Highly selective chemical dry etching of silicon nitride over silicon and silicon dioxide

A dry etch process is described for removing silicon nitride masks from silicon dioxide or silicon for use in a semiconductor fabrication process. A remote plasma oxygen/nitrogen discharge is employed with small additions of a fluorine source. The gas mixture is controlled so that atomic fluorine within the reaction chamber is maintained at very low flows compared with the oxygen and nitrogen reactants. Parameters are controlled so that an oxidized reactive layer is formed above any exposed silicon within a matter of seconds from initiating etching of the silicon nitride. Etch rates of silicon nitride to silicon of greater than 30:1 are described, as well as etch rates of silicon nitride to silicon dioxide of greater than 70:1.

TECHNICAL FIELD 
This invention relates to processes for making semiconductor devices, and 
more particularly, to a practical process for selective chemical dry 
etching of silicon nitride with respect to silicon and/or with respect to 
silicon dioxide. 
BACKGROUND OF THE INVENTION 
Silicon integrated circuits typically electrically isolate individual field 
effect transistors, bipolar transistors, and any substrate resistors and 
other elements with silicon dioxide ("oxide") regions at the surface of a 
silicon wafer. These oxide isolation regions can be directly formed by a 
thermal oxidation of a silicon wafer with an oxidation barrier such as 
silicon nitride ("nitride") masking off areas which will eventually 
contain transistors, substrate resistors, and other elements. This method 
of oxidation of selected regions of a silicon wafer has acquired the 
acronym LOCOS ("local oxidation of silicon"). 
Typical LOCOS includes using a thin oxide layer between the nitride mask 
and the silicon wafer to provide stress relief during thermal oxidation. 
However, thermal oxidation of silicon proceeds essentially isotropically, 
and the oxidation encroaches under the nitride mask along the pad oxide to 
form an oxide wedge. FIGS. 1-2 illustrate LOCOS with nitride mask 102 on 
pad oxide 104 which is on silicon wafer 106. FIG. 1 is prior to thermal 
oxidation and FIG. 2 is after thermal oxidation, which forms isolation 
oxide 110. As shown, wedges 110 warp nitride 102 and may generate defects 
in the adjacent silicon wafer due to the stresses generated. 
The stripping of silicon nitride mask material after the local oxidation of 
silicon is a possible source of damage to the pad oxide and/or the 
underlying device area silicon. For example, the pad oxide can suffer 
degradation during an over-etch or etchants can reach the underlying 
silicon substrate through imperfections in the pad oxide. Using current 
etch techniques, etching of silicon occurs at a significant rate and the 
result can be craters in the substrate, which are referred to as 
"pitting". 
Removal of the nitride mask 102 after LOCOS thermal oxidation requires a 
nitride etch which will stop on the pad oxide and thereby avoid damaging 
the underlying device area silicon. The standard nitride etch uses a bath 
of hot phosphoric acid (H.sub.3 PO.sub.4) which is highly selective to 
oxide. However, wet etches introduce undesired contamination of a wafer 
for two reasons: liquids typically cannot be purified sufficiently and the 
wafer must be removed from the oxidation chamber for the wet nitride 
stripping (plus pad oxide removal and cleanup) and then reinserted into 
the process chamber for subsequent steps, typically a thermal oxidation to 
form gate oxide. An all dry processing sequence for nitride stripping can 
avoid the wet etch and the removal/reinsertion contamination sources. 
Nitride can also be used in other integrated circuit processing steps which 
subsequently require isotropic stripping. For example, a wafer with a 
nitride backside seal and a frontside deposited protective oxide may 
require a selective nitride strip to avoid disturbing the frontside oxide. 
Current dry etch processes used for the nitride stripping step have the 
undesired effect of a significantly higher etch rate of silicon if the 
etchants reach the underlying substrate. For example, reference 
Kastenmeier et al. "Chemical Dry Etching of Silicon Nitride and Silicon 
Dioxide Using CF.sub.4 /O.sub.2 /N.sub.2 Gas Mixtures," J. Vac. Sci. 
