Cleaning method

A method for removing native oxides and other contaminants from a wafer surface while minimizing the loss of a desired film on the wafer surface. The method is carried out in a hermetically sealed reactor. A fluorine-containing gas or gas mixture is passed over the wafer during simultaneous exposure to ultraviolet radiation in the absence of added water, hydrogen, hydrogen fluoride or hydrogen containing organics, thereby avoiding the production of water as a reaction product. The addition of ultraviolet radiation and the elimination of water, hydrogen, hydrogen fluoride and hydrogen containing organics provides for the nearly equivalent (non-selective) removal of various forms of oxide and also provides for improved process control.

BACKGROUND OF THE INVENTION 
The present invention relates to the removal of an undesired material from 
the surface of a substrate with a desired material in place on the 
substrate while minimizing the loss of the desired material. It finds 
particular application in the etching, cleaning, or removal of silicon 
oxide and contaminant films from semiconductor surfaces or in topographic 
features of a semiconductor wafer. In particular, it relates to the 
removal of silicon oxides and other contaminants in a dry, gas-phase 
environment where ultraviolet (UV) light stimulation and a 
fluorine-containing molecular gas such as chlorine triflouride are used to 
etch different forms of silicon dioxide at similar rates without the 
significant generation of water as a reaction by-product. 
In semiconductor device processing, oxides of silicon are used in many 
different forms for many applications. Dense, thermally grown oxides of 
silicon are typically used as the primary gate dielectric film in MOS 
(metal oxide-silicon) transistors. Steam grown thermal oxides are commonly 
used as a field oxidation dielectric layer. Doped oxides such as 
phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) are 
commonly used as inter-metal layer dielectrics because they can be easily 
planarized with an elevated temperature reflow process. Spin-on-glass 
(SOG) is also used in dielectric applications where planarization is 
critical. An SOG is a siloxane-type polymer in an organic solvent which is 
deposited in liquid form and then cured at elevated temperature to form a 
solid silicon oxide film. 
During the processing of silicon based semiconductor devices, other types 
of oxide films may be formed as the result of exposure of silicon surfaces 
to chemical processing steps, or to the ambient environment. For instance, 
the well known RCA wet cleaning sequence is known to leave a 10-20 .ANG. 
"chemical" oxide on the surface. Exposure of a clean silicon surface to 
ambient atmosphere results in the growth of a 5-10 .ANG. "native" oxide. 
In many cases, these residual oxides are considered surface contaminants 
since they must be removed to reveal a pristine silicon surface to allow 
the formation of a high quality electrical interface. Interlayer metal 
contacts made through vias or "contact holes" in a dielectric layer such 
as BPSG are of high quality only when oxides and contaminants on the lower 
metal or polysilicon level are removed. Often the contamination in contact 
holes or on feature sidewalls which is the result of plasma or reactive 
ion etching processes may be comprised of a mixture of silicon oxides, 
silicides or oxides of metals, and organic contaminants. 
Very often, it is necessary to remove a chemical or native oxide, or 
post-etch residue contamination from a pattern feature bottom or from an 
exposed wafer surface in the presence of one or more of the many other 
types of silicon oxides mentioned above. It has long been known that 
vapors of HF/water mixtures will etch various silicon oxide films. This 
technology has been studied and commercialized (U.S. Pat. Nos. 4,749,440 
and 4,938,815). However, several limitations are sometimes encountered in 
the use of HF vapor phase etching of oxide films. These limitations can 
include the formation of non-volatile residues which must be rinsed away, 
and low etching rates for native and chemical silicon oxide films relative 
to doped silicon oxide films. In addition, water is generated as a 
reaction by-product, which can make the anhydrous HF processes difficult 
to perform controllably and repeatably. Water is in general among the most 
undesirable of chemical species to have present in a vacuum environment. 
The relative rate of etching (selectivity) of the HF vapor etching 
processes to many different types of oxide films has also been studied. 
The results show that native, chemical, and thermal oxides are typically 
removed at rates 10 times slower than the removal rates of PSG and BPSG 
doped silicon oxides. This is problematic in several common processing 
circumstances. First of all, it is commonly necessary to clean native 
oxide and other contaminants from the bottom of contact holes formed in 
films of BPSG. Using the current vapor phase processes, several hundred 
angstroms of the BPSG are removed before the silicon oxides and 
contaminants in the bottom of the contact hole are removed. Etching a 
large quantity of BPSG is unfavorable and may leave undesirable residues. 
