogen in the 
deposited layers, which is critical to the performance of a-Si:H, but 
disastrous if one is trying to grow epitaxial Si layers. No process used 
to deposit high quality a-Si:H films has proven successful in depositing 
epitaxial Si layers. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention is to provide a new and improved 
apparatus and method for growing epitaxial semiconductor layers on a 
substrate, which overcomes the problems in the prior art PECVD techniques 
above-noted, including inadequacy of the control over incident gas phase 
species, ion damage to the substrate, and the lack of excited metastable 
gas species at the substrate to enhance surface mobility of the adatoms 
and formation of the epitaxial layer. 
Another object of this invention is to provide a novel apparatus and method 
employing an improved RPECVD approach, by which epitaxial semiconductor 
layers can be deposited on a substrate maintained at a relatively low 
temperature. 
Still a further object of this invention is to provide a novel RPECVD 
apparatus and method for growing epitaxial diamond layers on a substrate. 
These and other objects are achieved according to the invention by 
providing a new and improved RPECVD apparatus and method for growing 
semiconductor layers on a substrate wherein an intermediate feed gas, 
which does not itself contain constituent elements to be deposited, is 
first activated in an activation region to produce plural reactive species 
of the feed gas. These reactive species are then spatially filtered to 
remove selected of the reactive species, leaving only other, typically 
metastable, reactive species which are then mixed with a carrier gas 
including constituent elements to be deposited on the substrate. During 
this mixing, the selected spatially filtered reactive species of the feed 
gas chemically interacts, i.e. partially dissociates and activates, in the 
gas phase, the carrier gas, with the process variables being selected so 
that there is no back-diffusion of gases or reactive species into the feed 
gas activation region. The dissociated and activated carrier gas along 
with the surviving species of the feed gas then flows to the substrate. 
The surviving reactive species of the carrier gas completely react and the 
surviving metastable specie of the feed gas completely order the activated 
carrier species on the substrate whereby the desired epitaxial 
semiconductor layer is grown on the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, it is first 
noted that the reactor design and operating criteria discussed hereinafter 
are based on the principle of a remote region of activation of a gas or 
mixture of gases. The activated gas (gases) then plays several roles 
leading to the deposition of the semiconducting film. Because of the 
central importance of "remote region of activation" to the present 
invention, this terminology is first defined referring to FIG. 1. 
FIG. 1 shows schematically a section of a flow tube 12. An intermediate 
feed gas stream (single gas, vapor, or mixture) enters at input inlet 10. 
In the region of activation 14, the feed gas has its chemical reactivity 
increased. Chemical reactivity of the feed gas can be increased in many 
ways. For example, one or more components of the feed gas may be ionized; 
one or more components of the feed gas may be dissociated into more 
reactive species, such as converting water vapor into hydrogen and oxygen; 
or the internal energy of the feed gas may be increased without 
ionization. This can be accomplished by many methods. Some of these 
methods can be internal to the flow tube. A sample of these internal 
methods might include heaters, or catalytic surfaces, and electron or ion 
bombardment sources. Some methods could be external to the flow tube. A 
sample of these external methods might include a broad range optical 
sources (with an appropriately transparent tube), microwave or radio 
frequency power sources, or simple heaters. Whatever the feed gas(es), the 
combined means for activation, or the reactive species formed, in the 
activation region 14, energy is coupled into one or more gases, and that 
energy can contribute to subsequent chemical reactions. 
In experimental studies performed to date, an external radio frequency coil 
14.sub.1, shown in FIG. 2, concentric with the flow tube was used to 
activate the gas stream. 
Referring to FIG. 1, the concept of a "remote" region of activation in the 
present RPECVD technique will be described. By remote region of activation 
is meant two things: (1) the substrate is not located in a remote region 
of activation; (2) in any remote region of activation, only gas(es) from 
the inlet of that region of activation is(are) present, other gas(es) that 
may be present in other regions of the apparatus can not reach a remote 
region of activation by diffusion or other processes that would allow such 
gas(es) to enter through the exit of a region of activation. To ensure 
this requires both a suitable reactor design and a proper selection of 
operating parameters. In the flow system of the present invention, shown 
in FIGS. 1 and 2, the design of the physical separation of the various 
regions of the reactor, coupled with the flow velocity of the gas stream 
(which of course depends on the selection of process parameters) in those 
regions, determines whether back-diffusion of gases into a region of 
activation can occur. 
