 |
Description  |
|
|
BACKGROUND OF THE INVENTION
This invention relates to a method of manufacturing semiconductor devices
and apparatus for the method and, in particular, to a method of etching a
layer on a wafer.
Recently, with an increasing degree of integration of a semiconductor
device, reduction of the number of particles has become an important
issue.
To decrease the particles, a vapour-phase cleaning method has been
investigated, in particular, vapour-phase hydrogen fluoride (HF) treatment
is investigated as an effective method to remove native oxide and obtain a
clean surface. An apparatus and method for removing native oxide layers
are disclosed in PCT International Publication No. WO91/06795.
On the other hand, a new phenomenon, that an etching rate greatly depends
on properties of the objective layers in vapour-phase etching, is also
observed. The phenomenon is described in a paper contributed by Nobuhiro
Miki and four others to the IEEE Transactions on Electron Devices, Vol.
37, No. 1 (January 1990), pages 107-115, under the title of "Gas-Phase
Selective Etching of Native Oxide". The Miki et al paper reports that an
etching rate of the objective layers considerably varies when a water
vapour concentration in the vapour-phase hydrogen fluoride is controlled
and that a change in etching rate depends on the properties, in
particular, compositions of the objective layers.
More particularly, the Miki et al paper reports that reducing of the water
vapour concentration in the vapour phase to 0.1 ppm or less enables
selective removal of objective layers consisting of phospho-silicate glass
(PSG). The process has the advantage that the objective layers can
selectively be removed without etching of layers other than the objective
layers.
However, the selective etching process of the Miki et al paper requires
strict control of a vapour-phase hydrogen fluoride concentration and of
the water vapour concentration. Reducing the water vapour concentration to
less than a few ppm as described in the Miki et al paper is very difficult
and costly for the following reasons. In order to reduce the water vapour
concentration, the concentration of impurities in the vapour-phase
hydrogen fluoride must be extremely reduced. Furthermore, an etching
chamber for the selective etching process must have a structure which
avoids mixing of the surrounding atmosphere and the like into the etching
chamber and adherence of water to the etching chamber. In addition,
etching the objective layers with vapour-phase hydrogen fluoride produces
water as a reaction product. Therefore, the water vapour concentration
undesiredly increases to adversely affect stable progress of the selective
etching process.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a silicon oxide etching
process and system that increases the difference in an etch rate caused by
a difference in quality or impurity concentration in the silicon dioxide
layers so that silicon oxide with a specific quality can be selectively
etched in a stable way at wide process conditions.
Herein, the etching rate difference means a difference between the etching
rate of the specific layer and that of an oxide layer.
According to an aspect of this invention, there is provided a method of
etching at room temperature a silicon oxide layer formed on a wafer in a
gaseous etching atmosphere consisting essentially of hydrogen fluoride or
a mixture of hydrogen fluoride and water vapour, the silicon oxide layer
being doped with an impurity into a doped oxide layer and left undoped as
an undoped oxide layer, the gaseous etching atmosphere being given a
reduced pressure with a water vapour partial pressure rendered lower than
a predetermined pressure in order to increase an etching rate difference
between the doped oxide layer and the undoped oxide layer and to
selectively etch the doped oxide layer of a specific impurity
concentration.
According to another aspect of this invention, there is provided a method
of etching a silicon dioxide layer formed on a wafer in a gaseous etching
atmosphere consisting essentially of hydrogen fluoride or a mixture of
hydrogen fluoride and water vapour, the silicon oxide layer being doped
with an impurity into a doped oxide layer and left undoped as an undoped
oxide layer, the silicon oxide layer being etched at a temperature higher
than room temperature in order to increase an etching rate difference
between the doped oxide layer and the undoped oxide layer and to
selectively etch the doped oxide layer of a specific impurity
concentration.
According to still another aspect of this invention, there is provided a
method of etching a silicon oxide layer formed on a wafer in a gaseous
etching atmosphere consisting essentially of hydrogen fluoride or mixture
of hydrogen fluoride and water vapour, the silicon oxide layer being doped
with an impurity into a doped oxide layer and left undoped as an undoped
oxide layer, the gaseous etching atmosphere being heated to a temperature
higher than room temperature in order to increase an etching rate
difference between the doped oxide layer and the undoped oxide layer and
to selectively etch the doped oxide layer of a specific impurity
concentration.
