Process for depositing aluminum nitride (AlN) using nitrogen plasma sputtering

A process for depositing a thin film of aluminum nitride (AlN) includes sputtering an aluminum target with energetic nitrogen ions generated in a nitrogen plasma. A single gas (i.e. nitrogen) is used as both the reactive gas and as the sputtering gas. The process is especially adapted for forming an etchstop layer for use in forming contact vias through a dielectric layer in semiconductor manufacture. The process is also useful in semiconductor manufacture for forming an aluminum nitride (AlN) film that may be used as a passivation layer, as a ceramic packaging material, as a mask for ion implantation, as a substrate material in hybrid circuits, and as a high bandgap window for GaAs solar cells.

TECHNICAL FIELD 
This invention relates to thin film processes and more particularly to a 
process for depositing a film of aluminum nitride (AlN) using nitrogen 
plasma reactive sputtering. The process of the invention is particularly 
but not exclusively adapted to semiconductor manufacture and for forming 
an etchstop layer. 
BACKGROUND OF THE INVENTION 
In semiconductor manufacture it often necessary to deposit a thin film over 
a substrate or over another film on the substrate. One film material that 
has characteristics suitable for semiconductor manufacture is aluminum 
nitride (AlN). Among the useful properties of aluminum nitride (AlN) are a 
high thermal conductivity, a close thermal expansion match to a Si 
substrate, and good mechanical strength. Table 1 lists the properties of 
aluminum nitride (AIN): 
TABLE 1 
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Properties of Aluminum Nitride (AlN) 
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Dielectric constant 8.8 
Resistivity (ohm-cm) 5 .times. 10e13 
Bandgap (eV) 6.2 
Grain size (um, bulk value) 
3-5 
Density (g/cm3, bulk value) 
3.25 
Thermal expansion 2.6 .times. 10e(-6) 
coefficient (1/K) 
Flexural strength (MPa) 
300 
Thermal conductivity (W/cmK) 
1.5 
Melting point (.degree.C) 
2400 
Refractive index 2.0 
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Such properties allow aluminum nitride (AlN) to be used in semiconductor 
device packaging and in semiconductor manufacture as an ion implantation 
mask. In these applications the electronic configuration of both aluminum 
and nitrogen allow the atoms to assume substitution sites in the silicon 
crystal lattice structure. Hence they will not introduce or nucleate 
stacking faults. These properties also allow aluminum nitride (AlN) to be 
used as a thermally conductive dielectric barrier in various semiconductor 
devices and to provide a high bandgap window for GaAs solar cells used in 
semiconductor devices. 
Despite these useful characteristics of aluminum nitride (AlN), its 
relatively high fabrication cost has prevented its wide use in the 
microelectronic industry. Being a high temperature material, in order to 
have suitable device characteristics, for some applications, aluminum 
nitride (AlN) requires formation and fabrication at temperatures so high 
that many of the materials involved in the process react and adversely 
affect the electronic properties of the film. Accordingly relatively 
complex and expensive manufacturing processes are required. U.S. Pat. No. 
4,152,182 to Rutz for instance, discloses such a process wherein aluminum 
nitride (AlN) is synthesized and grown epitaxially on an (Al.sub.2 
O.sub.3) substrate. Temperatures in excess of 1900.degree. C. are 
required. This high temperature process is required to provide a high 
quality aluminum nitride (AlN) film. 
It is also known to deposit an aluminum nitride (AlN) film using chemical 
vapor deposition (CVD) or sputtering. A (CVD) process, in general will not 
provide a film as high in quality as an epitaxially grown film but can be 
used to provide a film with better step coverage. U.S. Pat. No. 4,030,942 
to Keenan et al. discloses the use of an aluminum nitride (AlN) film as an 
ion implantation mask in semiconductor manufacture. A (CVD) process is 
used to deposit the aluminum nitride (AlN). The disclosed (CVD) process is 
relatively complicated and requires the use of several process gases 
including hydrogen, NH.sub.3 and stoichiometric quantities of aluminum 
chloride. With the use of such a large number of gases, impurities may be 
introduced into the deposited (AlN) film. These impurities have any 
adverse affect on the completed semiconductor devices. 
