Oxidation of silicon wafers to eliminate white ribbon

Disclosed is a process for eliminating the migration of nitrogen or nitrogen hydrides (e.g., NH.sub.3) to a Si-SiO.sub.2 interface site during silicon-nitride-masked oxidation--using an HCl additive to a pyrogenic oxidizing medium to react with the nitrogenous entity and so prevent formation of silicon nitride at this site [e.g., in a die zone intended for later reception of a precise oxide layer]--this improved technique resulting in increased yield, improved reliability and better electrical parameters.

INTRODUCTION 
The present invention relates generally to improvements in the manufacture 
of integrated circuit devices, and more particularly to improved 
techniques for developing oxide films on integrated circuit substrates. 
INVENTION BACKGROUND, FEATURES 
Workers in the art of making integrated circuit devices are well aware of 
the need for optimal techniques for developing silicon oxide films on an 
integrated circuit (wafer) substrate. This invention is intended to 
provide improved techniques for growing a silicon oxide film on such 
substrates. 
Silicon dioxide has played a major role in the fabrication of silicon 
[micro-electronic] devices and their operation since approximately 1958. 
If a wafer of silicon is heated in an atmosphere of oxygen or water vapor, 
a film of silicon dioxide is readily formed on its surface. This film is 
hard, durable [e.g., softening about 1400.degree. C.] and it firmly 
adheres to the silicon substrate. It makes an excellent electric insulator 
and is very convenient to use in the fabrication of integrated circuits, 
serving as a mask for the selective introduction of dopants. 
Convenient thicknesses of silicon dioxide can easily be grown in an 
oxidizing atmosphere at temperatures on the order of 1000.degree. to 
1200.degree. C.--thickness being rather precisely controlled by selecting 
the appropriate time and temperature of oxidation. For example, a 0.1 
micrometer layer of oxide will grow on a Si wafer exposed to an atmosphere 
of pure oxygen for about one hour at a temperature of 1050.degree. C. 
[substituting steam, for the pure oxygen, will grow a layer five times as 
thick]. As workers well known, such pyrogenic oxidation is popular for 
growing such precision silicon dioxide films; it is very convenient and 
inexpensive to use (several hundred wafers can be simultaneously oxidized 
in a single run). 
More particularly, workers are aware that during the preparation of certain 
types of integrated circuits, the silicon wafer substrates are oxidized to 
establish an SiO.sub.2 film adjacent silicon nitride mask areas (e.g., to 
place a thick "Field Oxide" there). It has been found that, in the course 
of growing such an oxide film (pyrogenically), a deleterious "white 
ribbon" effect can commonly occur injecting an undesired silicon nitride 
film under their mask areas. Such "white ribbon" films are commonly formed 
at a silicon/silicon oxide interface, where an SiO.sub.2 layer (thermal 
buffer) underlies the nitride mask during pyrogenic oxidation accompanied 
by the presence of nitrogenous products (e.g., believed to include 
ammonium evolved from the nitride mask; see article by Kooi, et al. page 
1117 et seq., Volume 123 of Journal Electro-Chemical Society, 1976). 
Such "white ribbon" films have become familiar as potentially disadterous 
to device yield. As explained below, they are believed to represent the 
reaction product of ammonium with silicon and constitute a thin silicon 
nitride film that is quite difficult, expensive and inconvenient to remove 
[e.g., a special etching might be carried out, but is contra-indicated 
because of the added time and expense it would involve and because it 
would likely damage the SiO.sub.2 film and the Si substrate.] 
It is believed, though not certain, that the nitrogen constituent of the 
"white ribbon" film finds its way to the film-site by diffusing through 
the silicon oxide to reach the silicon substrate. Nitrogen is readily 
diffusible through SiO.sub.2 at the prevailing high ambient temperatures, 
and ammonia gas would appear to be more effective in producing this 
nitride than molecular nitrogen. However, it is not certain whether these 
and/or some other "N entity" are operative. 