As shown in FIG. 1, the present invention includes a feed gas inlet 10 
through which a feed gas is entered into a plasma tube 12. In an 
activation region 14, the chemical reactivity of the feed gas is increased 
to produce reactive species of the feed gas which pass downstream of the 
exit plane 14.sub.2 in the downward direction shown in FIG. 1. Between the 
exit plane 14.sub.2 and the carrier gas inlet 18, the feed gas reactive 
species are filtered such that only the desired specie reaches the gas 
inlet 18 where it mixes and interacts with a carrier gas including a 
constituent element to be deposited introduced via the inlet 18 in a 
mixing and interaction region 20. 
In a working embodiment of the invention used to date, a radiofrequency 
coil 14.sub.1 concentric with the flow tube 12 has been used to create a 
"plasma" (glow discharge) of the feed gas in the activation region 14. 
Working examples have used either a pure noble gas plasma feed, as 
discussed hereinafter, or noble gas mixtures with hydrogen. The plasma 
environment in the activation region 14 contains many species, even with a 
simple feed gas like helium. In fact, the feed gas reactive species 
produced in the activation region 14 include ions, electrons, and a host 
of excited species all with different composite lifetimes which are 
influenced by various factors. The flow through the activation region 14 
carries the species downstream towards the carrier gas inlet 18 and a 
substrate 22 mounted in a deposition region 24 downstream of the inlet 18. 
The distance that the various species can travel before they are 
annihilated will depend on their composite lifetimes and the flow 
velocity. According to the invention, the flow velocity of the feed and 
carrier gases are controlled so as to control the relative abundance of 
selected of the reacted species at a given distance downstream of the 
region of activation, such as at the mixing and interaction region 20 at 
the carrier gas inlet 18. Thus, by controlling the gas flow rates, and by 
requiring the reactive species of the feed gas to pass from the exit plane 
to the mixing and interaction region 20, a spatial filtering region 26 is 
provided downstream of the exit plane 16, in which undesired reactor 
species are annihilated and only selected of the reactive species are 
passed downstream towards the mixing and interaction region 20. 
Spatial filtering as above described involves two aspects. First, some 
physical distance between the activation region which excites the feed gas 
and the region where the carrier gas is introduced must exist. And second, 
the lifetimes of the desired reactive species must be substantially longer 
than the lifetimes of those species not desired. Once these criteria are 
established, spatial filtering occurs because the pumping velocity of the 
reactive species determines how far downstream from the activation region 
various species will travel before they decay or be annihilated. For 
example, in a He discharge, electron impact excites He atoms into a host 
of excited electronic states. These states include 2.sup.3 P, 2.sup.1 P, 
3.sup.3 S, 3.sup.1 S, 3.sup.1 P, 3.sup.3 P, 3.sup.1 D, and 3.sup.3 D. All 
these states all have energies greater than the metastable 2.sup.3 S 
state. However, these states quickly decay to the ground state or one of 
the lower metastable states, 2.sup.1 S or 2.sup.3 S exponentially with a 
characteristic decay time. This decay time is less than 10.sup.-7 s. As 
the species are pumped from the discharge regions, the metastables and 
ground state He atoms are dominant. Of the two metastable states, the 
2.sup.1 S state has the shorter decay time or effective lifetime 
2.times.10.sup.-8 sec vs 6.times.10.sup.-3 sec for the 2.sup.3 S state. 
Thus, the host of highly excited He states in the plasma region have been 
spatially filtered to produce a desired flux of metastable 2.sup.3 S He 
atoms at the entrance to the gas mixing region. The unwanted excited 
species are completely attenuated exponentially along the length of the 
spatial filter compared to a factor of 3-150 attenuation (for plug 
velocities of 10-50 m/sec and spatial filter length of 0.3 m) for the 
desirable metastable specie. For this particular spatial filter design and 
system operating parameters, all activated gas feed species having 
effective lifetimes less than 4.times.10.sup.-3 sec will be completely 
annihilated in the spatial filter. 
The spatial filtering region 16 also acts as a backstreaming isolation 
region which in conjunction with the selected gas flow rates prevents 
injected carrier gas from the inlet 18 from back diffusing to the exit 
plane of 14.sub.2 of the activation region 14. 
The flux of activated noble gas species (and by activated it is 
specifically meant in the sense of internal energy and not kinetic energy) 
partially dissociates and activates (in the gas phase) the carrier gas. 
The flux of the activated noble gas species completely reacts and orders 
the activated carrier species onto the substrate 22 and results in the 
growth of an epitaxial semiconductor on the substrate. The flux of 
activated spatially filtered noble gas species enhances surface reactivity 
and reactant surface mobility in the growth of a single crystal epitaxial 
layer. The technique of the invention as applied to surface effects can be 
used in a low pressure process where the mean free path between the exit 
plane 14.sub.2 of the activation region 14 and the substrate 22 is such 
that no gas phase collisions occur. 