Second, it is common to use composite structures of different types of 
silicon oxide films. For instance, a BPSG layer sandwiched between two 
undoped silicon oxide layers is sometimes used as a dielectric film 
between metal layers. Cleaning of contact or other topographic features 
through this type of composite film with the current HF vapor technology 
causes enhanced lateral etching of the BPSG layer relative to the undoped 
silicon oxide layers. This results in an undercut profile which is 
difficult to fill with subsequent films without forming voids. For this 
case, a non-selective oxide removal process is most desirable, i.e., a 
process that etches native, chemical, and thermal oxides at nearly the 
same rate as doped oxides. Third, it is sometimes desirable to remove a 
thermal oxide film over a doped oxide without over-etching the doped oxide 
extensively. 
Attempts have been made to address the limitations of the aqueous HF vapor 
technology described above by substituting alcohol vapor in place of water 
vapor in combination with the HF gaseous reactant. However, water is 
generated as a by-product of the process, leading to many of the same 
limitations as the aqueous HF vapor technologies. Furthermore, the use of 
HF with alcohol vapor gives very high removal rates of BPSG relative to 
native oxide contamination. Also, the use of HF with alcohol vapor in the 
presence of BPSG can still result in problematic residue formation. 
Other attempts to remove silicon oxide films in a dry, gas-phase reaction 
environment have been made which do not utilize HF. The effluent from a 
plasma of nitrogen trifluoride (NF.sub.3) and hydrogen (H.sub.2) has been 
used to remove oxide films. Also, fluorine (F.sub.2) and hydrogen mixtures 
with UV illumination have been used to remove oxide films. The presence of 
hydrogen in the reaction chemistry still leads to the formation of water 
as a reaction by-product. 
Previous work with ClF.sub.3 (U.S. Pat. No. 4,498,953) indicated that 
thermal oxide removal rates with ClF.sub.3 exposure were not measurable. 
It was, in fact, reported that silicon oxide was successfully used as mask 
material in the etching of silicon by ClF.sub.3. This work did not utilize 
UV illumination. 
SUMMARY OF THE INVENTION 
The purpose of this invention is to overcome the limitations of current 
silicon oxide removal technologies which utilize hydrogen-containing gas 
mixtures such as HF/H.sub.2 O and ClF.sub.3 /alcohol which allow HF 
formation in situ to etch silicon oxides with and without UV irradiation. 
These limitations include the generation of substantial water as a 
reaction by-product, and the unfavorable removal of doped silicon oxides 
at rates which are very large compared to the removal rates of native or 
chemical oxides. In addition, these processes frequently lead to the 
production of colloidal residues of silicon oxide, metasilicic acid, and 
fluosilicic acid which must often be removed by wet rinsing before 
subsequent device processing. 
The invention in one aspect comprises a method of removing an undesired 
material from a substrate. Typically the undesired material comprises an 
unwanted silicon oxide, with which may be associated other metallic, 
organic and inorganic contaminants. These contaminants may be incorporated 
into the silicon oxide, at the Si/SiO.sub.2 interface, or on the 
SiO.sub.2, as a result of previous processing steps or incidental ambient 
exposure. The method comprises 
placing the substrate in a gaseous environment comprising at least one 
fluorine-containing gas selected from the group consisting of fluorine and 
fluorine containing gases, other than F.sub.2 O, which are free of 
hydrogen and which can be photodissociated, the gaseous environment being 
substantially free of water, hydrogen, hydrogen fluoride and hydrogen 
containing organic compounds, and 
exposing the substrate to UV irradiation in the presence of the gaseous 
environment until the undesired material has been removed. 
A further aspect of the invention is a method for cleaning or removing 
undesired silicon oxide material formed as a result of ambient exposure of 
the wafer surfaces or formed as an incidental result of wet or gaseous 
chemical processing steps, from the surfaces of a silicon wafer substrate 
with a desired doped silicon oxide material in place on the substrate 
while minimizing the loss of the desired material, comprising the steps 
of: 
(a) evacuating a hermetically-sealed processing chamber to a low base 
pressure, 
(b) introducing a silicon wafer substrate into the processing chamber, 
(c) introducing into said chamber and exposing the silicon wafer in said 
chamber to a gaseous environment comprising at least one 
fluorine-containing gas selected from the group consisting of fluorine and 
fluorine containing gases, other than F.sub.2 O, which are free of 
hydrogen and which can be photodissociated, the gaseous environment being 
substantially free of plasma products, water, hydrogen, hydrogen fluoride 
and hydrogen containing organic compounds, 
(d) exposing the gaseous environment and substrate in said chamber to 
ultraviolet light, 
(e) evacuating the processing chamber, and 
(f) removing the substrate from the processing chamber. 