The resultant nitride film has a "masking effect" against oxidation and 
thus will interfere with the superposition of an oxide layer. For 
instance, a "white ribbon" spot will obstruct a "gate oxide" of a MOS 
structure, making it too thin and resulting in unacceptably low 
gatebreakdown-voltage and device rejection. 
OBJECTS OF INVENTION 
Accordingly, it is an object of this invention to provide improved 
techniques for growing silicon oxide on a hot silicon substrate and to 
provide improved integrated circuits so coated. 
Another object is to grow such an oxide coating adjacent silicon nitride 
layers to be relatively free of undesired silicon nitride residue and the 
"white ribbon" effect under the nitride layer. It is a further object to 
provide such oxide coatings free of "white ribbons" as part of a pyrogenic 
oxidation and despite the presence of nitrogen and/or nitrogen compounds 
(especially ammoniated gas). 
These and other objects and related features of invention are accomplished 
by the improved techniques taught herein, such as by the addition of 
hydrogen chloride to the hot oxidizing gases used to grow SiO.sub.2 
--sufficient HCl to combine with any nitrogenous constituents 
present,--especially where the substrate includes a pure silicon surface 
adapted to receive a following precision thickness film, and more 
especially where this Si surface is covered by an SiO.sub.2 layer and 
where "white ribbon" strips are prone to form at the Si/SiO.sub.2 
interface. This also facilitates removal of gaseous products (of the 
HCl-ammonia reactions) from the oxide-growth zone. 
Also taught are techniques associated with the pyrogenic oxidation of a 
silicon substrate on which silicon nitride layers are superposed, on 
intervening SiO.sub.2 layers, wherein nitrogen and/or nitrogen compounds 
are prevented from migrating to the Si-SiO.sub.2 interface and forming 
silicon nitride films there; and particularly where sufficient hydrogen 
chloride is added to the oxidizing medium to react with free nitrogenous 
constituents present and so prevent such formation of silicon nitride at 
this interface; and further to convert such free "N entities" to a gaseous 
ammonium chloride product which is readily removed from the reaction zone 
at prevailing process temperatures. 
In a preferred embodiment, the invention involves an improved technique for 
preparing such an Si/SiO.sub.2 interface for removal of SiO.sub.2 and, 
formation of a thin, controlled-thickness silicon oxide film there, 
without "white ribbon" interference--and as involving prescribed pyrogenic 
oxidizing media at a prescribed processing temperature upon a prescribed 
silicon wafer substrate (this substrate being susceptible to formation of 
"white ribbon compounds" at its interface with a silicon oxide film and a 
"cation portion" of said compound being apt for combination with a 
prescribed anionic material), this technique being adapted to generate a 
prescribed vapor compound at said processing temperature and 
conditions--the technique involving the addition of sufficient of at least 
one additive reagent including said anionic material to so combine with 
said cationic material and form said vapor compound at said processing 
conditions--whereby to eliminate formation of said "white ribbon" 
interface compounds and to further facilitate the dissipation of the vapor 
compound product. 
In a particular useful form, this anionic material comprises chlorine and 
the vapor-compound comprises a chloride of nitrogen, with the white ribbon 
consisting of ultra-thin interface strips of a nitrogen compound. It is 
preferred that the additive reagent comprise hydrogen chloride and that 
the vapor compound generated comprise a gaseous chloride of nitrogen, such 
as ammonium chloride particularly. 
In a preferred form, the thin oxide film is to be pyrogenically grown (from 
a few hundred to a few thousand A.degree.) on prescribed first surface 
zones of a silicon wafer substrate, with each such zone disposed adjacent 
respective second zone including silicon oxide--these first zones, in 
turn, being protectively covered by a silicon nitride mask layer--the 
"white ribbon strips" being prone to form in the first zones of the 
substrate, under the mask layer, such as to be left as a troublesome 
"residue" once the mask layers are removed and prior to growing said thin 
oxide films. 