Technol. A 14(5), pp. 2802-2813 (1996). This article discloses that the 
silicon nitride etch rate in a chemical dry etch increases by a factor of 
two as oxygen is added to a CF.sub.4 microwave discharge. The addition of 
N.sub.2 to a CF.sub.4 /O.sub.2 plasma increases the etch rate by an 
additional factor of seven. Both oxygen and nitrogen are thus found to be 
necessary to enhance the etch rate significantly. However, the article 
assumes a regime where fluorine atoms are available in abundance, and the 
process described again comprises a process wherein etching of exposed 
silicon occurs much faster than etching of nitride. For example, reference 
Matsuo et al. "Role of N.sub.2 Addition on CF.sub.4 /O.sub.2 Remote Plasma 
Chemical Dry Etching of Polycrystalline Silicon," J. Vac. Sci. Technol. A 
15(4), pp. 1801-1813 (1997). This article discloses that the silicon etch 
rate (under conditions presented for the silicon nitride etch above) is a 
factor 10-15 higher than that of silicon nitride. 
There thus exists a need in the art for a chemical dry etch process for 
highly selective etching of silicon nitride over both silicon and silicon 
dioxide. The present invention provides such a process. 
DISCLOSURE OF THE INVENTION 
Briefly summarized, this invention comprises in one aspect a method for 
chemical dry etching silicon nitride. The method includes: exposing the 
silicon nitride to a source of oxygen; and simultaneous therewith, 
exposing the silicon nitride to a source of fluorine, the source of 
fluorine being controlled so that oxidation of silicon is favored over 
etching of silicon so that etching of the silicon nitride produces an etch 
selectivity of silicon nitride to silicon greater than 5:1, and so that 
the etch rate of the silicon remains less than 1 nm/min. 
As a more specific embodiment, the invention comprises exposing the layer 
of silicon nitride to a source of oxygen and a source of fluorine so that 
source of oxygen flow to source of fluorine flow is approximately 20:1 or 
greater (or 5:1 or greater if considering atomic fluorine flow), and the 
etch selectivity of silicon nitride to silicon is greater than 10:1. 
Further, the silicon nitride is preferably exposed to a gas mixture 
containing a source of oxygen and a source of nitrogen, wherein the source 
of oxygen has a flow rate in a range of 250-5000 sccm and the source of 
nitrogen has a flow rate in a range of 25-5000 sccm, wherein the source of 
fluorine is controlled so that etching of the silicon nitride proceeds in 
a fluorine-starved environment. 
Advantageously, chemical dry etching in accordance with the principles of 
this invention produces much higher silicon nitride to silicon and silicon 
nitride to silicon dioxide etch rate ratios than currently possible using 
existing techniques. Chemical dry etching in accordance with this 
invention is robust and silicon surface roughening is unobserved, and in 
fact, surface smoothing appears to occur. The technique presented herein 
employs simple, non-hazardous gases and repairs porous silicon dioxide 
thin films, e.g., less than 5 nm thick. Further, the technique minimizes 
the so-called "loading effect", since it is based on a surface mechanism, 
thus simplifying tool design.

BEST MODE FOR CARRYING OUT THE INVENTION 
Generally stated, this invention comprises a new approach for the stripping 
of silicon nitride (Si.sub.3 N.sub.4) layers in a silicon-based 
semiconductor fabrication process. (Unless otherwise indicated herein, 
"silicon" can refer to either single crystal silicon or polysilicon.) The 
approach employs a remote plasma oxygen/nitrogen discharge with small 
additions of a fluorine source, e.g., CF.sub.4 or NF.sub.3. Significant to 
the present invention is the maintenance of atomic fluorine in the 
reaction chamber at very low levels compared with prior chemical dry 
etching approaches. Further, in accordance with the present invention, 
maintenance of atomic fluorine at very low levels occurs in combination 
with maintaining an oxygen/nitrogen-rich environment within the reaction 
chamber. 
Etch rates of Si.sub.3 N.sub.4 of more than 30 nm/min can be achieved for 
CF.sub.4 as a source of fluorine. The etch rate ratio of Si.sub.3 N.sub.4 
to polycrystalline silicon can be 35 or more in accordance with this 
invention, while silicon dioxide (SiO.sub.2) is not etched at all. For 
NF.sub.3 as a fluorine source, Si.sub.3 N.sub.4 etches at a rate of 50 
nm/min, while the observed etch rate ratios to polycrystalline silicon and 
SiO.sub.2 are approximately 100 and 70, respectively. Within the chemical 
dry etching environment presented herein, in situ monochromatic 
ellipsometry shows the formation of an approximately 10 nm thick reactive 
layer on top of the polycrystalline silicon. This oxidized reactive layer 
beneficially suppresses etching reactions of the reactive gas phase 
species with the silicon. In particular, when low levels of a fluorine 
source are used in conjunction with these thick, oxide-like reaction 
layers, these etching reactions can be suppressed to a negligible value. 