A still further aspect of the invention is a method of removing an 
undesired material from a silicon, silicon oxide or gallium arsenide 
substrate, the substrate comprising on at least a portion of the surface 
thereof a silicon oxide material which is desired to be retained on the 
substrate, the desired silicon oxide material being selected from the 
group consisting of doped silicon oxide, deposited silicon oxide, and 
thermally grown silicon oxide, and the undesired material includes one or 
more members selected from the group consisting of silicon oxides formed 
as a result of ambient exposure of the substrate surfaces, silicon oxides 
formed as an incidental result of wet or gaseous chemical processing 
steps, and other trace silicon oxides, the method comprising 
placing the substrate in a gaseous environment comprising at least one 
fluorine-containing gas which is free of hydrogen and which can be 
photodissociated, the gaseous environment being substantially free of 
plasma products, water, hydrogen, hydrogen fluoride and hydrogen 
containing organic compounds, and 
exposing the substrate to UV irradiation in the presence of the gaseous 
environment until the undesired material has been removed. 
The invention is particularly applicable to the cleaning of silicon oxide 
and other types of contaminants in metal contact vias through BPSG or BPSG 
and thermal oxide sandwich structures since the BPSG film is not rapidly 
etched relative to the other silicon oxides during the contact hole 
cleaning process. The present invention addresses several of the 
limitations noted above for the etching or cleaning of oxide films in the 
dry, gas phase, and the removal of native oxides or other contaminants 
without the concurrent rapid removal of doped oxide films.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In its various embodiments one or more of the following objects may be 
obtained. 
One object and advantage of the present invention is to allow removal of 
either trace oxide films of under 30 .ANG. thickness or bulk oxide films 
of greater than 30 .ANG. thickness without the significant generation of 
water as a by-product of the reaction process. Trace oxide films may 
include native oxides grown in ambient, chemical oxide layers grown in 
oxidizing atmosphere (for instance an oxidizing plasma) or in liquid 
solution, or thin oxide layers grown by other means. Bulk oxide films 
include thermally grown oxides, steam grown thermal oxides, CVD oxides, 
plasma grown oxides, doped oxides, spin-on-glass and other thick silicon 
oxide containing films used in the fabrication of semiconductor devices. 
As a consequence of the substantially anhydrous and hydrogen free 
environment, HF is not expected to be formed in situ in the reaction 
process chamber to any substantial degree, and water is not formed as a 
significant by-product of the primary silicon oxide removal reaction 
pathway. 
A second object and advantage of the present invention is to allow the 
removal of native, chemical, and thermal oxide films at rates comparable 
to the removal rates of doped oxide films such as BPSG or PSG. The present 
invention does not use HF oxide removal chemistry which is sensitive to 
the level of water or water forming hydrides in the various oxide films, 
which causes them to etch at different rates. 
A third object and advantage of the present invention is to improve the 
endpoint control in the etching of silicon oxide films in a dry 
environment through the use of a modulated or shuttered UV lamp or UV 
laser of the appropriate power and wavelength. The etching rate of the 
various oxide films mentioned above can be controlled by adjusting the 
power of the impinging UV light. 
Another object and advantage of the present invention is to allow the 
cleaning of trace oxides and other contaminants from metal contact vias 
and from other topographic features formed in thick oxide films or in 
thick composites containing various oxide films. The nearly equal removal 
rates of different oxide films allows cleaning of the feature bottoms 
without significant under cut of the thick oxide film, or lateral 
under-cut of thick composite oxide layers. There is further no need for 
the adsorption of water or alcohol film in the via bottoms which is 
difficult to subsequently desorb from very small topographic features due 
to capillary forces. 
Another object and advantage of the present invention is to allow the 
cleaning of plasma etch residues which remain on the wafer after the 
etching of metal patterns. Current methods for removing these residues 
include the HF-water vapor process which has poor selectivity to 
underlying doped oxide films, or solvent based rinsing processes with 
problems of waste disposal. These residues are typically a mixture of 
different silicates and may be removed with the current invention with 
good selectivity to the underlying doped oxide layer due to the nearly 
equal removal rates of various oxides. 
The substrate materials which can be treated by the present process can 
generally be any type of substrate material, but commonly will constitute 
Si, SiO.sub.2 (including quartz) or gallium arsenide wafer substrates. 
Suitable fluorine containing gases usable in the invention include, in 
addition to fluorine, fluorine interhalogens, fluorides of sulfur and 
xenon difluoride. In alternate embodiments the present invention utilizes 
F.sub.2, ClF.sub.3, BrF.sub.3, SF.sub.6 or mixtures of fluorine containing 
gases with chlorine, for instance F.sub.2 /Cl.sub.2, ClF.sub.3 /Cl.sub.2 
or BrF.sub.3 /Cl.sub.2 mixtures, together with UV irradiation in the 
wavelength range of 180 to 600 nm, to remove various silicon oxides at 
nearly equal rates and without the generation of substantial water. The 
etching of doped silicon oxides (BPSG, PSG, BSG) occurs at rates similar 
to that of thermal, chemical, or native silicon oxides over a large, 
useful range of process parameters. Solid residues characteristic of HF 
gas or vapor phase etching processes are not observed to form. 