In addition to this fundamental problem, both of the cited references are 
relatively complicated and expensive processes not generally suited to 
large scale repetitive semiconductor manufacture. In view of the 
foregoing, there is a need in the art for an improved process for 
depositing a thin film of high purity aluminum nitride (AlN). Accordingly, 
it is an object of the present invention to provide a process for 
depositing aluminum nitride (AlN) using nitrogen plasma sputtering. It is 
a further object of the present invention to provide an improved process 
for depositing aluminum nitride (AlN) in a semiconductor manufacturing 
process having a quality suitable for semiconductor devices. Yet another 
object of the present invention is to provide a deposition process for 
aluminum nitride (AlN) that is simple, cost effective and repetitive. 
One application where the process of the invention is particularly suited 
is in the formation of an etchstop layer for semiconductor fabrication. As 
semiconductor device dimensions continues to shrink, the depth of an 
implanted area on a Si wafer substrate, such as an active area, becomes 
increasingly shallower due to the scaling in the device dimensions. This 
puts a limit on the amount of overetch allowed during a contact etch 
process through an oxide layer to the substrate, since excessive overetch 
will consume the Si in an implanted junction, resulting in device 
degradation. Unfortunately, overetch is often required in a contact etch 
process in order to (1) making sure that all contact holes are properly 
etched across the wafer and (2) in cases where two types of contact with 
two different desired contact depths are present on the same Si wafer. In 
the later case, in order to open the deeper contact, the shallower contact 
will be overetched. In order to eliminate Si consumption during any 
overetch step, an etchstop which consists of a thin layer of a desired 
material of a slower etch rate than that of the material to be etched 
(i.e. oxide) can be used. The etchstop layer is usually deposited above 
the implanted Si substrate (contact junction) and below the oxide layer 
through which the contact holes will be opened. In a contact etch process, 
when an etchstop of a much lower etch rate than that of oxide is used, the 
etch will stop on the etchstop layer during the overetch step, preventing 
the underlying Si junction from being consumed by the etch process. 
Therefore, with the help of an etchstop layer between the Si junction and 
oxide layer, an overetch can be allowed. The process of the invention is 
especially suited to forming an etchstop in this application. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a novel process for depositing a 
thin film of aluminum nitride (AlN) is provided. Simply stated, with the 
process of the invention an aluminum nitride (AlN) film is deposited in a 
conventional sputtering apparatus by sputtering an aluminum target with 
energetic nitrogen ions generated in a nitrogen plasma. A single gas (i.e. 
nitrogen) is used as both the reactive gas and as the sputtering gas. 
The process of the invention is especially suited to semiconductor 
manufacture for forming an aluminum nitride (AlN) film having a high 
purity. One such application, is in the formation of an etchstop layer 
during semiconductor fabrication. A representative process sequence can 
include the following steps: 
1. Device formation on a silicon substrate using standard processes. 
2. Aluminum nitride (AlN) deposition using nitrogen plasma reactive 
sputtering with nitrogen used as both the reactive gas and as the 
sputtering gas. 
3. Dielectric or doped oxide deposition. 
4. Photopatterning to define the regions in the oxide layer to be etched at 
contact etch step. 
5. Contact etch through the oxide layer using reactive ion etch. The etch 
will stop on the aluminum nitride (AlN) layer. Due to the high etch 
selectivity of aluminum nitride (AlN) to oxide (oxide etches much faster 
than aluminum nitride), the etch will stop on the aluminum nitride (AlN), 
leaving the underlying Si junction intact. 
6. Photoresist strip. 
7. Strip off the remaining aluminum nitride (AlN) layer using either wet or 
dry etch. 
8. Standard processes from then on. 
This is but one example wherein an aluminum nitride (AlN) layer can be 
formed in accordance with the invention during semiconductor manufacture. 
Other objects, advantages, and capabilities of the present invention will 
become more apparent as the description proceeds.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 1 a representative prior art method of depositing a 
thin film of aluminum nitride (AlN) on a semiconductor wafer is shown. The 
prior art method is a plasma enhanced chemical vapor deposition (PECVD) 
process. The prior art process simply stated uses a plasma established by 
a radio frequency induced glow discharge. Both an inert sputter gas such 
as argon and an nitrogen reactive gas are required. 
The apparatus for carrying out the prior art process includes a vacuum 
chamber 10 that encloses a lower grounded electrode 12 and an upper 
powered electrode 14. The powered electrode 14 may include a target formed 
of pure aluminum. Semiconductor wafers 24 to be processed are loaded into 
the vacuum chamber 10 by means of an access door 16 and are placed upon a 
grounded electrode 12. 