According to yet another aspect of this invention, there is provided a
method of etching a silicon oxide layer formed on a wafer in a gaseous
etching atmosphere consisting essentially of hydrogen fluoride or a
mixture of hydrogen fluoride and water vapour, the silicon oxide layer
being doped with an impurity into a doped oxide layer and left undoped as
an undoped oxide layer, the gaseous etching atmosphere being added in
order to increase an etching rate difference between the doped oxide layer
and the undoped oxide layer and to selectively etch the doped oxide layer
of a specific impurity concentration, with a gas which causes a liquid
adsorption layer comprising hydrogen fluoride and water and formed on the
undoped oxide layer to volatilize.
According to a different aspect of this invention, there is provided
apparatus for etching a silicon oxide layer formed on each of wafers in a
gaseous etching atmosphere consisting essentially of hydrogen fluoride or
a mixture of hydrogen fluoride and water vapour, the apparatus comprising:
(A) a reaction chamber in which an etching process takes place; (B) a
wafer boat for holding the wafers with a predetermined space left between
two adjacent ones of the wafers; (C) a transfer chamber which is located
below the reaction chamber and in which the wafer boat is loaded with the
wafers; (D) a boat transfer mechanism for transferring the wafer boat from
the transfer chamber to the reaction chamber; and (E) a sealing mechanism
for isolating the reaction chamber from the transfer chamber; the wafer
boat comprising heating means for individually heating the wafers.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B schematically show, each in a vertical section, side views
of a substrate for use in describing principles of the present invention;
FIG. 2 schematically shows in a vertical section a side view of apparatus
according to a first embodiment of this invention;
FIG. 3 schematically shows in a vertical section a side view of apparatus
according to a second embodiment of this invention;
FIG. 4 schematically shows a gaseous etching atmosphere supplying system
according to the present invention;
FIG. 5 shows a graph for use in describing etched thicknesses etched by a
method according to this invention and by a method used as a reference;
FIG. 6 shows a graph for use in describing etching rate ratios of a method
according to this invention;
FIG. 7 schematically shows in a vertically sectional view of a plurality of
stacked objective layers which are before etched by a method according to
this invention;
FIG. 8 schematically shows in a vertically sectional view of the stacked
objective layers which are after etched; and
FIG. 9 shows a graph for use in describing the distance mentioned in
connection with FIG. 5 and the etching rate ratio mentioned in connection
with FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1A and 1B, a wafer is partly depicted. The wafer is a
semiconductor silicon substrate 11 which is covered with a silicon dioxide
film 13.
In FIG. 1A, a liquid adsorption layer 15 is formed when the wafer 11 (the
same numeral being used) with the silicon dioxide film 13 is exposed to a
humid etching gas atmosphere which is composed of water vapour molecules
(shown at 17) and hydrogen fluoride molecules (shown at 19)
This liquid adsorption layer 15 is composed of HF and H.sub.2 O at an
initial stage. Therefore, a high etching rate is observed. This is caused
by the efficient HF ionization in the liquid adsorption layer 15. Hydrogen
fluoride anions 21 and water cations 23 are formed when HF is ionized in
the liquid adsorption layer 15. Silicon fluoride 27 and H.sub.2 O are
formed when HF ions react with the silicon dioxide film 13.
The etching mechanism of the silicon dioxide film 13 by using anhydrous HF
gas is shown in FIG. 1B. Under the anhydrous HF ambience, the adsorption
layer 15 does not form on the silicon dioxide film 13. Therefore, HF gas
is hardly ionized as compared with the HF and H.sub.2 O gas mixture. As a
consequence, the silicon dioxide film 13 is etched with a low etching
rate.
FIGS. 1A and 1B will be reviewed. Water exists in the gaseous etching
atmosphere as an impurity or as a product of the etching reaction. A
reduction of a pressure of the gaseous etching atmosphere will reduce the
water vapour partial pressure in the reaction chamber.