The gaseous environment substantially free of water, hydrogen, hydrogen 
fluoride and hydrogen containing organic compounds. Desirably water is 
present at less than 0.1% and other hydrogen containing gases at less than 
1%. Technical grade anhydrous fluorine containing gases, for instance 
technical grade ClF.sub.3, will generally suffice. Preferably the purity 
of all of the gases employed in the gaseous environment, as certified by 
the vendors, will be greater than 99% purity, more preferably 99.9% 
certified purity. 
In practice of the inventive method, a source of the fluorine-containing 
gas is connected to a processing chamber containing the substrate material 
to be etched or cleaned. The processing chamber suitably comprises a 
vacuum vessel constructed of chemically inert material, which is 
hermetically sealed from the ambient atmosphere and can be evacuated to 
better than 20 millitorr base pressure by means of suitable vacuum 
apparatus. The processing chamber is evacuated to a low base pressure, for 
example 20 millitorr. The substrate is desirably introduced into the 
processing chamber through an isolated load-lock chamber which can be 
pumped down to a similar base pressure. Introduction or removal of the 
substrate from the process chamber occurs through the load-lock chamber to 
prevent the introduction of atmospheric contaminants, particularly water 
vapor, into the process chamber. Alternatively, the substrate may be 
introduced into the chamber before evacuation. 
The process specifically occurs in the substantial absence of a plasma or 
plasma products, such as a downstream plasma effluent. 
The process chamber may also share a transfer interface with a vacuum 
cluster robotic transfer unit which allows sequential transfer of 
substrate materials to or from other process modules without exposure to 
ambient atmosphere. 
A fluorine-containing gas such as ClF.sub.3 is introduced into the process 
chamber to produce a gaseous environment in which the fluorine containing 
gas forms a substantial partial pressure over the substrate, suitably in 
the range of 0.001-760 torr. However, the fluorine containing gas will 
generally be introduced in mixture with one or more inert or facilitating 
gases. The inert gas may be any gas which is inert to the materials to be 
treated and which will remain in the gaseous phase under the processing 
conditions present. Suitable gases include nitrogen, argon, and helium. 
The facilitating gas is a gas which assists in the cleaning process, for 
instance helping to increase selectivity for particular contaminants or 
removal of specific materials or in providing an improved surface 
morphology. The facilitating gas may be, for instance, chlorine 
(Cl.sub.2), bromine (Br.sub.2), oxygen (O.sub.2) and mixtures of two or 
more of these gases. Other non-hydrogen containing gases which provide a 
source of atomic chlorine, bromine or oxygen by photodissociation may also 
be used, for instance CCl.sub.4, H-free chlorofluorocarbons such as 
CF.sub.3 Cl and H-free bromofluorocarbons such as CF.sub.3 Br. 
The gas mixture may be introduced into the processing chamber in a manner 
which creates a uniform radial laminar flow pattern over the substrate, 
for instance through a gas distribution showerhead. In this manner removal 
of etching products and contaminants is facilitated through entrainment in 
the laminar flow stream. However, the present invention may be 
accomplished using other reactive gas flow patterns or in an approximately 
stagnant gaseous environment. 
Both the fluorine-containing gas phase above the substrate and the 
substrate surface to be processed are illuminated with UV light, suitably 
through a UV transparent window in the processing chamber. Broad-band UV 
radiation in the wavelength range of 180-600 nm may be used, as may 
narrower band sources providing substantial output in the 180-420 nm 
range. Suitable sources are medium pressure Hg lamps and Xenon lamps. The 
UV radiation may be pulsed or continuous. A laser and suitable optics may 
also be used to generate the required UV photon flux. Substrate film and 
contaminant removal rates may be controlled to a large degree by the 
intensity of the UV radiation, the UV photon energy, the UV exposure time 
and/or the UV lamp or laser pulsing rate. 
Following treatment, the processing chamber is evacuated and the substrate 
is removed. 
FIG. 3 illustrates the chemical processes which facilitate silicon oxide 
etching and contamination removal in one preferred embodiment. Broadband 
UV illumination in the 180-400 nm wavelength region is known to 
efficiently photodissociate ClF.sub.3 resulting in the production of gas 
phase atomic fluorine (F). F is fairly long-lived in the gas phase and is 
convectively and or/diffusively transported to the various substrate 
surfaces. F is known to etch silicon oxides at a very slow, but finite 
rate at temperatures below 300.degree. C. However, the direct, 
simultaneous UV irradiation of the surface allows enhancements in the 
fluorine etching rate of various oxides by over an order of magnitude. 