Techniques according to this invention are especially useful in preventing 
white ribbon from interfering with the ultra-thin (UT) oxide films to be 
grown in active gate zones of a silicon wafer for MOS integrated circuits, 
particularly where, as mentioned, a nitride mask layer overlies an 
intervening insulation layer of silicon oxide in the gate zone and these 
mask and insulation layers are to be removed prior to growing such UT 
films there. 
This is particularly true when an ultra-thin SiO.sub.2 gate oxide is to be 
grown only after a first layer of silicon oxide is placed upon the entire 
wafer, followed by placement of the nitride mask "covers" over each gate 
zone and extending therebeyond--this being followed by a "field oxide 
growing" step wherein the original silicon oxide film is thickened, except 
under the nitride "covers" where, however, it extends a bit under the 
margin of these covers, and is gradually attenuated in thickness, to form 
a "beak"-- with the mentioned white ribbon spots formed on the substrate, 
in the gate zones and just beyond these "beak" portions. 
Such methods according to the invention have been found particularly 
effective when used for such purposes in growing oxide pyrogenically 
(preferably using steam at about 950.degree. C. or more) wherein a first 
silicon oxide layer (of about 1400 A.degree.) is grown. Then nitride mask 
layers (about 1500 A.degree.) are applied, and then the field oxide is 
expanded (to about 1.1 um thickness; HCl added to steam at 950+.degree.C.) 
and then the nitride "covers" removed along with the underlying silicon 
oxide layers overlying the gate zones so that the ultra-thin "gate oxide" 
may then be pyrogenically grown (on the order of 1000 A.degree. thick and 
free of "white ribbon").

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
As an example of how "white ribbon" is typically evolved, and of how 
techniques according to the invention can ameliorate this condition, the 
following description is presented, in conjunction with FIGS. 1-6. Workers 
will recognize this as involving the preparation of MOS integrated circuit 
devices --in particular the growth of a thick SiO.sub.2 layer ("field 
oxide") on a silicon wafer. Here, and elsewhere, it may be assumed that, 
except as otherwise mentioned, conventional techniques, materials and 
apparatus are employed to render conventional results. 
As indicated in FIG. 1 this may be understood as involving a silicon wafer 
substrate C--and particularly, a portion thereof adjacent an "active gate" 
area ("aa") flanked by adjacent "field" areas ("bb", "cc"). In general it 
should be understood that an Si.sub.3 N.sub.4 mask layer A covers the gate 
area and that a thick SiO.sub.2 "field oxide" layer B is to be grown in 
the field zones ("bb", "cc") flanking gate zone "aa"--then, the mask to be 
removed and a thin "gate oxide" grown in the gate zone. 
FIG. 1 shows the active gate area "aa" also covered insulating SiO.sub.2 
layer B [e.g., about 1400 A.degree.; this layer being, in turn, covered by 
the "masking" layer, A, of silicon nitride, here, preferably about 1500 
A.degree. of Si.sub.3 N.sub.4 ]. Silicon nitride layer A will be 
understood as functioning conventionally as a mask to prevent oxidation in 
active area "aa". Silicon oxide layer B serves to prevent the formation of 
dislocations which might be induced in the silicon substrate C if it were 
heated in direct contact with the nitride layer. (Thermal stress 
differential ameliorated by layer B). Oxide layer B will also be 
understood as influencing the shape of the "sunk-oxide" pattern, discussed 
below. Preferably, as illustrated, layers A and B are extended a bit 
beyond active zone "aa" [see dimension aa']. 
Pyrogenic Oxidation: 
As depicted schematically in FIG. 2, a relatively conventional pyrogenic 
oxidation is next invoked to grow the thick "field oxide" and produce the 
"sunk oxide" formation [see nitride layer A' depressed into wafer C above 
zone "aa"]. Here, a steam oxidation is preferred [10 hours at 950.degree. 