Three examples of specific semiconductive materials grown using the RPECVD 
technique according to the invention are next discussed. In these 
examples, there is no attempt to limit the invention to these specific 
features of remote region excitation technique, remote region feed gas, 
reactant feed gas, or specific reactor system design. 
In all three examples, reference is made to a schematic of a remote plasma 
enhanced chemical vapor deposition reactor, shown in FIG. 2. This 
representation of a RPECVD reactor primarily consists of a plasma tube 12 
in which is located the region of activation 14, and an activation source 
such as an rf coil 14.sub.1. The plasma tube 12 feeds into a deposition 
chamber 20.sub.1 in which is located a gas dispersal ring 18, and the 
substrate susceptor 28. Additional components include an electron gun 30, 
phosphorous screen 32, and a manipulator arm 34 used to perform Reflection 
High Energy Electron Diffraction (RHEED) characterizations of the 
substrate 22 and the epitaxial semiconductor film deposited thereon. The 
plasma tube 12 used consists of a 7.6 cm inside diameter pyrex tube. The 
plasma is driven by a 13.56-MHz rf generator with matching network. The 
substrates 22 are clamped to a graphite susceptor 28 heated internally by 
a tungsten halogen lamp (not shown). Substrate temperatures are calibrated 
using thermocouples (not shown) attached to the surface of a silicon 
substrate. Gasses are introduced through two separate gas feeds, the 
plasma feed gas inlet 12 and the carrier gas feed 18.sub.1 to the gas 
dispersal ring 18, which serves as the carrier gas feed inlet. The plug 
velocity of He or other noble gas through the 7.6 cm plasma tube 12 is 
high enough to prevent back-diffusion of GeH.sub.4, SiH.sub.4, or 
CH.sub.4. The plug velocities used are 3, 5, and 100 m/s for germanium, 
silicon, and diamond depositions, respectively. Also shown is an outlet 36 
for high vacuum pumping via a turbomolecular pump (not shown), an outlet 
38 for pumping the process gasses using a roots blower (not shown) 
together with a direct drive mechanical pump (not shown). Typical 
pressures are less than 5.times.10.sup.-10 Torr minimum base pressure when 
the process gasses are not flowing and 1-300 mTorr during epitaxial growth 
of a semiconductor layer. The vacuum intake to the roots blower is 
ballasted with a constant gas load to prevent antibackstreaming of oil 
vapors. Examples illustrating use of this process to epitaxially grow 
silicon, germanium, and diamond semiconductor layers are described below. 
Epitaxial growth of germanium is accomplished by flowing 200 sccm of He 
through the plasma tube and 20 sccm of 0.1% GeH.sub.4 in He through the 
gas dispersal ring 18. The pressure is controlled at 200 mTorr. To 
initiate deposition, 100 W is applied to the rf coil creating a He 
discharge plasma in the activation region 14. The substrate temperature is 
typically maintained between 225.degree.-450.degree. C., preferably at 
300.degree. C., during growth. 
Epitaxial growth is thought to occur through the following processes. The 
rf energy coupled to the plasma tube establishes a He plasma in the 
activation region 14. Through a variety of reactions many different 
species of excited He atoms and ions are created in the plasma, each 
having its own lifetime. These various species are caused to flow down 
from the plasma tube toward the gas dispersal ring 18 and the substrate. 
Each specie can be annihilated through a variety of mechanisms, and 
therefore, each specie has an average time that it can survive, or 
effective lifetime, until it is annihilated. This lifetime can be 
translated into an average distance it will travel below the plasma tube 
exit plane 14.sub.2 before it is destroyed. This distance is called the 
pathlength. The pathlength of a specie is determined by the effective 
lifetime of the specie and the plug velocity of the He gas flow. 
Consequently, the system and the growth parameters can be designed and 
chosen to cause undesired species to be spatially filtered in the spatial 
filtering region 16 and the desired specie to interact with the reactant 
molecules and to arrive at the substrate surface. In the present example 
the specie desired to interact with the reactant GeH.sub.4 is the 
metastable He(2.sup.3 S). These metastables play three important roles in 
the overall growth process: 
1. They dissociate the germane molecules through inelastic gas phase 
collisions; 
2. They have inelastic collisions on the growth surface of the film which 
enhances the surface mobility of the impinging species leading to epitaxy 
at low surface temperatures; and 
3. They can also play a role in dehydrogenation of surface reactants. 
These three functions for the metastables will be discussed in further 
detail below. 