Mechanisms which are important in this UV surface rate enhancement effect 
may include breaking of surface bonds by UV photons over 2 eV in energy, 
and the formation of electron-hole pairs at the substrate surface. In the 
present invention, UV light stimulation has been recognized to be the 
dominant factor in determining the overall oxide film etching rate 
according to the global reaction: 
EQU ClF.sub.3 +hv(180-400 nm).fwdarw.2F+ClF 
EQU 4F+SiO.sub.2 (s)+hv(180-400 nm).fwdarw.SiF.sub.4 +O.sub.2 (g) 
Water is not generated as a substantial reaction by product since hydrogen 
containing gases are not used, and residual chamber water levels are 
typically in the ppm range. Unlike HF chemistry which etches various 
silicon oxides at rates which are affected by the degree of hydrogen 
incorporation in the oxide film (doped oxide films have a higher degree of 
hydrogen incorporation), the etching of doped versus undoped silicon 
oxides using the above described UV/ClF.sub.3 process occurs at 
substantially similar rates (FIG. 2). 
By similar mechanisms F species are effective in removing carbonaceous and 
other trace contaminants by selective etching reactions or through 
entrainment mechanisms. Surface carbon residues may be removed according 
to the reaction: 
EQU 4F+C(s)+hv.fwdarw.CF.sub.4 (g) 
Thus, the present invention will remove carbon residues (undesired 
material) from the surface of a substrate. 
Undissociated ClF.sub.3 also reacts spontaneously with most organic 
residues. The present process will therefore remove most organic residues 
(undesired material) from the surface of a substrate. 
Both photolytically produced F species and ClF.sub.3 gas are effective in 
removing certain trace metals such as titanium, tantalum, tungsten, and 
molybdenum as well as their silicides and nitrides. Other trace metal 
contaminants such as iron, copper, and aluminum may be removed by adding a 
chlorine gas component into the above described mixture. Chlorine is 
effectively photodissociated into chlorine atoms by UV irradiation in the 
200-600 nm range. Chlorine atoms react spontaneously with some trace metal 
contaminants to form volatile metal chlorides. Removal of some types of 
involatile metals which are incorporated into contaminant films may be 
accomplished through entrainment in the product stream as the primary bulk 
of the film is etched and the metal particles are undercut. Therefore the 
present invention will remove such trace metals (undesired material) from 
the surface of a substrate. 
A particular feature of the inventive method is the facilitation of the 
removal of contaminants from high aspect ratio features while minimizing 
the loss of BPSG and avoiding the production of water and other residues 
as by-products. One application of this feature is the cleaning of trace 
silicon oxide and other contaminants from the surface of a silicon wafer 
at the bottom of a contact hole formed in a BPSG film. Post 
reactive-ion-etching (RIE) contaminants (undesired material) may also 
exist on the feature sidewalls. This feature is illustrated in FIG. 5 
which shows that contaminants would be removed from the silicon wafer 
surfaces with minimal removal of the desired BPSG film. 
Another application of the invention is the cleaning of trace silicon oxide 
and other contaminants from the surface of a silicon wafer at the bottom 
of a contact hole formed in a composite dielectric film formed from 
alternating layers of doped and undoped silicon oxide. This application is 
illustrated in FIG. 8 which shows that lateral under-cut of the doped 
oxide (BPSG) layer sandwiched in between undoped (thermal) oxide layers is 
avoided by the present invention while the contaminants are removed. 
The following non-limiting examples further illustrate the present 
invention. 
EXAMPLE 1 
Blanket films of thermally grown silicon oxide and of deposited BPSG were 
removed from a silicon wafer. The silicon oxide film was thermally grown 
to a thickness of 4000 angstroms on the surface of a silicon wafer by 
heating the wafer to 1000 degrees centigrade in the presence of hydrogen 
and oxygen. BPSG consisting of approximately 5% boron and 5% phosphorous 
was deposited to a thickness of 5000 angstroms onto the surface of a 
silicon wafer by a chemical vapor deposition (CVD) process. The BPSG films 
were subsequently annealed in oxygen at 1000 degrees centigrade for 30 
minutes. The silicon wafers were introduced into a vacuum process chamber 
through a load-lock at a base pressure of 20 mtorr. ClF.sub.3 gas was 
introduced with a nitrogen carrier in a radial laminar flow over substrate 
wafers at 1000 sccm. The equivalent ClF.sub.3 partial pressure above the 
substrate was varied from 50 torr to 90 torr. The total process pressure 
of the ClF.sub.3 /nitrogen gas mixture was held at 100 torr. The initial 
wafer temperature preceding UV lamp exposure was 150.degree. C. The 
ClF.sub.3 gas phase and the substrate were simultaneously exposed to 
direct UV illumination from a medium pressure mercury arc lamp for 1 
minute. The process chamber was then evacuated to its base pressure and 
the wafer was removed through the load-lock. The thickness of the 
remaining BPSG film was measured by spectroscopic reflectometry and 
compared with measurements made before processing to determine the amount 
of film removed during the simultaneous exposure of the wafer to ClF.sub.3 
and UV illumination. 
