Insulator deposition using focused ion beam

Methods are provided for depositing insulator material at a pre-defined area of an integrated circuit (IC) by: placing an IC in a vacuum chamber; applying to a localized surface region of the integrated circuit at which insulator material is to be deposited a first gas containing molecules of a dissociable compound comprising atoms of silicon and oxygen and a second gas containing molecules of a compound which reacts with metal ions; generating a focused ion beam having metal ions of sufficient energy to dissociate molecules of the first gas; and directing the focused ion beam at the localized surface region to dissociate at least some of the molecules of the first gas and to thereby deposit on at least a portion of the localized surface region a material containing atoms of silicon and oxygen. The dissociable compound comprises atoms of carbon and hydrogen, such as di-t-butoxydiacetoxy-silane. The compound which reacts with metal ions may be carbon tetrabromide or ammonium carbonate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to methods and apparatus for depositing 
insulator material using a focused ion beam, particularly a focused ion 
beam from a liquid-metal ion source. 
2. The Prior Art 
Systems for the treatment of integrated circuits and the like with a 
focused-ion-beam (FIB) are known. FIB systems having a needle and gas 
source for injecting gas at a surface region of an integrated circuit (IC) 
where the FIB is directed are also known. See, for example, U.S. Pat. No. 
5,140,164, the content of which is incorporated herein by this reference. 
A FIB system commercially available as the "IDS P2X FIBstation" from 
Schlumberger Technologies, Inc., San Jose, Calif., has a gas manifold with 
a plurality of controllable inlet valves and a positionable outlet needle 
for selectively injecting gases from any of a plurality of gas sources 
toward an IC surface region to be treated with the FIB. 
Various techniques have been developed for using such systems to effect 
semiconductor IC device-level repair and diagnosis. For example, the FIB 
can be used to mill away material. The rate and controllability of milling 
can be enhanced by injecting gases which preferentially mill particular 
materials, such as dielectric or metal. Such techniques can be used to 
selectively expose IC structure for probing or examination, cut holes 
through power and ground planes, and to selectively sever conductors. 
Techniques are also known for injecting a gas in the presence of the FIB 
at the IC surface to selectively deposit conductive material. These 
techniques can be used to construct or reconstruct conductors, and to 
deposit pads used for mechanical or electron-beam probing. 
An important limitation of current FIB techniques is that metal deposition 
can sometimes make electrical contact where it is not desired. The result 
can be unintended electrical contact between the deposited metal and 
exposed conductors surrounding the area of metal deposition. Metal 
deposition can often be time-consuming, such as when the presence of 
exposed conductors dictates that metal be deposited along a circuitous 
route to avoid unintended electrical connections. In many cases it is 
impossible to safely perform metal deposition at all, and the entire 
repair operation must be redesigned. 
It has been proposed to deposit film as an insulator for IC repair. See H. 
Komano et al. in Silicon Oxide Film Formation by Focused Ion Beam 
(FIB)-Assisted Deposition, JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 28, 
No. 11, Nov. 1989, pages 2372-2375. A film of SiO.sub.2 was formed by 60 
keV Si.sup.2+ FIB-assisted deposition. A mixture of tetramethoxysilane 
(SI(OCH.sub.3).sub.4) and oxygen gases was blown onto a sample surface 
through a 0.2-mm-inner-diameter nozzle. The beam diameter and current were 
0.2 .mu.m and 0.1 nA, respectively. The deposited film with 0.1 .mu.m 
thickness and 0.7 .mu.m width was reported to consist mainly of silicon 
and oxygen. The reported resistivity of the deposited film was 2.5 
M.OMEGA.-cm at 5 volts. The report correctly notes that the resistivity is 
not high enough for use as an insulator in actual devices. 