In prior work such as described in the above-referenced and incorporated 
article by Kastenmeier et al. entitled "Chemical Dry Etching of Silicon 
Nitride and Silicon Dioxide Using CF.sub.4 /O.sub.2 /N.sub.2 Gas 
Mixtures," the etching of silicon nitride in the afterglow of certain 
CF.sub.4 /O.sub.2 /N.sub.2 and NF.sub.3 /O.sub.2 discharges was 
investigated. In these prior approaches, the flow of the fluorine source, 
CF.sub.4 or NF.sub.3, was kept at a relatively high constant value, and 
O.sub.2 and N.sub.2 were added in varying amounts. The constant fluorine 
source values employed were such that the reaction chamber was flooded 
with fluorine atoms during the etching of silicon nitride. The etch rate 
of silicon nitride was found to be correlated to the density of nitric 
oxide (NO) in a linear relationship for both gas mixtures. No similar 
correlation was found between the silicon nitride etch rates and the 
density of atomic fluorine. 
In accordance with this invention, applicants have discovered that the F 
atom concentration (determined from the polycrystalline silicon etch rate 
and mass spectrometry) is higher by at least a factor of 20 than necessary 
to sustain the measured silicon nitride etch rates. Thus, we have 
concluded that the arrival of NO is the etch rate limiting step in silicon 
nitride etching, and fluorine atoms are available in abundance using prior 
techniques of silicon nitride etching. 
The etch rates of silicon dioxide for the same parameters were found to be 
independent of the NO concentration, but followed the F radical density 
very well. The CF.sub.4 based chemistry produces the desired etch rate 
ratio to silicon dioxide. With oxygen and nitrogen added to CF.sub.4, 
ratios of 10 or slightly higher can easily be obtained. The etch rate of 
silicon nitride typically is 30 nm/min, and that of silicon dioxide 3 
nm/min. The etch rates of silicon dioxide in the afterglow of NF.sub.3 
/O.sub.2 discharges were too high to achieve a selectivity to Si.sub.3 
N.sub.4 ; for example, typical etch rate values for SiO.sub.2 and Si.sub.3 
N.sub.4 are 60 nm/min and 80 nm/min, respectively. 
Etching of silicon always occurred at rates much faster than those of 
silicon nitride. Etch rates are as high as 300 nm/min for CF.sub.4 
/O.sub.2 /N.sub.2 and 700 nm/min for NF.sub.3 /O.sub.2 gas mixtures at 
parameters identical to those for which the etch rates above for silicon 
nitride and silicon dioxide are given. These high etch rates can be 
explained with the high fluorine atom density and the spontaneous reaction 
of a silicon surface with fluorine atoms. In the absence of a prohibitive 
reaction layer, the etch rate of silicon has been found to be linear to 
the fluorine density. Further, a decrease of the silicon etch rate is 
consistently observed for additions of 20% or more O.sub.2 to small flows 
of CF.sub.4 or NF.sub.3 plasma. This effect is due to the oxidation of the 
silicon surface in the presence of O or O.sub.2. The oxidation makes the 
silicon surface very similar to that of silicon dioxide during etching, 
thus, in the limit of very high flows of O.sub.2, and low flows of 
CF.sub.4 or NF.sub.3, the etch rates of both materials are nearly the 
same. 
Based upon the above observations, presented herein is a new process for 
the etching of silicon nitride that gives high selectivities to both 
silicon dioxide and silicon. A mixture of O.sub.2 and N.sub.2 is used as 
the primary discharge gas, to which small amounts of fluorine source 
(CF.sub.4 or NF.sub.3) are admitted. This is important in that the 
Si.sub.3 N.sub.4 etch rate becomes limited by the arrival of atomic 
fluorine, while the Si etch rate is limited by the thick oxide-like 
reaction layer that forms in the presence of such levels of oxygen and 
nitrogen. This allows for maximizing the Si.sub.3 N.sub.4 etch rate, under 
the constraint that the Si etch rate remains negligible. In accordance 
with the principles of the present invention, three mechanisms contribute 
to a high silicon nitride/silicon selectivity which can be exploited by 
choosing an O.sub.2 /N.sub.2 based chemistry, rather than a fluorine based 
chemistry. These three mechanisms are summarized below. 