FIG. 1 is a graph of the results of the process runs described in the 
preceding paragraph. Film removal over a one minute exposure of the films 
is plotted as a function of chlorine trifluoride fraction in nitrogen. 
Without chlorine trifluoride added to the nitrogen no film removal is 
detected. The removal of the BPSG film is equivalent within 20% to that of 
the thermal oxide film. Without UV illumination of either the gas-phase or 
the substrate, the removal of both thermal oxide and BPSG are below the 
detection limit. 
EXAMPLE 2 
Silicon wafers were processed as described in Example 1, except that small 
areas of the wafer surface were "shadowed" from UV illumination. The 
masking was with small pieces of anodized aluminum, approximately 0.5 inch 
square by 0.05 inch thick, which were placed over the wafer surface. Small 
alumina beads were used to maintain a spacing between the wafer surface 
and the masking piece such that the wafer surface under the mask was 
exposed to the gas and any gas-phase products of UV illumination without 
receiving direct UV illumination. For thermal oxide prepared as described 
in Example 1, and under process conditions of 250 sccm ClF.sub.3 flow, 750 
sccm nitrogen flow and 100 torr total pressure, film removal rates in the 
shadowed and unshadowed regions were typically 10-20% of the removal rate 
with direct UV illumination of the substrate as shown in FIG. 2. Also 
shown are results for etching of the shadowed and unshadowed regions under 
conditions of no UV illumination, where no significant oxide etching is 
detected. This example illustrates the overwhelming mechanistic importance 
of direct UV exposure of the substrate surface for rapid etching of oxides 
by photodissociated F atoms as illustrated in FIG. 3. 
EXAMPLE 3 
During processing as described in Example 1 a residual gas analyzer (RGA) 
mass spectrometer was used to sample the gases leaving the process 
chamber. The RGA was capable of measuring moisture levels down to 
approximately 1 part per million in the process chamber exhaust gas 
stream. FIG. 4 shows the results of this analysis during the process 
described in Example 1. The graph shows mass signals associated with 
water, nitrogen, chlorine trifluoride, and silicon tetrafluoride as a 
function of process time. The process starts with a flow of 100% nitrogen. 
At a process time of approximately 0.7 minutes, the flow of ClF.sub.3 and 
the UV illumination is started. Process pressure was 100 torr. Total gas 
flow rate was 1000 sccm. Initial substrate temperature was 150.degree. C. 
The gas mixture was 25% ClF.sub.3 and 75% N.sub.2 after 0.7 minutes. A 
background water signal is observed prior to processing. However, no 
substantial rise is seen in this signal as a result of the UV/ClF.sub.3 
etching of silicon oxide, indicating that water is not substantially 
produced as a reaction by-product. Silicon tetrafluoride is observed as a 
primary reaction product. 
EXAMPLE 4 
A silicon dioxide masking film was prepared over a p-doped silicon &lt;100&gt; 
substrate wafer. The masking pattern exhibited lines and contact vias with 
lateral dimensions ranging from 1-5 .mu.m. The wafers were briefly exposed 
to a liquid chemical cleaning process comprised of four steps in which the 
wafer surface was exposed first to an aqueous mixture of sulfuric acid and 
hydrogen peroxide, second to an aqueous mixture of dilute hydrofluoric 
acid, third to an aqueous mixture of ammonium hydroxide and hydrogen 
peroxide, and fourth to an aqueous mixture of hydrochloric acid and 
hydrogen peroxide. This chemical process leaves a 10 to 15 angstrom 
chemical oxide film over the exposed silicon regions. These wafers were 
exposed to UV light and a mixture of 2.5 sccm ClF.sub.3, 50 sccm Cl.sub.2, 
and 945 sccm nitrogen at 100 torr for time periods ranging from 30 seconds 
to 3 minutes. The wafer substrate temperature preceding UV exposure was 
100.degree. C. The wafers were removed from the process chamber, and the 
silicon dioxide mask was subsequently stripped using a liquid HF solution 
which is known not to attack the underlying silicon. The depth of etching 
which the UV/ClF.sub.3 +Cl.sub.2 process achieved in the underlying 
p-silicon substrate layer after removal of the thin film chemical oxide 
was then measured using a stylus profilometer. The results are plotted in 
FIG. 6, in which no etching in the p-silicon substrate was detected at 30 
seconds of process exposure. However, at process exposure times of 1 
minute and greater, silicon was removed in the unmasked line and via areas 
to an increasingly greater depth. Linear extrapolation of this rate data 
suggests that a UV process time of 30 to 45 seconds was required to remove 
the chemical oxide layer. Addition of chlorine into the process mixture 
results in much smoother surface morphologies in the underlying silicon as 
compared to UV/ClF.sub.3 -only processes. Also, Cl.sub.2 addition has been 
shown to enhance the removal of trace metallic contamination from the 
silicon substrate as illustrated in the following example. 