C. in wet oxygen atmosphere: dew point: 95.degree. C.]. This thickens the 
SiO.sub.2 film B' over the "field" areas "bb", "cc" [see layer B', with a 
maximum thickness "dd" on the order of about 1.1 um., preferably; 
narrowing to a "beak-like" point, or transition, under mask A' where it is 
joined with the original thin SiO.sub.2 layer over gate region "aa"]. The 
presence of original oxide layer B (FIG. 1) will be understood as 
influencing the shape of this "sunk-oxide" pattern, while a lateral 
oxidation effect produces the smooth beak-like point under the mask edge 
(where the mask is "lifted up"). 
As an example of such pyrogenic oxidation, consider the following: 
Several hundred silicon wafers (each approximately 3" diameter, about 200 
um. thick) are loaded into receiving slots in several (quartz) "boats", 
with a few millimeters between adjacent wafers. The boats are injected 
into a high temperature furnace consisting of a conventional elongate 
quartz tube surrounded by a cylindrical heating element with means for 
injecting, and for removing, prescribed streams of process gases. 
With the boats of wafers loaded into the open end of the tube and slowly 
pushed into the hottest portion, oxidation may begin [the initial tube 
atmosphere when the boats are so loaded consists principally of N.sub.2 
with a little O.sub.2, about 7%]. The temperature in the tube's process 
zone is typically closely controlled [e.g., to 1.degree. C. or better]. 
Often the entire procedure is computer-controlled--e.g., with a small 
"process control computer" monitoring furnace temperature, directing the 
insertion and withdrawal of boats, and controlling the internal atmosphere 
and other accessories of the furnace. 
Formation of nitride "white ribbon" under nitride mask: 
FIG. 3 is an enlarged partial version of FIG. 2 and schematically depicts 
what is believed to be the mechanism involved in forming of the nitride 
"white ribbon" layers r at the interface of silicon oxide layer B' and the 
top of silicon wafer C in zone "aa" (--where the ribbon underlies mask A', 
adjacent the mask edge and close to the point of the "oxide-beak"). 
For instance, nitride masking layer A', and the underlying (1400 A.degree.) 
oxide layer are typically removed (stripped by etching) as indicated 
schematically in FIG. 4--however, leaving the "white ribbon" nitride 
strips R,R' as a residue on wafer C in the critical active zone "aa". That 
is, the conventional treatments (e.g., etching) for removing the nitride 
mask and the underlying oxide unfortunately, leave the ribbon segments R, 
R' unaffected [and if a special "ribbon-etch" were used it would inflate 
production costs and likely degrade the wafer and SiO.sub.2 films]. 
Normally, the "gate oxide" layer should be formed to a very precise 
thickness over the active (gate) zone (e.g., see conventional gate oxide 
film B".sub.g in FIG. 5, shown undisturbed by "white ribbon"). As workers 
know, it is crucial to good MOS gate operation to form this oxide layer 
B".sub.g to a very precisely controlled, highly uniform thickness. And 
this problem is compounded by the fact that oxide B".sub.g is so 
ultra-thin (ordinarily only about 1000 A.degree.). 
But the "white ribbon" upsets all this and reduces the effective thickness 
of the gate oxide--as indicated in FIG. 5A, not only by interposition of 
the reduced-resistivity ribbon thickness R, R', but also by causing less 
oxide to grow above the ribbons (note depressions D, D' above ribbons R, 
R' in FIG. 5A). These depressions are, evidently, formed because "free 
silicon" is depleted at the ribbon sites (i.e., little or no Si available 
for forming SiO.sub.2 above the ribbons because it is tied-up in nitride 
form). Workers will readily appreciate how disastrous such a two-fold 
reduction in resistance and dielectric strength (at ribbon sites) can be. 
Workers will recognize that the presence of "white ribbon" is entirely 
unacceptable for such an embodiment since the white ribbon layers will 
necessarily prevent a proper critical-thickness "gate oxide" layer from 
being formed in this active zone "aa" and will interfere with formation of 
a precise, ultra-thin "gate oxide" film (on zone "aa") and degrade desired 
film properties. 