Oxidation of the silicon surface 
The first mechanism employed by the present invention is the oxidation of 
the silicon surface in the presence of a high concentration of O and 
O.sub.2 in the gas phase. This oxidation results from a shift in the 
balance between oxidation and fluorination of the Si controlled by 
maintaining low flows of the fluorine source with respect to the oxygen, 
e.g., 15Q.sub.(F source) &lt;Q.sub.O2. The resultant oxidized silicon 
surface, which forms in a few seconds, is very inert to the attack of 
fluorine atoms; and therefore, the silicon etch rate is significantly 
decreased. In addition to oxygen species, NO, which is produced by the 
O.sub.2 /N.sub.2 discharge, has been found to contribute to the oxidation 
of silicon surfaces. 
The equivalent flow of NO into the processing chamber that is sufficient to 
enhance this oxidation, while in the presence of oxygen and fluorine, is 
dependent on the level of fluorine introduced, as the oxidation and 
fluorination of the silicon are competing mechanisms. For example, in the 
systems containing high flows of a fluorine source (e.g., 400 sccm 
CF.sub.4), the fluorine density can be controlled by admixing oxygen to 
the plasma. (See above-referenced article entitled, "Role of N.sub.2 
Addition on CF.sub.4 /O.sub.2 Remote Plasma Chemical Dry Etching of 
Polycrystalline Silicon.") As the density of fluorine is decreased, the 
flow of NO required to enhance the oxidation, and hence quench the etching 
reaction, is decreased. In such a chemistry as proposed in the current 
invention, the fluorine density will be significantly lower than that 
produced by the 400 sccm of CF.sub.4 cited in this example. It is 
therefore safe to assume that any equivalent flow of NO in excess of 10 
sccm should be sufficient to produce such oxidation. Note that although 
such modest flows of NO are all that are required to boost the Si 
oxidation, there is no reason to limit these levels. Working in the regime 
where the chamber is flooded with nitrogen and oxygen species, yet where 
the Si etch reaction is fluorine starved, remains the secure method for 
achieving high etch selectivities. The surface of silicon nitride is also 
slightly oxidized during etching in the presence of O and O.sub.2, but 
this does not appear to influence the Si.sub.3 N.sub.4 etch rate. 
Decreased F atom density 
The second mechanism employed by the present invention is the limitation of 
the Si.sub.3 N.sub.4 etch rate by the availability of atomic fluorine. 
This entails maintaining the density of atomic fluorine in the reaction 
chamber at very low levels compared to conventional CF.sub.4 /O.sub.2 
/N.sub.2 and NF.sub.3 /O.sub.2 gas mixtures. For example, the parent gas 
of the fluorine source can be admixed in a flow ratio to oxygen &lt;20 in 
accordance with the present invention. The silicon etch rate will respond 
to the reduction in the atomic fluorine density via two mechanisms: (1) 
the thickness of the oxide-like reaction layer, created by the high flows 
of oxygen and nitrogen, will increase, which will in turn reduce the 
reaction probability of any fluorine atom with the silicon; and (2) the 
total number of fluorine atoms for the reaction will be reduced. The etch 
rate of silicon nitride, on the other hand, is not affected as strongly 
because (1) by design, fluorine is available in abundance for the desired 
etch reaction, i.e., the weaker dependence of the Si.sub.3 N.sub.4 etch 
rate than the Si etch rate on the F density can be exploited to starve the 
Si etching reaction, while maintaining sufficient fluorine to fuel the 
Si.sub.3 N.sub.4 etch reaction, and (2) the reaction layer thickness on 
silicon nitride is quite thin. 
Si.sub.3 N.sub.4 etch rate enhancement by NO 
The etch rate of Si.sub.3 N.sub.4 is proportional to the density of NO. The 
O.sub.2 /N.sub.2 chemistry described herein produces a significant amount 
of NO in its afterglow, which allows for high Si.sub.3 N.sub.4 etch rates 
in the presence of sufficient fluorine. Silicon etch rate is less 
influenced by NO addition, and in the surface oxidation limited regime of 
the present invention, no enhancement of the etch rate is observed. 
Etching in accordance with this invention naturally provides selectivity of 
Si.sub.3 N.sub.4 over SiO.sub.2 (in addition to selectivity of Si.sub.3 
N.sub.4 over Si). The etch rate of SiO.sub.2, which in the absence of ion 
bombardment depends only on the fluorine density, is expected to be 
similar to that of the silicon etch rate or smaller. In the balance of 
this disclosure, the setup of a chemical dry etching tool in accordance 
with this invention and the experimental procedure employed are described. 
Etch rates of silicon nitride, polysilicon and silicon dioxide are 
reported for CF.sub.4 and NF.sub.3 as sources of atomic fluorine. Gas 
phase experiments (Ar actinometry and mass spectrometry) are conducted to 
gain information about the concentration of reactive species. The surface 
of polysilicon during etching is examined by ellipsometry, and the close 
correlation between the decrease of the etch rate and the formation of a 
reactive layer on the silicon is demonstrated. 