The metastable He(2.sup.3 S) interacts with the GeH.sub.4 through inelastic 
gas phase collisions, and creates several reaction products. These 
products may include ionized and neutral GeH.sub.x species, where 
0&lt;x.ltoreq.4. The most probable products are GeH.sub.4 +, GeH.sub.3 + and 
GeH.sub.3 ; and the desirable product is GeH.sub.3. As the radicals 
condense on the substrate they must cross-link to form a germanium 
network. If this process is to form epitaxial layers of germanium, excess 
hydrogen carried by the free radicals must be liberated and the reactant 
species must have sufficient surface mobility to form an ordered solid. In 
the RPECVD process hydrogen removal occurs when He metastables collide 
with the growth surface. 
Epitaxial growth of silicon proceeds much in the same manner as growth of 
germanium. Again, growth is accomplished by flowing 200 sccm of He through 
the plasma tube 12 and by flowing 100 sccm He and 1 sccm SiH.sub.4 through 
the gas dispersal ring 18. A rf discharge plasma of 30 W is sustained 
during deposition. The deposition process occurs at a total pressure in a 
range of 50-300 mTorr, with 200 mTorr being preferred. The deposition rate 
is approximately 0.01 nm/s on a Si(100) 1.times.1 surface at 520.degree. 
C. Epitaxial growth has been achieved at temperatures as low as 
200.degree. C. with best results occurring at about 400.degree. C. The 
role of the metastable He in the epitaxial growth of silicon is thought to 
be much the same as described above for the epitaxial growth of germanium. 
Epitaxial growth of diamond may be accomplished by flowing a Noble gas (He, 
Ar, or Xe) through the plasma tube and methane, CH.sub.4, through the gas 
dispersal ring. One important factor that distinguishes growth of diamond 
from growth of either germanium or silicon is the poly-phasic nature of 
the deposited material. Depending upon the growth conditions, the 
deposited layers may be diamond, graphite, amorphous or glassy carbon, or 
mixtures of these materials. When a hydrocarbon such as methane is excited 
in a plasma, radicals of the form CH.sub.x are formed. As in the silane 
example, these radicals interact in the gas phase to form carbon-carbon 
bonds. The added complication in the carbon case results from the ability 
of carbon to form not one, but three hybridizations. Thus we get 
carbon-carbon bonding of the ethane form (sp.sup.3 hybridization), of the 
ethylene form (sp.sup.2 hybridization), and of the acetylene form (sp 
hybridization). The parallel between these gas phase precursors and their 
solid phase analogues is striking. Diamond (sp.sup.3 hybridization) has 
ethane type bonding, graphite (sp.sup.2 hybridization) has ethylene type 
bonding, and carbynes (sp hybridization) are chainlike compounds with 
acetylene type bonding. To grow smmiconducting diamond it is necessary to 
preclude the incorporation of wrong bonds of graphite-like or carbyne-like 
hybridization. The flux of gaseous precursors with incorrect hybridization 
onto the film surface is inevitable if the undesirable methane radicals 
(i.e., the ethylene and carbyne) are formed. Consequently, the design of 
the growth reactor and the choice of the growth parameters must be 
properly chosen to form precursors which upon condensation on the 
substrate promote sp.sup.3 hybridization and diamond growth. 
The growth of diamond proceeds technically in a similar way as does silicon 
and germanium. Typically, 500 sccm of He flows through the plasma tube 12 
with a 30 sccm dilute mixture of He, H.sub.2, and CH.sub.4 (4:10:1 by 
volume) flowing from the gas dispersal ring 18. A rf discharge of 80 W is 
sustained in the activation region 14 during deposition at a total 
pressure range of 10-1000 mTorr, typically less than 100 mTorr. The 
substrate temperatures is varied from 650.degree.-850.degree. C. The 
quartz plasma tube size is 1.5 in. o.d. insuring a high plug velocity 
necessary for transporting metastables and radicals to the substrate. 
Using these growth parameters, diamond films have been grown at the rate 
of approximately 2000 .ANG./hr. 
While the process technically is very similar to the silicon and germanium 
growth, the proper choice of noble gas and methane diluent is critical for 
promoting diamond growth. Because the energy of the He metastable is so 
high (.about.20 eV), the cross-section for collisional dissociation of the 
CH.sub.4 molecule is low. Thus, the depositional precursor species created 
by the He are CH.sub.4 +, CH.sub.3 +, or CH.sub.3, all of which are highly 
saturated CH.sub.x radicals Also the choice of methane diluent becomes 
more pertinent. Unlike the growths of silicon and germanium where the 
silane and germane were diluted in He, diamond growth is more facilitated 
with hydrogen dilution. The hydrogen serves two roles. First as a source 
of atomic hydrogen to the nucleating film, it more preferentially etches 
the graphitic bonds than the diamond bonds. Second, it moderates the gas 
phase chemistry promoting higher saturation of the CH.sub.x radicals. 