Also, a silicon-beam apparatus is believed to be inherently more complex 
than a system having a liquid-metal-ion-source (such as a Ga-ion source) 
and to have a beam less finely-focused than Ga-ion beam systems. It is 
unclear whether a silicon-beam apparatus would be suitable for the milling 
and metal-deposition operations needed for IC repair. Improved techniques 
for FIB-assisted insulator deposition are needed, preferably techniques 
which can be implemented in Ga-ion-beam systems now used for milling and 
metal-deposition in IC repair. The prior art is not believed to show the 
use of Ga-ion-beam deposition of insulative material, perhaps because the 
Ga ions are conductive. 
SUMMARY OF THE INVENTION 
Preferred embodiments of the invention offer methods and apparatus for 
depositing insulator material at a pre-defined area of an integrated 
circuit. Molecules of a compound containing silicon atoms and oxygen atoms 
are mixed with a reactive gas and injected at the surface region of the IC 
to be treated, while a FIB is directed at the region. The resulting 
material selectively deposited at localized regions of the IC surface is 
highly resisitive. 
Insulator deposition in accordance with the invention is useful when 
repairing a semiconductor IC with a charged-particle beam such as a FIB, 
thus enabling certain types of repair and minimizing repair time. For 
example, a FIB system is operated in accordance with the invention to 
deposit a layer of insulator on top of any exposed metal line, in order to 
protect the metal line from shorting to other metal lines and to enable 
any further FIB repair operation. 
These and other features of the invention will become apparent to those of 
skill in the art from the following description and the accompanying 
drawing figures.

DETAILED DESCRIPTION 
Deposition of insulator material in accordance with the invention is 
illustrated by examples of its use in modification of an IC. Operations 
which are difficult or impossible to achieve without insulator deposition 
are made possible in accordance with the invention. An example of such an 
operation arises when a probe pad is to be deposited which is in 
electrical contact with a conductor buried beneath a power plane, while 
keeping the probe pad electrically isolated from the power plane. 
Another example of such an operation arises when two lower-level conductors 
lying beneath a metal power plane are to be electrically connected without 
making electrical contact with the power plane. One way to do this is to 
first mill a large "window" through the power plane and through interlayer 
dielectric material beneath the power plane to expose the conductors. A 
metal line to act as a "jumper" connecting the two conductors can then be 
deposited on the interlayer dielectric. The procedure would be 
time-consuming because considerable material must be milled away to 
provide a large window. If the exposed edges of the power plane at the 
window perimeter are too near the deposition area, electrical leakage from 
the deposited metal to the power plane can result. 
The speed and reliability of the procedure can be improved in accordance 
with the invention by selective deposition of insulator material prior to 
depositing metal. Two small holes can be milled through the power plane to 
provide access to the two conductors. The exposed edges of the power plane 
at the perimeters of the holes can be covered with insulator, assuring 
electrical isolation even when depositing metal in a smaller window. 
Insulator deposition in accordance with the invention can also be used, for 
example, to avoid unwanted electrical contact with conductive structures 
when reconnecting a metal signal line after a section of the metal line 
has been removed. 
A FIB system suitable for carrying out the methods of the present invention 
is the DS P2X FIBstation, available commercially from Schlumberger 
Technologies, Inc., of San Jose, Calif. 
Insulator deposition in accordance with preferred embodiments of the 
invention is described with reference to examples of applications which 
are enabled by the ability to deposit insulating material and with 
reference to examples demonstrating a range of FIB-assisted 
insulator-deposition parameters. 
In the examples, chamber pressures were monitored using the Penning gauge 
of the P2X FIB station system. Two gases were used. The gas ratio was 
established by first determining parameters (temperature of the crucible 
holding a sample of precursor material) required to obtain a given chamber 
pressure of each gas individually for the given orifice, thereby producing 
a set of calibration data. Parameters selected from the calibration data 
are used to produce the desired gas ratio when injecting the mixture of 
gases. The partial pressures of the individual gases do not represent 
their exact molecular ratios, but do serve as an accurate reference scale 
of the gas mixture. Since the gas flux and pressure at the surface of the 
DUT were not measured, the calibration data and chamber pressures were 
used as an indirect indicator. 
