U.S. patent application number 10/512838 was filed with the patent office on 2005-08-04 for large area deposition in high vacuum with high thickness uniformity.
Invention is credited to Amorosi, Simone, Benvenuti, Giacomo, Halary-Wagner, Estelle.
Application Number | 20050166846 10/512838 |
Document ID | / |
Family ID | 29275950 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050166846 |
Kind Code |
A1 |
Benvenuti, Giacomo ; et
al. |
August 4, 2005 |
Large area deposition in high vacuum with high thickness
uniformity
Abstract
The invention relates to an effusing source for film deposition
made of a reservoir comprising one hole characterized by the fact
that the hole diameter is less than one order of magnitude than the
mean free path of the molecules determined by the pressure and its
thickness is at least one order of magnitude smaller than the
diameter. Preferably the source has several holes.
Inventors: |
Benvenuti, Giacomo;
(Ecublens, CH) ; Amorosi, Simone; (Ecublens,
CH) ; Halary-Wagner, Estelle; (Ecublens, CH) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
29275950 |
Appl. No.: |
10/512838 |
Filed: |
November 24, 2004 |
PCT Filed: |
May 2, 2003 |
PCT NO: |
PCT/CH03/00285 |
Current U.S.
Class: |
118/726 |
Current CPC
Class: |
C23F 4/02 20130101; C30B
23/066 20130101; C23C 14/24 20130101; C23C 14/243 20130101 |
Class at
Publication: |
118/726 |
International
Class: |
C23C 016/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 3, 2002 |
WO |
PCT/CH02/00241 |
Claims
1. Effusing source for film deposition made of a reservoir
comprising one hole characterized by the fact that the hole
diameter is less than one order of magnitude than the mean free
path of the molecules determined by the pressure and its thickness
is at least one order of magnitude smaller than the diameter.
2. Effusing source according to claim 1 designed to impinge
effusing molecules on a deposition area situated at a distance of
at least one order of magnitude larger than the hole diameter.
3. Effusing source according to claim 1 designed in such a way that
the ratio "number of molecules impinging on a deposition area"
versus "total number of effusing molecules" is more than 20%.
4. Effusing source according to claim 1 wherein the source is
designed in such a way that the distribution of impinging molecules
on a deposition area in the molecular regime only depends on the
source relative position and on its molecular angular
distribution.
5. Effusing source according to claim 1 wherein it includes a gas
phase collisions forbidden volume and pumping apertures in the
surface of said volume to reduce the contribution of walls surface
scattered molecules for certain angles.
6. Effusing source according to claim 1 comprising several
holes.
7. Effusing source according to claim 6 wherein the holes are
distributed on a ring or on several rings with a tilt angle.
8. Effusing source according to claim 6 wherein the holes are
distributed on a spherical surface such as an hemisphere that has a
diameter of at least one order of magnitude smaller than the
distance of the source to the deposition area.
9. Effusing source according to claim 6 with under-cosine
distribution of sources for small angles and over-cosine
distribution of holes for larger angles.
10. Effusing source according to claim 6 with fractal distribution
of holes.
11. Effusing source according to claim 10 with asymmetric fractal
distribution of holes.
12. Effusing source according to claim 6 with variable angular
distribution of holes and without any moving part.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to thermal or laser-assisted
(electron or ion beam-assisted) film deposition with chemical
precursors in the molecular regime.
STATE OF THE ART
[0002] The business and technology of large area vacuum coatings
have made significant progresses in the last two decades and are
still proceeding with great expectation for the future [1, 2]. In
particular, we can mention typical applications as architectural
and automotive glasses, solar cells, and micro-optoelectronics
applications that require larger and larger substrates to reduce
costs.
[0003] It is commonly believed that the alternative approach of
vacuum deposition is difficult to scale up to industrial
applications. Furthermore, vacuum deposition is also known to be
inherently more expensive and difficult with these two drawbacks
increasing as lower pressures are aimed at. However the use of
vacuum deposition has some advantages for industrial applications.