EXAMPLE 5 
The removal of metallic contamination from the surface of a silicon 
substrate was demonstrated with the disclosed invention and compared to 
metallic contamination removal with HF/alcohol and UV/Cl.sub.2 processes. 
FIG. 7 is a graph showing the results of this demonstration. Silicon 
wafers were first contaminated by applying a photoresist, and then 
removing that photoresist with an oxygen plasma process. Metallic 
impurities in the photoresist were left on the wafer (indicated as 
"contamination level" in FIG. 7). The surface concentration of these 
impurities was measured by total reflectance x-ray fluorescence with the 
detection limits as indicated in FIG. 7. Data for chromium, iron and 
copper are shown in FIG. 7. The HF/alcohol treatment consisted of exposure 
of the contaminated silicon surface to a mixture of 60% HF and 40% 
isopropanol in a N.sub.2 carrier at a total flow rate of 500 sccm, a 
pressure of 100 torr, and a temperature of 100.degree. C. for 5 minutes. 
The UV/ClF.sub.3 treatment consisted of exposure of the contaminated 
silicon surface to a mixture of 0.86% ClF.sub.3 and 99.14% N.sub.2 at a 
total flow rate of 500 sccm, a pressure of 50 torr and an initial 
temperature of 100.degree. C. for 2 minutes. The UV/Cl.sub.2 treatment 
consisted of exposure of the contaminated silicon surface first to the 
HF/alcohol treatment described above and then to a mixture of 9% Cl.sub.2 
and 91% N.sub.2 at a total flow rate of 550 sccm, a pressure of 100 torr 
and an initial temperature of 100.degree. C. for 2.5 minutes. Both 
HF/alcohol and UV/ClF.sub.3 achieve about a 10.times. reduction in the 
level of contamination of the metals shown in FIG. 7. The use of 
UV/Cl.sub.2 achieves about a 100.times. reduction in those contaminants. 
The addition of Cl.sub.2 to the UV/ClF.sub.3 process will achieve the 
metals removal demonstrated by UV/Cl.sub.2 alone while maintaining the 
oxide removal advantages of the UV/ClF.sub.3 process of the present 
invention. 
EXAMPLE 6 
A mixture of F.sub.2 and Cl.sub.2 was used in place of ClF.sub.3 in the 
process of Example 1 described above. Films of thermally grown silicon 
oxide and of deposited BPSG, as described in Example 2, were removed from 
a silicon wafer. F.sub.2 and Cl.sub.2 were flowed in a nitrogen carrier in 
a radial laminar flow over substrate wafers at 1000 sccm. The total 
process pressure of the F.sub.2, Cl.sub.2 and nitrogen gas mixture was 
held at 100 torr. The initial wafer temperature preceding UV lamp exposure 
was 100.degree. C. The gas mixture and the substrate were simultaneously 
exposed to direct UV illumination from a medium pressure mercury arc lamp 
for 1.5 minutes. The process chamber was then evacuated to its base 
pressure and the wafer was removed through the load-lock. The thickness of 
the remaining film was then measured by spectroscopic reflectometry and 
compared with measurements made before processing to determine the amount 
of film removed during the process. 
FIG. 9 is a graph of the results of the process runs described in the 
preceding paragraph. Film removal over a 1.5 minute exposure of the films 
is plotted for a mixture of 50% F.sub.2 in nitrogen and a mixture of 50% 
F.sub.2 and 25% Cl.sub.2 in nitrogen. For the UV/F.sub.2 process, the 
thermal silicon oxide was removed at 600 angstroms/minute, while the BPSG 
was etched twice as fast at 1200 angstroms/minute. For the UV/F.sub.2 
+Cl.sub.2 process, the chlorine addition suppressed thermal oxide etching 
to about 10 angstroms/minute, while the BPSG etching rate was reduced to 
92 angstroms/minute. This data demonstrates that the properties of silicon 
oxide removal with UV/F.sub.2 and UV/F.sub.2 +Cl.sub.2 processes are 
similar to those seen for UV/ClF.sub.3 and UV/ClF.sub.3 +Cl.sub.2 
processes, although each method retains specific advantages. 
EXAMPLES 7-11 
A significant aspect of these examples is the efficient removal of carbon 
with the combination of halogen and oxygen gases under UV illumination as 
compared to each gas alone. Also, the surface treated with F.sub.2 
+O.sub.2 seemed to be resistant to recontamination by adventitous 
atmospheric hydrocarbons. 