In the growth of diamond, H.sub.2 is used in surface reactions such as the 
etching of graphitic bonding units and in gas phase reactions to convert 
sp and sp.sup.2 bonded hydrocarbon radials to sp.sup.3 bonding forms. For 
this purpose the H.sub.2 source gs is activated by inversion to atomic 
hydrogen, H(H.sub.2 +energy.fwdarw.2H). In the basic reactor design shown 
in FIG. 2 this activation is carried out in one of two ways. By one 
method, Case 1, the H.sub.2 gas enters with the carrier gas stream through 
the gas disposal ring 18. The H.sub.2 is activated through interaction 
with energetic species of the feed gas that have passed through the 
spatial filter 14. Because this technique relies on energetic species of 
the feed gas (ex. metastable He (2.sup.3 S)), to activate both the 
hydrogen and the methane, dramatic reduction in deposition rate is 
observed as H.sub.2 :CH.sub.4 flow ratios are increased beyond 20:1. This 
limitation places a severe restriction on the range of gas mixtures for 
which reasonably efficient diamond deposition rates can be achieved 
(rates.gtoreq.1.ANG./sec). 
The second method (Case 2) of operation seeks to overcome this limitation. 
The H2 is introduced with the noble gas feed through the plasma region. 
This allows direct activation of the H.sub.2 (H.sub.2 +energy.fwdarw.2H) 
by the plasma. However if this scheme is used at powers typical for pure 
noble gas plasmas (80 W), dramatic reduction in the production of noble 
gas metastables by the plasma is observed. This is due to the fact that 
H.sub.2 dissociation occurs at lower energies than He metastable 
production. Thus the presence of H.sub.2 shifts the electron energy 
distribution of the plasma region to lower energies where metastable noble 
gas production is inefficient. If it is attempted to overcome this 
limitation by brute force (i.e. just by increasing the plasma power) a 
practical limitation is experienced in that high power H.sub.2 noble gas 
plasmas etch the plasma tube releasing (for the usual case of quartz or 
pyrex tubes) silicon, oxygen, and a variety of trace contaminants that 
travel downstream with the gas flow and are incorporated in the growing 
diamond film. 
A comprehensive solution to the problem involves an important extension of 
the concept of remote region of activation, that is, the use of multiple 
remote regions of activation. In this specific case two separate plasma 
tubes are operated. One with a H.sub.2 gas flow and plasma conditions 
optimized for H atom production, and one with a noble gas flow (e.g., He) 
with plasma conditions optimized for metastable production. This scheme 
avoids the tube errosion and contamination problems noted in Case 2 above, 
because mixed H.sub.2, noble gas plasmas are not formed and H.sub.2 plasma 
power densities are significantly reduced (reduction factor&gt;10). At the 
same time the separate H.sub.2 plasma is more efficient (by orders of 
magnitude) in the creation of atomic hydrogen as compared to the 
metastable activated scheme (Case 1 above). In addition since the cross 
section for energy exchange between excited metastable noble gas species 
and atomic hydrogen is much less (order of magnitude) then the same cross 
section for molecules H.sub.2, the multiple remote regions of activation 
concept allows both higher deposition rates (rate.about.7.ANG./sec) and a 
much broader range of accessible effective H:CH.sub.4 ratios than Case 1 
above, thus allowing improvements both in film quality and deposition 
rate. 
It should be noted that this is one specific example of the broad concept 
of multiple remote regions of activation, where such regions can differ in 
many ways including source material (e.g., gas feed as in the example 
above), means of activation (RF, thermal, etc.) spatial filter design, and 
so on. Indeed, the multiple activation region concept permits optimization 
of spatial filter designs which for mixed gas sources otherwise would of 
necessity involve design compromises based on the differing criteria for 
exclusion of the unwanted reactive species of two different parent gases. 
Further commenting on the spatial filtering employed according to the 
present invention, the design of the spatial filter in large part provides 
the flexibility in the remote region of activation scheme. First, it is 
noted that the region of spatial filtration works in both directions. 
First it is optimized to transmit the desired excited species from the 
region of activation (e.g., R.F. plasma) to the mixing region while 
suppressing the transmission of other excited species created in the 
region of activation. Second and equally important, it supports the remote 
aspect of a region of activation by allowing the suppression of 
back-diffusion of an species present in the deposition chamber. Thus 
without a spatial filter the region of activation cannot be remote. 