That is, as indicated schematically in FIG. 5A, it will be understood that 
ultra-thin "gate film" B".sub.g (e.g., only about 1000 A.degree. nominal 
thickness SiO.sub.2 grown during about 45 minutes oxidation at about 
1100.degree. C.) would be reduced in thickness and will exhibit obvious 
weakened, reduced-thickness areas D, D' above white ribbons R, R'. These 
weakened areas can be expected to have such a radically reduced resistance 
and thickness across the gate film and create problems of low 
gate-breakdown-voltage and device failure, as workers in the art will 
readily appreciate. 
Any further processing of the device is beyond the scope of this 
discussion, but might include conventional MOS transistor formation [e.g., 
depositing a polycrystalline silicon film; defining gate, source and drain 
areas, etc.]. 
Pecularities of "white ribbon": 
While one cannot presently be certain, it appears that the silicon nitride 
"white ribbon" is produced, generally speaking, when the hot oxidizing 
medium reacts with any silicon nitride present [e.g., with layer A, FIGS. 
1 and 2]. This probably produces [among other things] ammonium, (and 
possibly nitrogen and other nitrogen compounds) and SiO.sub.2 
constituents. The nitrogenous reagents thus formed could include ammonia 
gas or some ammonium compound (and other hydrides of nitrogen), and/or 
pure (atomic) nitrogen. The nitrogenous reagents are believed to migrate 
to the Si/SiO.sub.2 interface to produce the ribbon nitride film [i.e., 
Si.sub.3 N.sub.4, which, as mentioned, cannot readily be removed]. As 
workers know, this "white ribbon" is readily visible (usually with an 
optical microscope; hence, it's name) and might, for an embodiment like 
the foregoing, be about 1 um. (10.sup.-6 meter) wide. The ribbon location 
suggests a narrow region of non-oxidized silicon just beyond the point of 
the "oxide-beak"; this supports the theory of migration of nitrogen (or 
its compounds) to that site (see site II, FIG. 3). 
Workers will remember that the silicon substrate in this active gate zone 
(adjacent site II) is covered only by a relatively thin Si.sub.3 N.sub.4 
oxidation-mask layer A', Also, some silicon nitride is presumably formed 
in the ribbon area [at site II, the Si/SiO.sub.2 interface] during the 
local oxidation process above-mentioned. This nitride may, of course, be 
pure Si.sub.3 N.sub.4 and/or may comprise related nitrogen compounds. 
Various theories have been offered to explain the "white ribbon" effect 
"e.g., see above cited article, Kooi, et al.]. One theory is based on the 
transfer of (free) nitrogen from the nitride mask layer A [FIGS. 1 and 2, 
e.g., from site I in FIG. 3] to the Si/SiO.sub.2 interface [as indicated 
at site II in FIG. 3, for instance]--pure N.sub.2 readily passing through 
SiO.sub.2 layer B'. It will, of course, be assumed that the diffusion of 
steam, normally, through layer B' allows it to interact with the silicon 
of wafer C to produce SiO.sub.2 plus H.sub.2, there, conventionally [see 
path i, FIG. 3]. 
In addition, it is theorized that a "lateral" diffusion (of N.sub.2, etc.) 
takes place under the edge of mask A' [see path ii], so that when the hot 
steam reaches a given point along the interface between the mask and 
SiO.sub.2 layer B' [site I or, at least some of it], it will react with 
the Si.sub.3 N.sub.4 to produce SiO.sub.2 and NH.sub.3 (thus helping to 
form the ultimate beak-shaped pattern indicated--while also oxidizing 
silicon from substrate C, as for path i). This oxidation of the nitride 
layer A' will likely occur not only at the top side of layer A' but also 
on its underside (under the mask edges, there forming NH.sub.3 and/or 
nitrogen atoms or some other NH compound). And, the reactive nitrogen (or 
N compounds) so formed is assumed to diffuse relatively easily through the 
SiO.sub.2 layer to reach the Si wafer C along the active gate zone [that 
is, proceeding along path iii to site II, FIG. 3] and form the white 
ribbon there. 