FIG. 3 is a schematic of one embodiment of a chemical dry etch (CDE) 
apparatus, generally denoted 200, which can be used in accordance with the 
present invention. CF.sub.4, N.sub.2 and O.sub.2 source gases in this 
example pass through respective mass flow controllers 202 and a gas inlet 
to an applicator 204. The mixtures of O.sub.2, N.sub.2 and fluorine source 
(CF.sub.4 or NF.sub.3) are excited using an Astex DPA-38 2.45 GHz 
microwave source 206 with a coupling tube to applicator 204. In most 
experiments, the plasma was ignited with a sapphire coupling tube. A fiber 
optic cable for optical emission experiments of the discharge is mounted 
on the housing of the applicator. One spectrograph useful in this 
investigation is a 30 cm optical multichannel analyzer. 
The species produced in the plasma travel through a transport tube to the 
cylindrical reaction chamber. As one example, the length of the tube might 
be fixed at 75 cm, and its inside provided with a Teflon liner. Samples, 
e.g., of size 1 inch by 1 inch, are glued on a carrier wafer 211, which is 
placed on an electrostatic chuck 212 in the reaction chamber 210. The 
materials used for this investigation are low-pressure chemical vapor 
deposition (LPCVD) Si.sub.3 N.sub.4, and thermally grown SiO.sub.2. 
Surface modifications of Si have been studied using crystalline Si and 
polycrystalline Si, and etch rates of silicon are determined herein using 
polycrystalline silicon. The temperature of the sample is monitored with a 
fluoroptic probe which contacts the backside of the sample. The probe is 
kept constant at 10.degree. C. for all experiments. Helium at a pressure 
of 5 Torr is fed between the surface of the electrostatic chuck 212 and 
the carrier wafer 211 in order to obtain good heat conduction. Etch rates 
are measured in situ by monochromatic ellipsometry (wavelength 632.8 nm). 
Some samples discussed herein were moved to the surface analysis chamber 
in the UHV wafer handling system without exposure to air. A quadrupole 
mass spectrometer 220 is mounted on top of the reaction chamber such that 
the distance orifice-discharge is the same as the distance 
sample-discharge. 
Most experiments described herein were conducted with a sapphire applicator 
at 1000 W microwave power and a chamber pressure of 600 mTorr. Flows of 
O.sub.2 and N.sub.2 were kept constant at 800 sccm and 110 sccm, 
respectively. These parameters are referred to herein as the "standard 
conditions". 
Etch rates and selectivities 
Pursuant to this invention, Si.sub.3 N.sub.4 etch rates have been measured 
as a function of CF.sub.4 addition to an O.sub.2 /N.sub.2 plasma (see FIG. 
4). The etch rate increases linearly with the flow of CF.sub.4. The varied 
parameter for the two curves in FIG. 4 is the amount of oxygen fed into 
the discharge. A lower flow of O.sub.2 (300 sccm) produces an etch rate 
about 15 nm/min higher than the higher flow of O.sub.2 (800 sccm). The 
highest etch rates obtained with 46 sccm of CF.sub.4 in O.sub.2 /N.sub.2 
are 25 nm/min for 800 sccm of O.sub.2, and 39 nm/min for 300 sccm of 
O.sub.2. 
The etch rates of polycrystalline silicon were found to depend strongly on 
the initial surface conditions of the sample, and on the etch time. These 
etch rate variations can be explained by the presence of a native oxide 
layer on the Si surface, or the formation of an oxygen-rich surface layer 
during etching in accordance with this invention. A layer of native oxide 
on a polycrystalline silicon film suppresses the etching almost 
completely. As an example, the etch rate of poly-Si at standard conditions 
and 30 sccm of CF.sub.4 is as low as 0.07 nm/min with the native oxide 
layer present. If the native oxide is removed immediately before 
processing by dipping the sample in HF for 30 s, the etch rate is 0.49 
nm/min, which is almost one order of magnitude higher. Note that for 
O.sub.2 flows of 300 sccm, despite the increased Si.sub.3 N.sub.4 etch 
rate and high selectivity, the magnitude of the silicon etch rate is too 
high for a reliable process. 