Indeed, high-throughput techniques, such as MOCVD, can produce
large quantities of materials, while the precise in-situ control of
thickness, purity and composition that can be achieved through MBE
has been invaluable in developing the technology. A hybrid
technique (called CBE, MOMBE, or GSMBE), consisting in molecular
beams of CVD precursors, could provide a good solution: high
control, possibility to grow good quality devices and higher growth
rates than those typically obtained in conventional MBE. For these
reasons, by 1990 most of the major electronics and
telecommunication companies had a research effort in Chemical Beam
Epitaxy [3].
[0004] Several patent documents discuss the increase of deposit
uniformity both in thickness and in chemical composition. For
vacuum deposition with chemical precursors, two precursor transport
regimes have to be distinguished: Chemical Vapour Deposition (CVD)
in the viscous or the transition regime and Chemical Beam Epitaxy
(CBE) in molecular regime (also called Metal-Organic Molecular Beam
Epitaxy (MO-MBE) or Gas Source MBE (GSMBE)). As a general rule,
most of the patents are for CVD systems to solve the classical
problems reported in literature. In the molecular regime very few
patents have been deposited.
[0005] As a summary, we can report the following points:
[0006] CVD processes have a much higher production capacity, but
suffer from poor utilization efficiency of the expensive and toxic
reactants gases (L. M. Fraas, in U.S. Pat. No. 4,828,021). In
opposition, MBE devices are not easily scalable to meet the needs
of the growing market.
[0007] It should be desirable to have an apparatus and a process
that could be extended to the general use of any metal organic gas
in a vacuum environment to optimise the growth of uniform
semiconductor layers on large area substrates while minimizing the
disposal problems of the toxic reactant gases and the reaction
by-products.
[0008] A greater degree of control over uniformity of coating
and/or etch rate, with an ability to alter uniformity profiles in a
selective and controlled manner, is required in deposition
techniques (J. J. Schmitz reports U.S. Pat. No. 5,387,289).
[0009] Different gases in the source or in reaction chamber are
liable to react very rapidly with each other (i.e. arsine and
triethyl-indium). They cannot be mixed in advance and should be
introduced simultaneously, but separately. Concentric annular
arrays of sources have been proposed to provide different gases
(Frijlink U.S. Pat. No. 4,961,399), but several problems are still
open.
[0010] Rotation, or planetary motion, was also introduced to
provide uniform thickness (Frijlink U.S. Pat. No. 4,961,399), but
light processes are incompatible with such a motion.
[0011] Plurality of vapour streams to generate partially
overlapping molecular angular distributions has been proposed
(Chuong Van Tran U.S. Pat. No. 6,082,296). Zinck reports a source
with an array of micro-channels to use as a gas source for MBE
(U.S. Pat. No. 5,188,671). With this over-cosine collimated source,
the growth rate can be increased as the local pressure at the
surface of the wafer can be over 100 times higher than the residual
pressure.
[0012] A state of the art is given by Tanabe for vacuum coatings of
large area substrates and high precursor efficiency use (patent EP
0 982 411 A2). This is the closest result found for patent to our
proposed invention, but uniformity is only about 5-10% and is hence
not suitable to many applications.
[0013] What can be said is that no global solution has been found
up to now and the above cited patent document have only solved some
of the specific mentioned problems.
[0014] The following points and basic understanding are reported by
non-patent literature. In particular, viscous and transition
regimes are poorly, if not at all, compatible with light (electron
ion)-assisted processes. Furthermore, flexibility is poor to
achieve good thickness uniformity (inter-correlation of deposition
parameters), while up-scaling to larger substrates or upgrading to
new materials is not straightforward and can be very difficult. We
will hence focus on the molecular beam regime.