In order to realize the flexibility of the scheme, it is important to 
realize that the spatial filter is not simply a time delay where merely 
the intrinsic lifetime of the various reactive species is allowed to 
change the relative distribution of reactive species at different planes 
(times) downstream of the region of activation. If this was true, the 
technique would be restricted to enhancing only the intrinsically longer 
lived species and would be unable to separate species with nearly equal 
intrinsic lifetimes. 
Fortunately the intrinsic lifetime is only one component of the crucial 
parameter, the effective lifetime. The effective lifetime can be 
controlled in many cases. Consider the example presently disclosed herein 
for epitaxial growth of semiconductors (Si, Ge, C), oxygen and oxygen 
containing gases (e.g., H.sub.2 O) are common contaminants of feed gases 
such as He. Oxygen is readily activated to a metastable state in an R.F. 
plasma. In addition metastable oxygen species have lifetimes far longer 
(factor of 100) than noble gas metastables. Thus a small amount of oxygen 
or oxygen containing contaminants in the feed gas will have a 
disproportionately large effect on the growing film. However by 
incorporating a wall element 16.sub.1 providing an aluminum wall surface 
in a portion of the spatial filter we effectively quench the metastable 
oxygen while having no appreciable effect on the metastable noble gas 
flux. Wall interactions are extremely important (dominate at pressure&lt;10 
Torr) in determining effective lifetimes, thus flow dimensions and 
materials of construction of the spatial filter can be used to engineer 
the relative effective lifetimes and the resultant transmission 
characteristics of the spatial filter. Note that in this case (oxygen in 
noble gas) the spatial filter is designed to eliminate the normally longer 
lived species. 
An additional way to design the spatial filtering selectivity is to insert 
a baffle plate 16.sub.2 downstream of the plasma region generally midway 
in the spatial filtering region to increase the back pressure. In this 
way, those reactive species which have effective lifetimes which decrease 
as a function of pressure will have a greater susceptibility to 
annihilation. By locating the baffle plate 16.sub.2 generally midway in 
the spatial filtering region, sufficient forward flow of feed gas reactive 
species is produced to prevent back-diffusion. 
Because of the importance of wall interactions, the downstream end 16.sub.3 
of the spatial filter tubulation is normally extended through the chamber 
wall. Thus, the wall material of choice for gas stream interactions is 
maintained, independent of the considerations governing the selection of 
deposition chamber material. In the interior of the deposition chamber, 
the large increase in cross section relative to the spatial filter 
tubulation 16.sub.3 and the flow pattern resulting from the high velocity 
gas stream introduced through the spatial filter exit plane minimize any 
effect of wall interactions in the deposition chamber. This design feature 
is referred to as a reentrant source tubulation. It is shown in FIG. 2. 
One of the foremost problems in low temperature chemical vapor deposition 
is the removal of hydrogen from the nucleating film. For CH.sub.4 the 
spontaneous desorption of hydrogen occurs around 1000.degree. C. For 
SiH.sub.4 it is around 500.degree. C. For GeH.sub.4 it is around 
350.degree. C. Growth of diamond, silicon, or germanium below these 
respective temperatures then requires some other process besides thermal 
desorption to rid the deposited layers of hydrogen. One approach is to 
supply the surface which some other source of energy, photons, electrons, 
ions, etc. 
According to the present invention, a flux of metastables is supplied to 
the deposition surface. The same metastables which dissociate bonds in the 
gas phase can liberate hydrogen bonded on the nucleating solid. This is 
accomplished by keeping the carrier gas concentrations low to prevent 
total quenching of the metastable in the reaction zone. However, because 
quenching of the metastables is necessary to form precursor species which 
can deposit at lower temperatures, there is a compromise made between the 
deposition rate determined by how many metastables are quenched and the 
dehydrogenation rate determined by how many metastables survive the 
reaction zone and are incident on the substrate. 
One approach developed according to the present invention uses a 
sequentially pulsed growth technique where one deposits for some 
determinant period of time with the carrier gas flowing, removes the 
carrier gas, and dehydrogenates for some determinant period of time. The 
growth period is sustained long enough to deposit a monolayer of material. 
The dehydrogenation period is sustained long enough to rid the deposited 
monolayer of hydrogen. Because the carrier gasses are not quenching the 
metastable flux, the metastable flux to the surface would be maximum and 
the dehydrogenation time minimized. One might also expect that the 
metastable flux to the surface to impart energy to the adsorbed atoms and 
increase their mobility on the growth surface. In general, the higher the 
surface atom mobility is, the better the crystal will grow. The following 
are key operating parameters of the pulsed growth sequence technique of 
the present invention: 
Deposition Sequence 
Ar 200 sccm plasma tube 
Ar 50 sccm ring feed 
SiH.sub.4 10 sccm ring (2% SiH.sub.4 in He) 
Pressure 0.200 Torr 
rf power 40 W 
substrate temperature 200.degree. C. 
deposition time 1 min 
Dehydrogenation Sequence 
Ar 200 sccm plasma tube 
Ar 50 sccm ring feed 
SiH.sub.4 O sccm ring 
Pressure 0.200 Torr 
rf power 40 W 
substrate temperature 200.degree. C. 
dehydrogenation time 30 sec 
The overall sequence includes repeated alternate performances of the above 
noted deposition and dehydrogenation sequences. 