In addition, the silicon dioxide film 13 is essentially hydrophilic so that
the silicon dioxide film easily becomes wet. When the water vapour partial
pressure is high, the water is adsorbed on the surface of the silicon
oxide film 13 forming a liquid layer, such as the liquid adsorption layer
15.
On the other hand, when the water vapour partial pressure is low, the water
vapour is stabler than water in the liquid phase. Therefore, the silicon
dioxide film does not become wet at the surface in the gaseous etching
atmosphere with a low total pressure.
Continuing review of FIGS. 1A and 1B, attention will be directed to an
objective layer which is formed on the wafer or the substrate and is
etched with a gaseous etching atmosphere. The silicon dioxide film 13 is
the objective layer when impurities, for example, phosphorus (P) and boron
(B), are contained therein. The objective layer is therefore made of a
layer material comprising silicon dioxide. Either the humid etching
atmosphere or the relatively dry etching atmosphere serves as the gaseous
etching atmosphere. Therefore, the gaseous etching atmosphere consists
essentially of hydrogen fluoride or a mixture of hydrogen fluoride and
water vapour. A reaction takes place between the impurities and both the
hydrogen fluoride anions 21 and water cations 23 irrespective of whether
the gaseous etching atmosphere is the humid etching atmosphere or the
relatively dry etching atmosphere. As a result of the reaction, hydrates
of the impurities are produced on the objective layer. The hydrates are a
hygroscopicity, resulting in the liquid adsorption layer 15 formed on the
objective layer. Therefore, a high etching rate of the objective layer is
attained even if the gaseous etching atmosphere is the relatively dry
etching atmosphere.
Table 1 shows those hydrates of various chemical elements by percent by
weight except for hydroxide radicals which result from a properly etched
phosphorus silicate glass (PSG) layer containing 3.9 mol percent of
phosphorus (P) and from an overetched boron phosphorus silicon glass
(BPSG) layer containing 10 mol percent of boron (B) and 4.4 mol percent of
phosphorus. In the hydrates, the chemical elements are boron, pentavalent
phosphorus (P(V)) in a radical ion PO.sub.4.sup.3-, trivalent phosphorus
(P(III)) in another radical ion HPO.sub.3.sup.2-, and silicon (Si) of such
objective layers and fluorine (F) of the gaseous etching atmosphere.
TABLE 1
______________________________________
P(V) P(III) (wt %)
B (PO.sub.4.sup.3-)
(HPO.sub.3.sup.2-)
Si F
______________________________________
PSG -- 97.30 2.67 0.01 0.02
P: 3.9 mol %
(properly etched)
BPSG 5.07 86.34 1.48 0.01 7.1
P: 4.4 mol %
B: 10 mol %
(over etched)
______________________________________
In Table 1, it should be noted that the hydrates mostly comprise phosphoric
acid (H.sub.3 PO.sub.4) compound. Like other compounds, the hydrates of
the phosphoric acid are highly hygroscopic. The phosphoric acid hydrates
easily form the liquid adsorption layer 15. The hydrogen fluoride
molecules 19 and the water molecules are ionized in the liquid adsorption
layer 15 into the hydrogen fluoride anions 21 and the water cations 23.
The hydrogen fluoride anions 21 react with the objective layer together
with the water cations 23. Therefore, the objective layer is etched with a
high etching rate.
When the silicon dioxide film 13 is a thermal oxide layer, the liquid
adsorption layer 15 is not made on the surface in the relatively dry
etching atmosphere. The thermal oxide layer is etched with a low etching
rate. Therefore, the objective layer should preferably include the
impurities.
Selective etching can be realized without strict control of water vapour
partial pressure in the gaseous etching atmosphere with a low total
pressure.