[0015] In the molecular regime, gas-phase collisions are assumed
negligible compared to molecules to walls collisions. Equivalently,
we can say that the mean free path of molecules is large compared
to the chamber geometry. This line of sight travelling of molecules
leads to exact modelling of their distribution on whatever surface
is used as a deposition area and is independent of parameters such
as substrate temperature or precursor molecular species. Due to the
closed relation with light propagation, several authors have
compared particle effusion with light in a diffusive medium [6, 7].
In molecular beam conditions, we can suppose to be in conditions
that are similar to a very transparent medium for light. In
particular, the gas inlet system, consisting in one or more
effusive sources with given molecular angular distribution and
relative position compared to the deposition area, can be assumed
theoretically as the unique parameter influencing molecular
impinging rate distribution on the deposition area [8, 9].
[0016] Experimentally however, several effects can modify this view
[10]. Among these effects, we can list vapour pressure
uncertainties, the cosine emission law from walls when the walls
are not totally inert against the vapours (surface diffusion [11]
and specular reflexions of molecules on surfaces [12]). These
effects are however very limited to the working temperatures below
100-200.degree. C. required avoiding chemical precursor adsorption
and sufficient vapour pressure. Another effect that can make fail
attempts to produce uniform impinging rate on the deposition area
is that molecules can bounce on walls inside the deposition
chamber. These scattered molecules would hence lead to uncontrolled
deposition. This drawback can be reduced with liquid nitrogen
cryopanels that will condense both unused precursor and
by-products. Analytical modelling [13, 14], Monte Carlo simulations
[15-17], and experimental work [9, 18, 19] have been used
extensively to achieve parameters optimisation. Very good agreement
is usually achieved between experiments, simulations and analytical
modelling if the previously described precautions are taken.
[0017] Sources flow angular distribution and their relative
position to the deposition area can hence be assumed, practically
and not only theoretically, as the key parameter to achieve high
uniform thickness. Effusive sources are defined as apertures
between the main chamber, in which precursor molecules in the form
of molecular beams will impinge on the deposition area, and a
reservoir (in our case a single pre-chamber common to all the
sources for a given precursor) in which higher-pressure conditions
occur ranging from the viscous, through the transition, to the
molecular regime. Evaporation sources are used in physical vapour
deposition processes and generally require very high temperatures
necessary to reach sufficient vapour pressure, while gas sources
use volatile chemical precursors resulting in high enough pressures
even at temperatures as low as 100.degree. C.
[0018] The difficulty in the molecular flow regime is to achieve
simultaneously both high thickness uniformity and high growth rates
[4], which are usually related to high efficiency use of precursor
[4, 5], and high initial investment costs [1] as reducing the size
of the reactor usually allows reduced equipment costs, but also
leads to poor thickness uniformity.
[0019] An important aspect to be considered in the deposition
process using effusive sources is the growth rate, which is tightly
related to the source flow throughput. This flow can be assumed
proportional to the aperture area of the source and to the vapour
pressure of the gas at a given temperature even if some exceptions
exist to this rule (for example collisions inside small capillaries
reducing effusion [20]).
[0020] Large aperture sources are usually required both in physical
evaporation and gas sources configuration. The effusing area can
however be either a single large area source or distributed among
several smaller sources. Small apertures have not been used very
often in physical vapour deposition sources because difficulties
appear due to regulation of each source. These sources are
furthermore expensive and unstable as the vapour condensation and
evaporation can modify the effusion. Gas phase sources are
user-friendlier and allow the use of smaller sources without the
described drawbacks, but have only been investigated recently.
[0021] The advantage of small sources lays in the more precise
control of flow throughput as discussed by Vassent [10]. The limit
is given for ideal sources (very small effusion orifices compared
to molecular mean free path related to pressure (about 0.5 mm at
10.sup.-2-10.sup.-3 mbar)) by the pressure (temperature) control of
the cell. In large aperture cells thermal equilibrium is not
warranted, while small apertures, like in Knudsen effusion, work as
thermal blackbody.