Remote plasma enhanced chemical vapor deposition, RPECVD, according to the 
present invention, avoids the problems associated with conventional PECVD 
techniques. There are three primary differences between RPECVD and PECVD. 
First, the parent gas molecules are not excited in the plasma region but 
instead react with excited, metastable gas species that flow from the 
plasma region. These metastable species have well-defined metastable 
energy states that are 4-20 eV above their ground state, depending upon 
which noble gas is used. By selecting the appropriate noble gas, it is 
possible to tune the energy increment used in the metastable specie. The 
coupling of this energy into the parent molecules during collisional 
events determines the gas phase species. The plug velocity of gas through 
the plasma tube prohibits back-diffusion of parent molecules into the 
plasma region. Because there is a fairly limited number of collisional 
by-products, the RPECVD process offers more control the type of species 
that is incident on the growth surface during a deposition. Table I shows 
a list of the collisional by-products obtained when different noble gas 
metastables collide with methane (see Bolden et al., J. Phys. B.: 3,71 
(1970) and Balamuta et al, J. Chem. Phys. 79,2822 (1983)), as follows: 
TABLE I 
______________________________________ 
NOBLE 
GAS SPECIES BY-PRODUCT 
______________________________________ 
He CH.sub.4.sup.+, CH.sub.3.sup.+, CH.sub.2.sup.+ 
Ar CH.sub.3, CH.sub.2 
Xe CH.sub.3 
______________________________________ 
A second primary difference between PECVD and RPECVD is that in RPECVD, 
unlike in conventional PECVD, the substrates are well removed from the 
plasma region, minimizing the plasma densities near the substrate. This 
should result in virtually no sheath fields between the substrate and the 
plasma in contrast to immersion systems. Ions created by Penning processes 
in the vicinity of the substrate see no large sheath fields to accelerate 
them. Furthermore, with typical deposition pressures between 100 and 300 
mTorr, the ions are thermalized, reducng their incident energy on the 
substrates. Considering the damage and embedment that has been observed in 
silicon from even moderately low energy ions (&lt;50 eV), reduction of ion 
flux and energy is certainly an advantage offered by RPECVD. This feature 
allows extremely low deposition temperature unconstrained by annealing 
considerations. 
A third difference between the disclosed RPECVD apparatus to grow epitaxial 
layers and typical PECVD apparatus is the ultra-high vacuum capability of 
the RPECVD apparatus. As explained below, this ultra-high vacuum 
capability of the RPECVD, base pressure less than 5.times.10.sup.-10 Torr, 
is required to obtain epitaxial layers of sufficient quality for 
electronic device applications. On the other hand, the PECVD apparatus has 
a base pressure typically of 1.times.10.sup.-6 Torr, and never better than 
1.times.10.sup.-8 Torr. These base pressures for the PECVD systems, are 
completely inadequate to grow epitaxial semiconductor layers. 
Important features of the RPECVD epitaxial growth process that clearly 
distinguish it from the SiO.sub.2 RPECVD deposition process are: 
1. In RPECVD epitaxial growth, no element of the deposited layers passes 
through the plasma region. In the SiO.sub.2 process, O is activated by the 
plasma and becomes incorporated in the deposited layers. 
2. Furthermore, the reaction between the metastable species exiting the 
plasma tube and the reactant injected by the gas dispersal ring is very 
different in the two examples. For RPECVD epitaxial growth, the 
metastable-reactant interaction results in the dissociation of the 
reactant or parent group. In the case of SiO.sub.2 deposition, this 
interaction results in the formation of chemical groups. 
3. The effective lifetimes and, therefore, the pathlengths of the 
metastable species exiting the plasma tube are quite different in the two 
cases: for metastable oxygen the pathlength is 1-2 meters, whereas, for 
metastable He*, the pathlength is a few centimeters. 