FIGS. 1A and 1B will continuously be referred to. It will be assured that
the objective layer is the thermal oxide layer. Selective etching can be
attained by heating either the gaseous etching atmosphere or the substrate
11 to be etched, because of the following reasons. Existence of the liquid
adsorption layer 15 on the silicon dioxide film 13 influences the etching
rate of the silicon dioxide film 13. Heating either the gaseous etching
atmosphere or the substrate 11 gives heat energy to the water molecules 17
and hydrogen fluoride molecules 19 of gaseous etching atmosphere. When
given the heat energy, the water molecules 17 and hydrogen fluoride
molecules 19 are stabler than water in the liquid phase. Therefore, the
liquid adsorption layer 15 is hardly formed on the silicon dioxide film
13. It will now be assumed that the objective layer includes the
impurities. The liquid adsorption layer 15 is formed on the objective
layer which now contains the hydrates. The hydrates have a low vapour
pressure. Heating neither the gaseous etching atmosphere nor the substrate
11 can remove the hydrates. Therefore, the etching rate of the objective
layer does not depend on heating. The objective layer is etched with a
high etching rate.
Herein, a reduction of a total pressure of the humid etching atmosphere
produces the relatively dry etching atmosphere, for the following reasons.
The pressure of the humid etching atmosphere consists of a hydrogen
fluoride partial pressure and a water vapour pressure. Reduction of the
total pressure of the humid etching atmosphere reduces not only the
hydrogen fluoride partial pressure but also the water vapour pressure.
Reduction of the water vapour pressure makes the humid etching atmosphere
relatively dry. When the total pressure of the humid etching atmosphere is
given a reduced total pressure, a similar effect results on the etching
rate as in the relatively dry etching atmosphere with respect to the
etching rate of the silicon dioxide film 13.
The selective etching can be obtained by adding to the gaseous etching
atmosphere a gas which lowers a boiling point of the atmosphere and causes
a liquid adsorption layer 15 to volatilize. The gas is, for example,
ethanol, methanol, n-propanol, isopropyl-alcohol, tertiary butylalcohol,
or acetone and will be called a volatilization accelerating gas. By
addition of the gas, the silicon dioxide film 13 is etched with a low
etching rate. However, the objective layer is etched with a high etching
rate, because the liquid adsorption layer 15 contains the hydrates.
After an etching reaction takes place in the gaseous etching atmosphere,
the water molecules 17 and the hydrates are produced on the wafer 11 or on
the objective layer. The water molecules 17 and the hydrates form the
liquid adsorption layer 15. Water molecules 17 can be easily removed by
heating the substrate 11. However, the hydrates can not be removed only by
heating but can be removed by addition of a halogen gas, such as fluorine,
to the gaseous etching atmosphere at the same time of heating. By addition
of halogen gas, a reaction takes place between hydrates and the halogen
gas. As a result of the reaction, the hydrates volatilize into the gaseous
etching atmosphere.
Referring to FIG. 2, the description will be made as regards apparatus
according to a first embodiment of this invention. The apparatus is for
etching the objective layer described with reference to FIGS. 1A and 1B
and formed on each of a plurality of wafers 30 (now designated by a
different reference numeral) in a gaseous etching atmosphere of hydrogen
fluoride or a mixture of hydrogen fluoride and water vapour. The apparatus
comprises a reaction chamber 31, a wafer boat 33, a transfer chamber 35, a
moving or transfer mechanism 37, and a flange or sealing member 39.
In the reaction chamber 31, the objective layers of the wafers 30 are
etched. The wafer boat 33 holds the wafers 30 with a certain space left
between two adjacent ones of the wafers 30 in the manner illustrated.
In the transfer chamber 35, the wafer boat 33 is first placed together with
the wafers 30. The transfer mechanism 37 transfers the wafer boat 33 with
the wafers 30 from the transfer chamber 35 to the reaction chamber 31. The
flange 39 is provided in the transfer chamber 35 and isolates the reaction
chamber 31 from the transfer chamber 35. The mentioned wafer boat 33
comprises a heating unit to heat the individual wafers.