In FIG. 5 the etch rates of poly-Si are plotted as a function of time. The 
native oxide layer was removed before the experiments by HF dipping. Both 
samples were treated at standard conditions, with CF.sub.4 and NF.sub.3 as 
a source of fluorine. The etch rate of a "clean" Si surface (no native 
oxide and no reactive layer) can be as high as 20 nm/min. As etching 
proceeds, a reactive layer pursuant to the present invention forms on the 
Si and impedes etching reactions. The final etch rates are 0.50 nm/min for 
both CF.sub.4 and NF.sub.3. The etch rate decreases faster if CF.sub.4 is 
used as fluorine source. Extrapolation of the curves to the x axis, using 
the initial slopes, yields 12 s for CF.sub.4, and 40 s for NF.sub.3 as 
estimates for the decay time. All etch rates reported in the following are 
"final" etch rates, taken after 300 s of etching. 
FIG. 6 shows the etch rates of silicon for the "standard parameters" as a 
function of the flow of CF.sub.4. The etch rates increase with the flow of 
CF.sub.4. Strong variations of etch rate with the amount of O.sub.2 fed 
into the plasma are also observed. The etch rate is significantly 
suppressed if 800 sccm of O.sub.2 is used as proposed herein. No etch rate 
suppression is found at a low flow (300 sccm) of O.sub.2. At 30 sccm of 
CF.sub.4, for example, the etch rate for 300 sccm of O.sub.2 is 19 times 
higher than that for 800 sccm. 
The etch rate ratio obtained from FIG. 4 and FIG. 6 is plotted in FIG. 7. 
The best selectivity is achieved for a high flow of O.sub.2 and low or 
intermediate fluorine additions. 
The etch rates of silicon dioxide are plotted in FIG. 8. Etching occurs 
only at the low flow of O.sub.2 (300 sccm), and etch rates never exceed 1 
nm/min. The other etch rates are too small to be detected by ellipsometry 
(&lt;0.05 nm/min). Therefore, the etch selectivity of Si.sub.3 N.sub.4 to 
SiO.sub.2 at the higher flow of O.sub.2 is extremely high (&gt;500 nm/min, or 
infinity). Note that in accordance with present invention, a flow of 
oxygen anywhere in the range of 250-5000 sccm is believed to produce 
practical chemical dry etching when used in combination with an atomic 
fluorine flow of approximately the same ratios as used in the stated 
standard conditions. Nitrogen flows of 25-5000 sccm could be implemented, 
again in accordance with maintaining a N.sub.2 :O.sub.2 ratio between 0.1 
and 1.0. Simply stated, although 800 sccm of oxygen and 110 sccm of 
nitrogen are presented as preferable examples, those skilled in the art 
will recognize that various combinations of flow of nitrogen and oxygen 
may be used without departing from the scope of the present invention. The 
goal is to produce a flooding of the reaction chamber with oxygen and 
nitrogen and a commensurate limiting of atomic fluorine within the 
chamber, for example, within the range of 2 to 20 sccm. Such an atomic 
fluorine level can be attained by limiting the CF.sub.4 source flow to 
less than 50 sccm. 
NF.sub.3 as fluorine source 
NF.sub.3 was also used as an alternate fluorine source to CF.sub.4. The 
plasma chemistry of NF.sub.3 is significantly different than that of 
CF.sub.4. The dissociation energies for NF.sub.3 are lower than those for 
CF.sub.4. This results in a higher degree of dissociation of NF.sub.3 in a 
discharge. In fact, in the high density microwave discharges employed for 
the work reported here, 100% dissociation is typically achieved. The 
dissociation of CF.sub.4 for similar discharge parameters varies between 
40% and 60%. Therefore, the F production of free fluorine radicals from 
NF.sub.3 is higher than from CF.sub.4 and thus a lower flow rate of 
NF.sub.3 source would be needed to produce, e.g., a 15 sccm flow rate of 
atomic fluorine within the reaction chamber. Moreover, the concentration 
of NO in the afterglow of the discharge might be affected by the 
additional N atom feed. This enhancement of the NO production can result 
in more pronounced Si oxidation, while at the same time increasing the 
Si.sub.3 N.sub.4 etch rate, thus improving the selectivity. 
The etch rates of Si.sub.3 N.sub.4, poly-Si and SiO.sub.2 as a function of 
the flow of NF.sub.3 are shown in FIG. 9. All experiments were performed 
under "standard conditions", i.e., the flow of O.sub.2 was kept constant 
at 800 sccm. The etch rates of Si.sub.3 N.sub.4 are proportional to the 
flow of NF.sub.3, and significantly higher if NF.sub.3 is used instead of 
CF.sub.4. An addition of 46 sccm of CF.sub.4, e.g., yields an etch rate of 
24 nm/min, whereas the etch rate for the same flow of NF.sub.3 is twice as 
high (49 nm/min). 