Inside the vacuum chamber 10 electrodes 12 and 14 have generally planar 
surfaces and are parallel to one another. Both grounded electrode 12 and 
powered electrode 14 are electrically insulated from the walls of the 
chamber 10. Powered electrode 14 is adapted to dispense the inert gas from 
an inert sputter gas source 18 and the nitrogen (N.sub.2) gas from a 
nitrogen gas source 20 into the space between the electrodes 12 and 14. A 
vacuum source 26 is used to evacuate the vacuum chamber 10. An RF power 
source 28 is coupled to the upper electrode 14. 
In use the nitrogen (N.sub.2) and the inert sputter gases are introduced 
into the space between the electrodes 12 and 14 and are excited to a high 
energy state by the RF energy emitted from RF power source 28. Synthesis 
of an aluminum nitride (AlN) compound on the wafer 24 is accomplished by 
the release of aluminum (Al) atoms from the upper electrode 14 caused by 
the inert gas molecules striking the upper electrode 14. The free aluminum 
atoms combine with the energized nitrogen (N.sub.2) gas molecules and are 
reactively deposited on the wafer to form a thin film of aluminum nitride 
(AlN). This process, although effective, is relatively expensive and 
complicated and is not generally suited to large scale semiconductor 
manufacture. In addition, the use of multiple gases introduces undesirable 
contaminants into the aluminum nitride (AlN) film. 
The process of the present invention simplifies this prior art process by 
using only one gas, nitrogen (N.sub.2), as both the sputter gas and the 
reactive gas. The process of the invention can be carried out in a 
standard sputter apparatus such as that shown in FIG. 2. With reference to 
FIG. 2, the sputter apparatus 30 includes a vacuum chamber 32 coupled to a 
vacuum source 34 and to a source of nitrogen (N.sub.2) gas 42. Inside the 
vacuum chamber 32 is an aluminum target 36. The target 36 is electrically 
grounded. Semiconductor wafers 38 are mounted on a support surface 40 
within the vacuum chamber 32. 
In use nitrogen gas (N.sub.2), is introduced into the vacuum chamber 32 and 
is ionized to a positive charge. The positively charged nitrogen atoms are 
attracted to the grounded target 36 and accelerate toward it. Upon 
striking the target 36 aluminum atoms are released. These aluminum atoms 
combine with nitrogen (N.sub.2) molecules in the vacuum chamber 32 to form 
a thin film of aluminum nitride (AlN) on the surface of the wafer 38. 
The nitrogen (N.sub.2) gas thus serves the dual purpose of both the sputter 
gas and the reactive gas. This process offers several advantages for 
semiconductor manufacture. Among these advantages are: 
1. The amount of impurity incorporation into the deposited aluminum nitride 
(AlN) film is reduced because only one gas source and one gas inlet is 
required. 
2. The deposition process is simplified and adaptable to use in large scale 
semiconductor manufacture with reduced production costs. 
3. The deposition process can be done in any conventional sputtering 
machine and an RF power source is not required. 
In semiconductor manufacturing aluminum nitride (AlN) can be used for 
replacing alumina and beryllium oxide substrates used in microelectronic 
packaging. In this application aluminum nitride provides a better thermal 
expansion match to silicon. Additionally aluminum nitride (AlN) can be 
used as a high bandgap window for GaAs solar cells, as a mask for ion 
implantation and lift-off techniques, as a thermally conductive dielectric 
barrier, and as a ceramic package. Moreover, aluminum nitride is 
particularly favored as a substrate material in hybrid circuits. 
FIG. 3 is a cross-sectional scanning electron microscope of an aluminum 
nitride (AlN) layer 44 deposited by the process of the invention as a 
passivation layer. The aluminum nitride layer 44 has been deposited upon a 
silicon substrate 46 having patterned semiconductor devices 48 formed 
thereon. As illustrated in FIG. 3 a high quality aluminum nitride (AlN) 
film 44 is evenly distributed over the semiconductor devices 48. 
Additionally, step coverage extends in an area 50 between the 
semiconductor devices 48, and along the sidewalls 52 of the semiconductor 
devices 48, indicating a reasonably good step coverage obtained by this 
process. These new and unexpected results are provided by the simple yet 
unobvious process of the invention. 
FIGS. 4 and 5 are auger spectroscopy confirmations of a 250 .ANG. thick 
aluminum nitride (AlN) film deposited on Si substrate in accordance with 
the invention. FIGS. 4 and 5 illustrate the results of such an auger 
analysis. FIG. 4 shows the depth profile of AlN on a Si wafer. As is 
apparent from this data both Aluminum (Al) and Nitrogen (N) are present at 
the surface of the wafer as well as to a depth of between about 200-300 
.ANG.. Additionally, as is apparent in FIG. 5, the concentration of 
contaminants such as carbon (C) is relatively low. 