The reaction chamber 31 comprises an external tube 41, an inner tube 43 in
the external tube 41, and a reaction tube base 45 which fixes the inner
tube 41 to the external tube 41. A gas inlet tube 47 is connected to the
external tube 41 for introducing the gaseous etching atmosphere into the
reaction cheer 31. The external tube 41 is surrounded by a reaction tube
heater unit 49 for heating the reaction chamber 31 together with its inner
contents. The inner tube 43 has a gas outlet tube 51 and through holes 53
through a tube wall 55 of the inner tube 43. The gas outlet tube 51 is
connected to a top of the external tube 41 and thence to a gas exhaust
system (not shown) for evacuating the reaction chamber 31. A combination
of the gas outlet tube 51 and a space between the external and the inner
tubes 41 and 43 serves as an introducing arrangement or means. The
reaction tube heater unit 49 may be an electric heater or a heating pipe
for circulating hot water.
The transfer chamber 35 comprises a vessel 57. The vessel 57 has a wafer
inlet and outlet opening 59 and a gas outlet 61. The transfer mechanism 37
comprises a movable body 63, and a ball screw 65 placed vertically for
moving the movable body 63 upwards and downwards. A motor 67 is placed
outside of the vessel 57. The motor 67 rotates the ball screw 65 so that
the movable body 63 moves upwards and downwards together with the wafer
boat 33. The wafer boat 33 is held by a wafer boat leg 69 with a
predetermined space. The wafers 30 are transferred from outside of the
vessel 57 through the wafer inlet opening 59 to the wafer boat 33. The gas
outlet 61 is connected to the exhaust system for evacuation of the vessel
57. The leg 69 includes a wafer boat heater unit 71. The wafer boat heater
unit 71 heats the leg 69, resulting in heating both of the wafer boat 33
and the wafers 30 placed on the wafer boat 33. The wafer boat 33 may be
made of a thin carbon heater or a thin silicon carbide heater, which heats
the boat 33 directly.
When the wafer inlet opening 59 opens to a surrounding atmosphere of one
atmospheric pressure, the opening 59 must be isolated by a gate valve (not
shown). After transference of the wafers 30 to the wafer boat 33, the
reaction chamber 31 and the transfer chamber 35 are evacuated through the
gas outlet 61. The wafer boat 33 is moved into the etching chamber 31 by
the transfer mechanism 37 provided in the transfer chamber 35. When the
flange 39 isolates the reaction chamber 31 from the transfer chamber 35
perfectly, the wafer boat 33 is placed at a predetermined position. The
reaction chamber 31 and the boat 33 are heated by the heater units 71 and
49 up to a predetermined temperature, such as 70.degree. C.
Through the introducing arrangement, the gaseous etching atmosphere is
introduced into the inner tube 43 through the gas inlet tube 47, the space
between the external and the inner tubes 41 and 43, and the through holes
53 which are formed through the tube wall 55 at positions between upper
and lower levels of each wafer 30 support on the boat 33. The gaseous
etching atmosphere supplies for the objective layer etching. After
completion of the chemical reaction, introduction of the gaseous etching
atmosphere is suspended. The gas etching atmosphere is exhausted from the
inner tube 43 through the outlet tube 51. When the pressure of the
reaction chamber 31 becomes equal to that of the transfer chamber 35, the
flange 39 is moved downwards to open. The boat 33 is moved downward
together with the flange 39. The wafers 30 are taken out from the opening
59 outside the apparatus.
Turning to FIG. 3, the description will be directed to apparatus according
to a second embodiment of this invention. The apparatus comprises similar
parts designated by like reference numerals.
In FIG. 3, the apparatus differs from the apparatus of FIG. 2 in the
following. The inner tube 43 has no through holes in the tube wall 55 but
an aperture 75 at an upper end of the tube wall 55. The aperture 75 is
opened to the space between the external tube 41 and the inner tube 43.
The gas outlet tube 51 is placed at a side portion of the external tube 41
rather than at the top of the inner tube 43. In addition, the introducing
arrangement or means comprises the gas inlet tube 47. Through the
introducing arrangement, the gaseous etching atmosphere is introduced into
the inner tube 43 through the gas inlet tube 47. The gaseous etching
atmosphere gives rise to the predetermined chemical reaction with the
objective layers. After completion of the chemical reaction, the gaseous
etching atmosphere including the reaction products are exhausted through
the aperture 75, the space between the external and the inner tubes 41 and
43, and the gas outlet tube 51 from the reaction chamber 31.