In these examples organic contamination was removed from the surface of a 
silicon wafer utilizing the methods described in the present invention. 
Contamination was introduced on the surface of a boron-doped, &lt;100&gt; 
surface orientation, silicon wafer by applying a highly diluted mixture of 
positive novolac photoresist in acetone onto the spinning wafer. The 
resulting organic film was approximately 100 .ANG. or less in thickness as 
indicated by ellipsometry and by low angle X-ray photoelectron 
spectroscopy analysis (XPS). Wafers were exposed to several different 
process involving difference gas mixtures with UV illumination. The gas 
mixtures included oxygen (O.sub.2) only, chlorine (Cl.sub.2)+nitrogen 
(N.sub.2), fluorine (F.sub.2)+N.sub.2, Cl.sub.2 +O.sub.2 and F.sub.2 
+O.sub.2. Conditions of each process are listed in Table 1. 
TABLE 1 
______________________________________ 
Process conditions 
Example Pressure 
Temp. Duration 
No. Process Flows (sccm) 
(torr) 
(C.degree.) 
(minutes) 
______________________________________ 
7* O.sub.2 O.sub.2 : 1000 
400 130 3 
8* Cl.sub.2 Cl.sub.2 : 50 N.sub.2 : 950 
400 130 3 
9* F.sub.2 + N.sub.2 
F.sub.2 : 10 N.sub.2 : 490 
400 130 3 
10* Cl.sub.2 Cl.sub.2 : 50 O.sub.2 : 950 
400 130 3 
11* F.sub.2 + O.sub.2 
F.sub.2 : 10 O.sub.2 : 490 
400 130 3 
______________________________________ 
Comparative examples 
Results of XPS analysis of the silicon surface before and after processing 
are shown in Table 2. XPS analysis indicated the reduction of carbon from 
a level of 84% to a level of &lt;1% with the F.sub.2 +O.sub.2 process. The 
relative abundance of elements within 100 .ANG. of the surface of the 
wafer before and after the cleaning process are indicated in Table 2. 
Subsequent analysis of the silicon surface treated with F.sub.2 +O.sub.2 
after 15 hours of ambient exposure indicated no significant change in the 
carbon content. 
TABLE 2 
______________________________________ 
Surface analysis results. 
Elemental abundance by XPS (%) 
Example Process C O Si F Cl 
______________________________________ 
control none 84 14 2 &lt;1 &lt;1 
7* O.sub.2 60 33 4 nd nd 
8* Cl.sub.2 60 N.sub.2 
5 1 nd 34 
9* F.sub.2 + N.sub.2 
17 34 26 23 nd 
10* Cl.sub.2 9+ O.sub.2 
45 43 &lt;1 2 
11 F.sub.2 + O.sub.2 
&lt;1 51 43 18 &lt;1 
______________________________________ 
*Comparative examples. "nd" indicates analysis not done 
The amount of silicon potentially removed with the F.sub.2 +O.sub.2 process 
was determined independently by exposing a wafer with an oxide mask and 
open areas of silicon to the cleaning process. After exposure to the 
process, the oxide mask was removed and the amount of silicon removed 
during the cleaning process was determined by stylus profilometry as 
described in Example 4 above. The results of that test indicate that less 
than 50 .ANG. of silicon are removed during this 180 second process. 
The roughness of the surface was also measured after the cleaning process 
by atomic force microscopy (AMF). The surface of the wafer used in the 
original experiment described above was analyzed near the center of the 
wafer. AFM measurements were taken at 2 1-micron square sites. The 
roughness was measured as &lt;1 .ANG. Ra. This is equivalent to the roughness 
of a new silicon wafer. Maintenance of a smooth surface during cleaning 
processes is critical since surface roughness has been shown to 
significantly degrade the performance of semiconductor devices. 
EXAMPLE 12 
Further tests as in examples 7-11 using UV and an 8% (volume basis) mixture 
of F.sub.2 in O.sub.2 gave significant etching of silicon whereas UV/4% 
F.sub.2 /O.sub.2 results indicated that a passivating thin (7-9 .ANG.) 
silicon oxyfluoride was formed (approximately 30% F and 70% O) with 
negligible etch of silicon, negligible hydrocarbon recontamination 
following 12 hours exposure to ambient atmosphere, and with a surface 
roughness comparable to RCA cleaned silicon control. These results 
indicate that UV/F.sub.2 /O.sub.2 processes, particularly at a F.sub.2 
concentration of less than 8%, are suited to pre-gate insulator surface 
conditioning applications. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof, reference being 
made to the appended claims rather than to the foregoing description to 
indicate the scope of the invention.