[0022] Another reason that makes small gas sources more attractive
is that sources are generally tilted to the normal of the
deposition area to increase uniformity [21]. Physical reasons may
limit this tilting angle that is not always optimal for evaporation
sources due to liquid geometry level considerations [5]. Finally,
smaller sources compared to a single large source are interesting
as light induced or light enhanced deposition is incompatible with
substrate motion. For sources that are stable over the time
interval required for a few revolutions of the substrate, substrate
rotation is equivalent to a continuous ring of identical sources
[18].
[0023] In addition to the growth rate and geometrical distribution
of sources as discussed here above, another important parameter has
to be discussed: the angular distribution of molecules effusing
from the source. Literature on the subject is extensive from first
works of Knudsen at the beginning of the XX.sup.th century and will
not be reported here. A complete review is given by Stekelmacher to
understand effusion mechanism and the different effusion sources in
different regimes [22].
[0024] The first quality of a source with respect to angular
distribution is reproducibility and stability. One of the problems
in MBE sources is that the source angular distribution may vary to
a large extent as a function of the filling level. This leads to
poorly controllable angular distributions of effusing molecules
with time [23]. Gas sources are generally more reliable, but care
must be taken to control molecular angular distribution that can
depend on pressure. Another error that can account for different
angular distributions is that the source dimension aperture can
have a deep impact on the molecular distribution. In opposition to
the previously discussed case where the orifice aperture was
compared to the mean free path, the source aperture geometry must
now be compared to the distance between the source and the
deposition area. Leiby has studied this for molecules effusing from
circular and rectangular apertures [14]. He demonstrated that for
large distances of the source from the deposition area compared to
the source diameter, every source can be considered as a point
source. Point sources are more easily dealt with as they can be
assumed as ideal sources (the angular distribution is constant as a
function of the distance to the deposition area). Winkler [24], for
example, reports a formalism to consider non point-like circular
sources that follows a procedure previously described by Olander
[25].
[0025] Point sources can arbitrarily be subdivided into three
classes as reported by Cale [8]: cosine sources (Knudsen sources)
over-cosine and under-cosine sources depending on their degree of
collimation. The most investigated sources are the over-cosine
sources [22] [26] mainly because they can reach higher growth rates
[4] and optimise precursor use. However, such collimated sources
lead to several problems. Among these we can list a general
increase in source-deposition area distance to keep uniform
impinging rates and higher sensitivity to misalignments [10].
Under-cosine sources in opposition to more focused sources have
been poorly discussed in literature.
[0026] The obtained results reported by the scientific community
with regard to thickness uniformity deposition are in general
difficult to evaluate if the parameters of reactor size versus
substrate size are introduced. Uniformity of a few percent can be
achieved with a single source and a rotation of the substrate. An
order of magnitude improvement in the film thickness uniformity can
be obtained with planetary motions of the substrate or by
introducing suitable masks [27] or both. However, thickness
uniformity is closely related to the deposition area diameter, the
precursor efficiency use, and growth rates [4]. As a general rule,
thickness uniformity decreases rapidly with deposition increasing
area and with reduced distance of the sources to the deposition
area. Jahan et al. report a thickness uniformity of about 3% on a
2-inch diameter substrate [28]. Aers reports a uniformity of 3-4%
over a 3-inch substrate [29]. Spring Thorpe achieved a thickness
uniformity of 3% on a 76 mm diameter substrate, but uniformity
quality degrades rapidly to 20% as the crucible empties [5]. All
these references however, do not report any source-deposition area
distance neither an evaluation of the ratio between the molecules
impinging on the deposition area compared to the total effused
molecules, which are important parameters for an optimized cost
effective deposition system. A systematic approach and a good
understanding of these parameters are still lacking.