The etch rates of poly-Si and SiO.sub.2 (see FIG. 9) show a different trend 
for NF.sub.3 as compared to CF.sub.4. The etch rates of both materials 
increase with the flow of CF.sub.4, but they assume a plateau value if 
NF.sub.3 is used. The etch rate ratios resulting from this data are 
plotted is FIG. 10. The selectivity of Si.sub.3 N.sub.4 to both materials, 
poly-Si and SiO.sub.2, increases with the flow of NF.sub.3. Those skilled 
in the art will note that the degree of disassociation, and hence the 
density of fluorine available for etching reactions, can be enhanced by 
increasing the microwave power. Further, the proposed flows of fluorine 
source can be altered by one skilled in the art with respect to the oxygen 
and nitrogen flows by manipulating the degree to which the source gasses 
are disassociated. 
Gas phase experiments 
This section gives a summary of gas phase experiments which were performed 
to determine the dependence of the reactive species concentration on the 
process parameters. Due to technical difficulties, the experiments 
reported in this section were not conducted under standard conditions. A 
quartz applicator was used instead of the sapphire one, the microwave 
power was reduced to 500 W, and the flow of O.sub.2 was kept constant at 
600 sccm. The etch rates under these conditions were measured and compared 
to the etch rates for standard conditions. The etch rates of Si.sub.3 
N.sub.4, when CF.sub.4 was used as F source, are 50% of those reported 
above for the standard conditions. The etch rates for NF.sub.3 are 
identical. Polycrystalline Si shows the same etch characteristics for both 
parameter settings. Silicon dioxide was not etched when CF.sub.4 was used, 
and etched at a rate of 1 nm/min or less if NF.sub.3 was used. The close 
agreement between the etch rates for the parameters used in this section 
and the standard parameters suggests that the gas phase results can be 
applied qualitatively to the "standard conditions" noted above. 
The fluorine generation in the plasma was monitored using Ar actinometry. 
FIG. 11 shows the ratio of the F emission at 703.7 nm and the Ar line at 
811 nm. The F concentration is approximately 4 times higher if NF.sub.3 is 
used as compared to CF.sub.4. This is consistent with the observation that 
NF.sub.3 dissociates to a higher degree than CF.sub.4 in discharges. This 
fact is shown for CF.sub.4 and NF.sub.3 in FIG. 12 for the present 
experiments. The dissociation is determined by mass spectrometry from the 
intensity ratio I.sub.PlasmaOn /I.sub.PlasmaOff of the CF.sub.3.sup.+ (amu 
69) and the NF.sub.2.sup.+ (amu 52) peaks. The dissociation of CF.sub.4 
ranges between 20% and 30%, whereas that of NF.sub.3 is always 100%. 
The densities of atomic fluorine and NO in the reaction chamber, as 
determined by mass spectrometry, are shown in FIG. 13 and FIG. 14. 
NF.sub.3 produces an F atom density two orders of magnitude greater than 
that produced by CF.sub.4. Significant amounts of NO are present in the 
afterglows of both gas chemistries. In the case of CF.sub.4, however, the 
density decreases by almost 50% as a small amount of CF.sub.4 (15 sccm) is 
added to the O.sub.2 /N.sub.2 discharge, and then decreases further as 
more CF.sub.4 is injected. The same initial decrease is observed for 
NF.sub.3 added to O.sub.2 /N.sub.2, but then, in contrast to the CF.sub.4 
case, the NO density increases. The higher densities of F and NO for 
NF.sub.3 as compared to CF.sub.4 can account for the increased Si.sub.3 
N.sub.4 etch rates, and also for the different surface kinetics during 
poly-Si etching, as described below. 
Surface analysis 
The surface modifications during the etching of silicon have been 
investigated in situ by monochromatic ellipsometry and after the 
experiment by x-ray photoelectron spectroscopy. 
A. Ellipsometry measurements 
The formation of the reactive layer on top of the polycrystalline silicon 
to be etched can be monitored in situ during etch rate measurements. FIG. 