Thus the invention provides a simple yet unobvious method of depositing a 
film of aluminum nitride (AlN) in a semiconductor manufacturing process. 
For use as a passivation layer the aluminum nitride (AlN) layer may be 
deposited after the pattern definition of the last metal layer. For use as 
packaging material the aluminum nitride (AlN) layer may be deposited after 
bond pads are open and the wafer is ready for packaging. The process may 
also be used to form a mask for ion implantation, as a high bandgap window 
for GaAs solar cells, and as a substrate material in hybrid circuits. 
The process of the invention is also particularly suited to the formation 
of an etchstop layer during semiconductor fabrication. A representative 
process sequence for such an application is shown in FIGS. 6A-6G. With 
reference to FIG. 6A a semiconducting substrate such as a silicon wafer 60 
has a large number of semiconductor devices 62 formed thereon by standard 
techniques such as patterning, doping, and ion implantation. Item 62 may 
also be viewed as a junction to a device. 
Next and as shown in FIG. 6B a layer of aluminum nitride (AlN) 64 is 
deposited in accordance with the invention by sputtering a target 
substrate of aluminum in a vacuum chamber using an energized nitrogen gas 
plasma as both a reactive gas and a sputter gas. A thin film of aluminum 
nitride (AlN) 64 (i.e. 1000 .ANG.) formed by this process has a high 
purity as previously explained. 
Next, as shown in FIG. 6C a dielectric or an oxide 66 is deposited over the 
aluminum nitride (AlN) layer 64. 
Next, as shown in FIG. 6D, photoresist 68 is deposited upon the oxide 66 
and patterned to define regions 70 in the oxide layer 66 to be etched for 
forming contact vias to the devices 62. 
Next, as shown in FIG. 6E, a via or contact opening 72 is etched through 
the oxide layer using a process such as a reactive ion etch. The etch will 
stop on the thin aluminum nitride (AlN) layer. Due to the high etch 
selectivity of aluminum nitride (AlN) to oxide (oxide etches much faster 
than aluminum nitride (AlN)) the etch will stop at the aluminum nitride 
(AlN) layer 64, leaving the underlying silicon junction to the device 62 
intact. 
Next, as shown in FIG. 6F the photoresist 68 is stripped. Next, as shown in 
FIG. 6G the exposed aluminum nitride (AlN) in the contact via is stripped 
using either a wet or dry etch. Standard processes can then be used to 
complete formation and contact to the semiconducting devices 62. 
This etchstop application of sputtered AlN has been tested by the 
inventors. A layer of 1000 .ANG. aluminum nitride (AiN) film was first 
deposited atop of 150 mm diameter, &lt;100&gt; single crystal Si wafers. The AlN 
deposition was carried out in a DC magnetron sputter machine using high 
purity nitrogen gas. The wafers were next deposited with 18,000 .ANG. of 
boron-phosphorus-silica-glass (BPSG). After an etch mask (photoresist) was 
defined in the BPSG layer by the standard photolithography technique, the 
oxide contact etch was carried out on these wafers with various overetch 
times. The oxide etch process used was a standard reactive ion etch 
technique using CF4 and CHF3 based chemistry. A cross-sectional scanning 
electron microscope (SEM) was used to determine the etch selectivity of 
(AlN) to oxide, as well as the effectiveness of a sputtered aluminum (AlN) 
film as an oxide etchstop. It was observed that aluminum nitride (AlN) was 
etched 10 times slower than that of (BPSG). For the wafers using an 
aluminum nitride (AlN) etchstop layer, there was no Si consumption 
observed for an overetch time as long as 45 seconds, while for the wafers 
without an aluminum nitride (AlN) etchstop layer, some Si consumption was 
observed. 
Table 2 shows that the aluminum nitride (Aln) deposition rate using this 
process as a function of sputter deposition power. 
TABLE 2 
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(AlN) deposition using nitrogen as sputter and 
reactive gas as a function of sputter power. 
deposition rate (A/min) 
sputter power (Kilowatt) 
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70 1.8 
210 2.4 
280 3.0 
420 3.6 
720 6.0 
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conditions: 
N2 pressure (during sputtering) = 7.0 (millitorr) 
N2 gas flow rate: 100 (cm.sup.3 /minute) 
Although only certain embodiments of the invention have been described 
herein, it will be apparent to one skilled in the art that changes and 
modifications may be made thereto without departing from the spirit and 
scope of the invention as claimed.