SUMMARY OF THE INVENTION
[0027] The invention relates to a novel small point gas source
(using chemical precursors) for vacuum deposition in the molecular
regime able to lead to highly uniform thickness on large areas with
small reactor size and high precursor efficiency use. The source
design is also compatible with light-assisted (electron or
ion-assisted) deposition. As alternative to deposition, homogeneous
etching can be achieved if it relies on impinging rate of etching
chemical on a given area.
[0028] The reactor design is such that the source relative position
and its angular distribution of effusing molecules are the only
parameters accounting for their distribution on the substrate.
Because of the line of sight propagation of the molecules in the
molecular regime, the distribution of impinging molecules on the
substrate can be calculated mathematically.
[0029] In particular, the reactor is composed of a precursor
reservoir heated (for example by a thermo-regulated oil-bath up to
temperatures of 200.degree. C.) connected to a pre-chamber with a
ring shape allowing irradiation of the substrate through its
centre. Four or more sources, consisting in holes in a vacuum tight
sheet separating the pre-chamber from the deposition chamber, where
the substrate is positioned, are responsible of the molecular
impinging distribution on the substrate. The effusing rate is
controlled by regulating the pressure and the temperature of the
pre-chamber resulting in only one single cost efficient system
control for all the sources. All the system is baked at a
temperature at least 20.degree. C. higher than the temperature of
the precursor reservoir to avoid precursor condensation. A
cryo-panel is used to condense all the molecules effusing from the
sources that do not collide directly on the substrate and of the
by-products resulting from the chemical decomposition of the
precursor. A pumping system is used to achieve a vacuum between
10.sup.-12 and 10.sup.-3 mbar in the deposition chamber and a
vacuum between 10.sup.-6 and 10 mbar in the pre-chamber.
[0030] Furthermore, the characteristics of the sources have been
chosen for the following reasons:
[0031] 1. Gas sources:
[0032] Low temperatures in the effusing cell.
[0033] Multiple sources are easily achievable (cost effective
solution).
[0034] Small sources do not get clogged.
[0035] Stability in time.
[0036] All orientations are possible independently on the filling
level of the source.
[0037] Generally higher throughput.
[0038] Higher reproducibility.
[0039] Ideal conditions to model mathematically.
[0040] 2. Small sources (small diameter compared to mean free path
of molecules in the pre-chamber):
[0041] Thermodynamic equilibrium is guaranteed.
[0042] Ideal to model mathematically.
[0043] 3. Point sources (small diameter compared to distance of the
source to the deposition area):
[0044] No variation of angular distribution as a function of
distance to the deposition area.
[0045] Ideal to model mathematically.
[0046] 4. Large aspect ratio between hole thickness and hole
diameter:
[0047] No opaque regime that reduces effusion.
[0048] Effusing flow oriented normal to the surface. No
complications due to poor controlled holes orientation.
[0049] 5. New ad hoc molecular angular distributions:
[0050] Non-focused sources to minimize misalignments effects on
thickness uniformity for deposition area.
[0051] Non-focused sources to minimize reactor size: a larger
acceptance angle is possible compared to more focused
distributions.
[0052] Reduced precursor flow outside the deposition area (i.e. for
angles lower and/or greater than critical angles (asymmetric
sources)).
[0053] 6. Single pre-chamber connected to all the effusing
sources
[0054] Simple and reproducible
[0055] Cost effective solution
[0056] Graded composition possible by precursors
inter-diffusion
[0057] 7. Disposition of the sources on one or more rings with
possibly a tilt between the rings
[0058] Avoid substrate rotation and/or planetary motion
[0059] Different materials co-deposition with controlled
composition and thickness uniformity
DETAILED DESCRIPTION OF THE INVENTION
[0060] A. The Effusion Source
[0061] A typical effusion source is a hole of 0.5 mm drilled in a
foil of 0.05 mm of thickness between a reservoir (pre-chamber) with
a pressure of 10.sup.-3-10.sup.-2 mbar and the deposition chamber
with a pressure below 10.sup.-3 mbar. The hole dimensions however,
depend on the pressure in the pre-chamber and on the substrate
size, and could vary from 0.001 and 50 mm. Furthermore, the
thickness of the hole is about one order of magnitude (or more)
smaller than the diameter, while the distance of the source to the
deposition area is one order of magnitude (or more) larger than the
diameter of the hole.