15 shows the evolution of the ellipsometric variables .psi. and .DELTA. as 
a function of time. For these experiments, a stack consisting of poly-Si 
(total thickness 250 nm) on top of SiO.sub.2 (100 nm) on a Si (100) 
substrate has been etched. .psi. and .DELTA. can be calculated by assuming 
values for the thickness and the optical parameters of each layer of the 
stack. Also, the reactive layer can be included in those calculations as a 
film of variable thickness on top of the poly-Si. In FIG. 15 the results 
of these calculations for three different thicknesses of the reactive 
layer (0 nm, 10 nm and 12.5 nm) are included. Etching of the poly-Si 
corresponds to an increase of the value of .DELTA.. At the same time, 
information about the reactive layer thickness is obtained from .psi.. An 
increase of .psi. means an increase of the reactive layer thickness. 
The top panel of FIG. 15 shows the etching of polycrystalline Si and 
formation of the reactive layer under standard conditions with 30 sccm of 
CF.sub.4 added. Immediately after the plasma is ignited and tuned 
("start") the reactive layer grows at considerable rate, and Si is removed 
at the same time. The time interval between two measurement points is 1.1 
s. The reactive layer assumes a steady state thickness of approximately 10 
nm. At this thickness value the etch rate of Si is slowed down to 0.4 
nm/min. At the moment the discharge is terminated ("end"), the overlayer 
thickness has increased to about 12.5 nm. The etch time from start to end 
was 475 s, and 16.7 nm of Si was removed in that time. After termination 
of the discharge ("post-plasma"), more surface modifications occurred. 
NF.sub.3 was used instead of CF.sub.4 for the experiment shown in the 
bottom panel. As in the case of CF.sub.4, growth of the reactive layer and 
poly-Si etching occur simultaneously. However, some significant 
differences are observed: The formation of the reactive layer happens at a 
slower rate than in the case of CF.sub.4 etching, and the reactive layer 
does not achieve a steady state thickness, but continues to grow. Also, 
the post-plasma modifications show a different trend. 
XPS measurements 
Photoemission spectra have been taken from crystalline Si substrates which 
have been etched under standard conditions. FIG. 16 shows the Si(2p) 
spectra of a representative sample, etched under "standard conditions". 
The spectrum which was taken with an electron emission angle of 15.degree. 
with respect to the surface shows a relatively strong peak at high binding 
energy (104 eV), which corresponds to a high degree of surface oxidation. 
If the e.sup.- emission angle is changed to 90.degree., the Si(2p) 
emission from the bulk (99.6 Ev) increases and becomes stronger than the 
emission from oxidized Si. The relative intensity of the two peaks allows 
one to estimate the thickness of the oxidized overlayer. For the spectra 
shown, this thickness is about 2 nm. 
Conclusions 
A novel remote plasma Chemical Dry Etching (CDE) process which enables 
etching of Si.sub.3 N.sub.4 over silicon and SiO.sub.2 with an etch rate 
ratio greater than 10:1 (and even greater than 30:1) has been 
demonstrated. It uses high flows of O.sub.2 and N.sub.2, and relatively 
small additions of CF.sub.4 or NF.sub.3 as a source of fluorine. 
From the data presented above, it can be concluded that the formation of an 
etch-inhibiting reactive layer on top of the Si is the dominant mechanism 
for achieving a high Si.sub.3 N.sub.4 /Si etch rate ratio. Conventionally, 
the etching of the virgin silicon surface proceeds at a rate of 
approximately 20 nm/min. In accordance with the present invention, in situ 
ellipsometry shows the formation of a reactive layer on top of the 
polycrystalline silicon during the etch process within a matter of 
seconds. The etch rate of silicon is thus decreased to a level comparable 
to that of SiO.sub.2 after the reactive layer has formed, and the 
significantly decreased F atom density, therefore, facilitates etching of 
silicon nitride with high selectivity over silicon. The silicon nitride 
etch rate is further boosted by high NO density in the chamber. 
Those skilled in the art will note from the above discussion that chemical 
dry etching in accordance with the principles of this invention produces 
much higher silicon nitride to silicon and silicon nitride to silicon 
dioxide etch rate ratios than currently possible using existing 
techniques. Further, the highly selective chemical dry etching processes 
presented comprise practical implementations for removing a silicon 
nitride mask from a silicon-based wafer. This chemical dry etching 
approach is robust and silicon surface roughening is unobserved, and in 
fact, surface smoothing appears to occur. The technique presented employs 
simple, non-hazardous gases and repairs porous silicon dioxide thin films. 
Further, the technique minimizes so-called "loading effect", since it is 
based on a surface mechanism, thus simplifying tool design. 
While the invention has been described in detail herein in accordance with 
certain preferred embodiments thereof, many modifications and changes 
therein may be effected by those skilled in the art. Accordingly, it is 
intended by the appended claims to cover all such modifications and 
changes as fall within the true spirit and scope of the invention.