Referring to FIG. 4, an etching gas supplying system comprises first,
second, and third gas flow controllers 81(1), 81(2), and 81(3), first,
second, and third valves 83(1), 83(2), and 83(3), a gas mixing chamber 85,
first, second, and third inlet pipes 87(1), 87(2), and 87(3) connected to
the gas flow controllers 81 (suffixes omitted) and between the gas flow
controllers 81, a gas heating chamber 89, first and second connecting
pipes 91 and 93 connected to the reaction chamber 31, and the outlet pipe
95. A temperature control system 97 is for controlling the temperature of
the etching gas supplying system.
Free ends of the first, the second, and the third inlet pipes 87 (suffixes
omitted) are connected to three raw gas reservoirs (not shown) for water
vapour, hydrogen fluoride, and the volatilization accelerating gas,
respectively. In the gas mixing chamber 85, the raw gases are mixed into
the gaseous etching atmosphere.
For heating the gaseous etching atmosphere, heated pipes may be used in
place of the connecting pipes 91 and 93 and the gas heating chamber 89.
Furthermore, a heater unit (not shown) may be placed adjacent to the inlet
tube 47 (FIG. 2 or 3) inside the reaction chamber 31. In this event, the
heating chamber 89 is unnecessary with the connecting pipes 91 and 93
directly connected to each other.
Referring again to FIGS. 1A, 1B, and 2 to 4, the apparatus is put into
operation as follows. FIGS. 5, 6, 7, 8, and 9 will additionally be
referred to.
EXAMPLE 1
After the wafers 30 were loaded in the reaction chamber 31 and the reaction
chamber 31 was evacuated to an ultimate pressure (0.1 Pa), the valve 83(1)
was opened to introduce water vapour in the reaction chamber 31 up to a
water vapour pressure of 300 Pa. Subsequently, the valve 83(2) was opened
to introduce hydrogen flouride with a purity of 99.999% into the reaction
chamber 31 up to a hydrogen flouride partial pressure of 600 Pa.
Subsequently, the reaction chamber was isolated. The total pressure of
etching gases was 900 Pa, with which silicon oxides were etched. In a
similar way, an etching environment consisting of 600 Pa hydrogen flouride
only was used.
As a reference, silicon oxide layers were etched in an aqueous solution of
hydrogen flouride. The aqueous solution of hydrogen fluoride was obtained
by mixing hydrogen fluoride and pure water in a ratio of 1:30 by weight
percent.
The hydrogen fluoride solution was kept at room temperature in a vessel
made of polytetrafluoroethylene (not shown). To investigate the etching
rate of silicon oxide 1000 angstroms, thermal oxide was grown on 6-inch
N-type silicon substrate and in addition 10,000 angstroms PSG was
deposited on other 6-inch N-type silicon substrate.
In FIG. 5, the etched thickness of the PSG and the thermal oxide versus the
etching time is shown for the three different etching processes mentioned
above. The black circles and the white circles represent the etched
thermal oxide thickness and the etched PSG thickness respectively, in the
hydrogen flouride vapour etching process. The black squares and the white
squares represent the etched thermal oxide and the etched PSG thickness
respectively in the hydrogen flouride and water vapour mixture. The black
triangles and the white triangles represent the etched thermal oxide
thickness and the etched PSG thickness respectively in the aqueous
hydrogen flouride solution,
The etching rate is determined from the slope of the line representing the
dependence of the etched oxide thickness and the etching time. The etching
rate ratio of PSG to thermal oxide was found to be about 20 for the
hydrogen flouride solution. However, in both low pressure vapour phase
etching processes the etching rate ratio was higher than 1000.
In this example, the reaction chamber was isolated after introducing the
process gases. However, continuous supply of etching gas in combination
with continuous removal of byproducts is also possible.