[0062] B. Disposition and General Description of the Sources
[0063] The combination of several sources may allow substrate
rotation avoidance. Several holes are uniformly distributed on an
annular geometry (see FIG. 1). The formula that describes the
distribution of impinging molecules on a planar surface for several
cost distributions of the effusion sources distributed on a ring is
the following: 1 I ( ) = I 0 cos n ( ) I tot = I 0 i = 1 p [ h h 2
+ R 2 + r 2 - 2 Rr cos ( - i 2 p ) ] n
[0064] For Knudsen effusion sources n=4 (cosine sources).
[0065] To illustrate the concept of reactor size reduction with
opportune thickness uniformity, several examples of angular
distribution shaping (as reported in the summary of the invention
point 5) are provided in Table-1. Analytical modelling of precursor
transport is applied to find the adequate parameters providing
molecular impinging uniformity better than 1% on a 150 mm substrate
for various arbitrary cos.sup.n distributions, distance h of the
sources to the substrate, and radius R of the ring on which are
distributed the sources (see Table-1). With the parameters reported
in Table-1, the distributions are identical with an error less than
0.1%. These cos.sup.n sources (with n<4 under-cosine sources)
are not existing sources nor are they to be considered optimal
sources, but are used only to show that the trend of the reactor
size is reduced with decreasing the focusing of the source.
1TABLE 1 Geometrical disposition of the holes (both R, h varied) as
a function of the effusing beam angular distributions (cos.sup.n)
to achieve uniformity better than 1%. Distribution Cos.sup.1
Cos.sup.2 Cos.sup.3 Cos.sup.4 Cos.sup.5 Cos.sup.6 Cos.sup.8 R value
(mm) 99.2 113.2 119.57 122.4 124 127 129 h value (mm) 57 100 133
159 181.5 205 242
[0066] Introducing a tilting angle (see FIG. 2), the dimensions can
be further decreased. The more general equation is: 2 I tot = I 0 i
= 1 p [ - x s ( x - x s ) - y s ( y - y s ) + h x s 2 + y s 2 tan (
( x - x s ) 2 + ( y - y s ) 2 + h 2 ) ( x s 2 + y s 2 ) ( 1 + tan )
]
[0067] Similar distributions (within still an error of less than
0.1%) are achieved with parameters reported in Table-1 and Table-2.
We can see that the dimensions can be further reduced with a small
tilting angle of the sources. Furthermore, several tilted
concentric rings could be considered equivalent to planetary
motion.
[0068] It can be shown that this angle is smaller and less critical
on uniformity distribution for under-cosine sources.
2TABLE 2 Geometrical disposition of the holes (R, h and .PHI.) for
Cos.sup.1 distribution to achieve distribution uniformity better of
1% on a 150 mm wafer. Tilt angle (.pi./2 - .PHI.) 0.degree.
3.degree. 6.degree. 9.degree. R value (mm) 122.4 98 96 80.8 h value
(mm) 57 48 38 17.2
[0069] C. Molecular Beam Shaping
[0070] The second point discussed is how to achieve the desired
distribution shaping of the sources required for the already
discussed reasons in point 5 of the summary of the invention. In
particular, under-cosine distributions for small angles and
over-cosine distributions for greater angles corresponding to
regions outside the substrate (see FIG. 3) are aimed at. A tilt
angle could be considered the first step in molecular beam
modification (relative to the substrate), but this method is very
limited. Two other solutions are proposed, as examples, but should
not be considered exhaustive.