4. The Si epitaxial material is a crystalline material which needs 
long-range order to obtain good semiconducting properties. Conversely, the 
dielectric material SiO.sub.2 is an amorphous material, and, therefore, 
has no long-range or crystalline order. In RPECVD epitaxial growth, 
metastable He* promotes long-range order through increasing the surface 
mobility of the adatoms. For SiO.sub.2 deposition, metastable O* promotes 
the further oxydation of disiloxane adsorbed on the substrate surface, 
which reduces the surface mobility of adatoms and enhances the formation 
of amorphous material. While these differences are illustrated for RPECVD 
of epitaxial Si and amorphous SiO.sub.2, they apply to the general 
techniques of RPECVD of single crystal epitaxial layers of semiconductor 
materials and RPECVD of amorphous dielectric materials. Because these 
differences between the two deposition processes are quite fundamental, 
they therefore require different reaction chamber designs, especially with 
respect to cross-section, distance from the plasma tube to the substrate 
and plug velocity, and process parameters, including pressure, flow 
velocity, excitation levels and reactant concentrations. 
In any type of epitaxial process the order and cleanliness of the starting 
surfaces are of paramount importance. This is especially true in any low 
temperature epitaxial process where adsorbed atoms may not have enough 
mobility unless the energy is provided by some other source other than 
thermal. In the RPECVD apparatus and process according to the invention, 
two techniques that use the flux of excited or dissociated species from 
the plasma region to clean substrates of residual contaminants have been 
developed. The first technique involves dissociation of molecular hydrogen 
in the plasma region and transport of atomic hydrogen to the substrate 
surface. There the hydrogen reacts with residual carbon and oxygen atoms 
forming volatile compounds which leave the surface. Typical operating 
conditions for this process are 80-100 sccm H.sub.2 plasma, 4-5 mTorr, 35 
Watts, 300.degree. C. substrate temperature, and 20 scc time duration. 
Because atomic hydrogen may react with the glass walls of the plasma tube, 
this process has been refined. Now, metastable species of Ar, generated in 
the plasma region 14, interact with hydrogen flowing from the ring feed 18 
to form atomic hydrogen. The plug velocity of the Ar is kept high to 
prevent hydrogen from back-diffusing into the plasma region. Typical 
operating conditions for the refined cleaning process are 200 sccm Ar 
plasma, 50 sccm H.sub.2 ring, 100 mTorr, 50 Watts, 300.degree. C. 
substrate temperature, and 30 s time duration. Here as before, the atomic 
hydrogen reacts with the residual contaminants on the substrate to form 
volatile compounds and leave the surface. Without these effective hydrogen 
cleaning procedures, none of the epitaxial work would be possible. As with 
the epitaxial growth, it is the flux of particular selected species from 
the excitation region to the substrate that is the key to these processes. 
Following use of the cleaning procedure and before limitation of the 
epitaxial growth process it is very important to prevent re-contamination 
of the substrate surface by undesirable gas species which constitute the 
background or "base" pressure of the growth apparatus. These contaminants 
can impinge the substrate from a variety of sources including the several 
surfaces present in the growth apparatus. Therefore, it is very important 
to minimize the concentration of these contaminant species in the 
apparatus through maintaining a very low base pressure, less than 
5.times.10.sup.-10 Torr. Base pressures larger than this value, through 
contamination and disruption of the crystalline order of the cleaned 
substrate surface, will significantly degrade the quality of the epitaxial 
grown layer. For example, at a base pressure of 5.times.10.sup.-10 Torr, 
10 percent of the substrate surface, will be covered with new contaminants 
after only 4 minutes. Yet, a 10 percent surface contamination will create 
an unacceptably large crystalline point defect density between 10.sup.20 
and 10.sup.21 in the epitaxial layer. Consequently, the requirement of 
base pressure less than 5.times.10.sup.-10 Torr is a minimum requirement. 
It should further be understood that while the RPECVD technique of the 
invention employs a feed gas activation region which certainly is 
physically remote from the deposition region, it is also chemically remote 
because back-diffusion into the activation region is prevented and only 
selected activated gas species such as the metastable noble gas species 
arrive at the substrate surface. 
Other considerations important to the present RPECVD apparatus and method 
growth of epitaxial layers of semiconductor materials include: 
(1) Design of an ultra high vacuum, pressures less than 5.times.10.sup.-10 
Torr, reaction vessel to maintain surface cleanliness after the in situ 
cleaning procedures. 
(2) Incorporation of RHEED equipment into the reactor design to allow 
qualification of the surfaces prior to and after the cleaning procedures 
and/or the depositions. 
(3) Use of a reentrant plasma tube to eliminate metal exposure to the 
plasma environment. It is well known that metals in active plasma 
environment become eroded leading to metal contamination. 
(4) Installation of a complete vacuum system bakeout system from the 
pump-mouths to the gas bottles. This allows system base pressures to be 
less than 5.times.10.sup.-10 Torr. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.