EXAMPLE 2
The etching rate ratio R of BPSG and thermal oxide is shown in FIG. 6 as
function of the hydrogen flouride partial pressure for etching in a low
pressure environment consisting of hydrogen flouride vapour and water
vapor in which the hydrogen flouride partial pressure is varied and the
water vapour partial pressure is fixed at 300 Pa (curve 103) or 600 Pa
(curve 101). From FIG. 6, it is understood that the etching rate ratio R
is lower for higher hydrogen flouride partial pressures and that the
etching ratio R is generally higher for the low water vapour partial
pressure.
The BPSG layer can be selectively etched at a wide range of hydrogen
flouride partial pressures when the water vapour partial pressure is low.
EXAMPLE 3
To investigate the influence of the water vapour pressure, the reaction
chamber was evacuated to 0.1 Pa, subsequently the reactor chamber was
filled with water vapour to a pressure between 0 and 2000 Pa after which
the hydrogen flouride was supplied in such a way that the hydrogen
flouride partial pressure in the reaction chamber equaled 600 Pa. The
etching process was carried out at 10.degree. C.
In FIG. 7, the structure of the used samples is shown. Referring to FIG. 7,
the samples consisted of 1000 angstroms thermal oxide film 113 grown on an
N-type silicon substrate 111, on this 200 angstroms silicon layer 115 was
sputtered, followed by the formation of a 1000 angstroms boron silicate
glass (BSG) layer 117 with low pressure chemical vapour deposition
(LPCVD). On top of the BSG layer 200 angstroms silicon layer 119 was
sputtered on top of which 1000 angstroms silicon oxide layer 121 was
formed with LPCVD. On top of the silicon oxide layer 121, 200 angstroms Si
layer 123 was sputtered. On top of which 1000 angstroms BPSG layer 125 was
formed with LPCVD. On top of the BPSG layer 125 again 200 angstroms
silicon layer 127 was sputtered on top of which 1000 angstroms PSG 129 was
formed. Finally, a 200 angstroms silicon layer 131 was sputtered on top of
the PSG layer 129. The structure described above. After etching for 30
seconds in an environment consisting of 600 Pa hydrogen flouride and 300
Pa water vapour, is shown in FIG. 8. This figure is drawn based on a
scanning electron microscope photograph. From this figure, it is clear
that the BPSG and PSG were etched much more than the other oxides.
In FIG. 9, the etched thickness of the BPSG, the PSG and the thermal oxides
are shown together the etched thickness ratio of BPSG and thermal oxide
versus the water vapour partial pressure. The left ordinate represents the
etched thickness, the right ordinate the etching ratio and the abscissa
the water vapour pressure.
From this figure, it is understood that the etched thickness of the PSG and
BPSG layers increases monotonously with increasing water vapour pressure.
However, a sudden increase in the etched thickness of thermal oxide is
observed when the water vapour pressure is increased above 600 Pa.
However, strictly speaking, one cannot say that explosive etching of
thermal oxide starts at a water vapour pressure of 600 Pa. In this typical
example of etching a multilayer oxide, hydrogen flouride starts reacting
with the layers that show a high etching rate like BPSG and PSG. In the
reaction the hydrogen from the hydrogen flouride reacts with the oxygen in
the oxide film and water is formed at the wafer surface. Therefore, the
water vapour partial pressure close to the wafer increases aria the
selectivity is easily lost. In this example, the reaction chamber was
isolated after supplying the etching gas mixture. Therefore, the water
vapour formed in the etching process increased the water vapour partial
pressure with a result of a changed etching atmosphere and mechanism.
As a countermeasure, another method was used as well. In this other method,
the etching gases were supplied to the reaction chamber at low pressure
while at the same time the reaction chamber was evacuated. In other words,
the etching process was carried out with a constant supply of etching gas
and a constant removal of reaction byproducts. With this method, it was
possible to minimize the influence of the water vapour, formed in the
etching process, on the etching mechanism. In fact, an etching rate ratio
for BPSG and thermal oxide of around 1000 was obtained for a water vapour
partial pressure up to 850 Pa with the hydrogen flouride partial pressure
set at 600 Pa. However, as desc | | |