[0071] First Source Design
[0072] The first design is based on selecting and promoting
molecules escaping the source with a given angle. Two different
types of molecules will escape the source: the molecules that had
the last collision inside the pre-chamber with another molecule and
those that had the last collision on a surface inside the
source.
[0073] On one hand, a volume below the effusing aperture is a
forbidden region for gas phase collision. On the other hand, a
pumping aperture will act as a trap for surface scattered
molecules. Counterbalancing both effects could lead to shaped
distributions. Furthermore, variable pressure configuration could
lead to variable angular distributions without any moving part or
modifications of the set-up.
[0074] An example to reduce molecules effusing at small angles is
reported in FIG. 4 with a cone-like shaped forbidden volume. Any
kind of structure could serve to reach this purpose. In the
cone-case, the apex may be cut and a hole provides a pumping
aperture. If the cone is positioned under the hole at a distance
b.noteq.0, we will have a progressive increase of the volume, as
angle will increase that will depend on the ratio between the
distance b and the mean free path .lambda.. In particular, variable
pressure configuration will lead to variable effect of the cone as
the mean free path is changed. A particular case of this
configuration could be a negative parameter b; i.e. the cone exits
the pre-chamber through the hole. As multiple sources are to be
used, mechanical complexity of multiple cones can be avoided by
producing a continuous structure.
[0075] Second Source Design
[0076] As a general rule, any kind of molecular angular
distribution can be achieved by opportune disposition of several
holes a 3-D surface. However, only in the case in which these
sources are dispatched close together compared to their distance to
the deposition area we can consider them as a single point source
with the advantage of easy mathematical modelling. Furthermore, the
total area of the holes should be small compared to the area
separating the 3D surface from the pre-chamber to avoid gas
depletion and pressure gradients. Furthermore, each single hole
must satisfy the rules introduced previously in the summary of the
invention.
[0077] A particular shape of interest is a hemisphere (see FIG. 5).
In particular asymmetric sources are easily produced leading to gas
waste reduction.
[0078] Figures Summary:
[0079] FIG. 1:
[0080] Disposition of effusion holes with annular geometry (1). R
is the radius of the ring on which are distributed the holes, r is
the distance from the centre on the substrate (2), and h is the
distance of the substrate from the holes containing plane (3).
[0081] FIG. 2:
[0082] .PHI. is the tilt angle of the surface on which is the
source S and is oriented towards the z-axis.
[0083] FIG. 3: Shape of the ideal angular distribution (1), to
achieve high impinging rate uniformity and small reactor size,
compared to Knudsen effusion (2). Both curves are normalised to
achieve same number of effusing molecules. In the ideal curve (1),
60.degree. is assumed to be the cut-off plane that discriminates
between deposition on and outside the deposition area. A rapid
decrease of the molecules occurs after this critical angle and the
under-cosine distribution (4) is modified resulting in an
over-cosine distribution (3). Asymmetric sources could also prove
useful.
[0084] FIG. 4: The source is composed of a hole (1), a volume (2)
that reduce the region where gas phase collisions are allowed, and
a pumping aperture (3) that reduce the surface from which scattered
molecules can exit the source. Gas phase collisions are restricted
to larger angles in FIG. 4a. Surface scattered molecules are
reduced for small angles FIG. 4b. When the parameter b is not null,
a variation in pressure induces a variation of the mean free path
.lambda.. A different contribution of the cone to angular
distribution is hence achievable as a function of pressure in FIG.
4c. The structure can also exit the source as reported in FIG.
4d.
[0085] FIG. 5a: Fractal source composed of a distribution of
effusing holes on a hemispherical surface.
[0086] FIG. 5b: Asymmetric fractal source with preferential
orientation of the molecular beam.
II. BIBLIOGRAPHY
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[0089] 3. Tsang, W. T., Progress in chemical beam epitaxy. Journal
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[0090] 4. Scheinowitz, D. A., et al., Comparative study of
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