Now, at the outer edge of mask A', it is assumed that little or no Si.sub.3 
N.sub.4 is formed because of the competing oxidation reaction: of Si with 
hot H.sub.2 O vapor. However, at some distance in from the edge of mask A' 
this will not be so--and, there (site II), Si will be free to combine with 
the "in-migrating" NH.sub.3 to form a Si.sub.3 N.sub.4 ribbon (--this 
presumably at a point where the concentration of steam H.sub.2 O drops low 
enough to allow the Si to so react; or, more probably, Si will react there 
with a mixed compound of silicon, nitrogen, oxygen and possibly, 
hydrogen--thus forming up the "white ribbon" film). 
Thus, as indicated in FIG. 4, when the nitride mask A' and its underlying 
SiO.sub.2 film B' will be understood as removed, the nitride ribbons R, R' 
nonetheless remaining [typically, up to 10 to 20 A.degree. thick]. 
Consequently, the "gate oxidation" [layer B".sub.g in FIG. 5A] will not be 
effective in gate zone "aa" and defective gate characteristics can be 
expected. 
At times, the white ribbon effect may not be observed because the white 
ribbon film is removed by an "over-etching" operation (presumably 
unintentional). 
HCl Addition: 
As a feature of this invention, it was discovered that adding a minor 
concentration of hydrogen chloride (HCl) to the oxidizing (hot steam) 
medium can essentially eliminate the white ribbon effect. For instance, 
with an embodiment like the foregoing, it is feasible to add about 2% HCl 
to the steam oxidant [at 950.degree. C.] to grow the field oxide (B', FIG. 
2). This has been observed to effectively eliminate "white ribbon". 
Presumably, this is because enough HCL is available adjacent the normal 
ribbon site to react with the available white ribbon formers (N.sub.2 or N 
compounds--these reacting in the manner of a Lewis Base with the 
HCl-"Lewis acid" to form NH.sub.4 CL gas--a reaction product which, 
happily, is readily vaporized at the prevailing ambient process 
temperature). 
While workers might be inclined to contemplate various substituents for 
this HCl additive (to oxidant) many will be found undesirable. For 
example, other halogen acids appear unsatisfactory and chlorine alone is 
contra-indicated as being too noxious and too corrosive to handle (as well 
as obviously unable to serve as the described "Lewis Acid"). 
The NH.sub.4 CL vapors are readily swept out of the reaction zone and thus 
kept from affecting oxidation [NH.sub.4 CL is vaporized at about 
520.degree. C., whereas the oxidation proceeds at about 
950.degree.-1000.degree. C. as mentioned above]. Thus, presumably, the 
free nitrogenous moiety is prevented from reaching the silicon substrate C 
(site II, FIG. 3), and cannot form the white ribbon nitride there. 
The result is illustrated schematically in FIG. 6 where the stripping steps 
[removal of nitride layer A' and removal of SiO.sub.2 layer B' in the 
active gate region] will be understood as leaving the gate region "aa", 
atop silicon wafer C, free of any white ribbon films--and thus the 
subsequent growth of ultra-thin gate oxide film B".sub.g (FIG. 5) can 
proceed unimpeded, unmarred by nitride anomalies or thickness attenuation. 
It will be understood that the preferred embodiments described herein are 
only exemplary, and that the invention is capable of many modifications 
and variations in construction, arrangement and use without departing from 
the spirit of the invention. 
Workers will understand that others have suggested various other ways of 
using HCl, or like additives, for other purposes, including that of 
forming oxide on a circuit substrate [e.g., see U.S. Pat. Nos. 3,549,411, 
3,574,677, 3,692,571, Re 28,385 and Re 28,386]. 
Some modifications of the invention come to mind. For example, other 
additive agents, besides HCl, may be used for gaining the same effect. The 
means and methods disclosed herein are also applicable to techniques for 
growing related precision oxide insulator-films for other applications. 
Also, the invention is applicable with a plurality of such oxide 
films--either superposed or side-by-side. 
The above examples of possible variations of the present invention are 
merely illustrative. Accordingly, the present invention is to be 
considered as including all possible modifications and variations coming 
within the scope of the invention as defined by the appended claims.