Deposition is conducted by scanning the FIB over a specified "box" (e.g., a 
region of 5 .mu.m.times.5 .mu.m or 5 .mu.m.times.10 .mu.m or other 
specified dimension) while gas is injected at the surface of the region 
through a needle. Regardless of the box dimension, 500 horizontal scan 
lines are used to cover the region, and one full sweep of the box is 
performed in 30 milliseconds (ms). 
EXAMPLE 1.1 
The section views of FIGS. 1A-1D and top views of FIGS. 2A-2D illustrate. 
In a two-metal-layer CMOS sample device, a conductor 100 surrounded by 
native silicon dioxide (SiO.sub.2) insulation 105 lies beneath a power 
plane 110 which is in turn covered by a layer 115 of native SiO.sub.2 and 
a passivation layer 118 of native silicon nitride (Si.sub.3 N.sub.4). A 
hole 120 of 4 .mu.m.times.4 .mu.m was milled through the power plane to 
expose insulation 105, as shown in FIGS. 1A and 2A. Referring to FIGS. 1B 
and 2B, a 10 .mu.m.times.10 .mu.m.times.1 .mu.m pad 125 of insulator 
material was deposited by directing a focused gallium (Ca) ion beam of 250 
pA beam current at 15 keV beam energy for 30 minutes while a precursor gas 
of CBr.sub.4 and DBTS mixed at 1:1 ratio was delivered onto a localized 
region of the device surface through a 10 cm long gas injector having an 
0.8 mm inside diameter. The tip of the injector was maintained at a 
distance of 0.5 mm from the device surface. The chamber pressure changed 
from 1e-6 torr prior to deposition to 3e-5 torr during deposition. 
The injected gas mixture consisted of: (1) a SiOx precursor gas, C.sub.12 
H.sub.24 O.sub.6 Si, Di-T-Butoxydiacetoxy-silane (DBTS), available for 
purchase from United Chemical Technologies, Inc., Bristol, Pa., and (2) a 
Ga-bonding gas, CBr.sub.4, carbon tetrabromide, available for purchase 
from Aldrich Chemical, Milwaukee, Wis. 
After depositing pad 125, a hole 130 of 2 .mu.m.times.2 .mu.m was milled 
through the deposited insulator material and through native SiO.sub.2 to 
expose conductor 100 as shown in FIGS. 1C and 2C. Metal was then deposited 
to fill hole 130 and to form a bridge 135 in electrical contact with 
conductor 100 as shown in FIGS. 1D and 2D, and extending over a portion of 
insulator pad 125 and onto the native Si.sub.3 N.sub.4. 
FIG. 3 is a tracing of a voltage contrast image of a section taken through 
the deposited insulator material (but not through the deposited conductive 
material), i.e., along line III--III of FIG. 2D. The deposited insulator 
material 125 is outlined by the bold line 140 in FIG. 3. The deposited 
conductive material is visible in section at 145. 
FIG. 4 is a tracing of a voltage contrast image of a section taken through 
the deposited insulator material and conductive material, i.e., along line 
ID--ID of FIG. 2D. The deposited insulator material 125 is outlined by the 
bold lines 150 and 155. The deposited conductive material is visible in 
section at 155. 
The example demonstrates the ability to establish contact through power 
planes or ground planes using a combination of milling, deposition of 
insulator material in accordance with the invention, and deposition of 
conductive material. Such contact structures can be used for further 
jumper formation as needed. The deposited insulator material offers good 
isolation and good electrical characteristics, verified by acquisition of 
full 5-volt e-beam waveforms. Any substantial leakage from the lower 
conductor to power- or ground- planes would result in amplitude of the 
acquired e-beam waveforms substantially less than 5-volts. 
EXAMPLE 1.2 
Another portion of the two-metal-layer CMOS device was used to demonstrate 
the ability to cut a top-layer (M2 layer) conductor to expose a 
bottom-layer (M1 layer) conductor, and to re-connect the top-layer 
conductor while using insulator deposition in accordance with the 
invention to avoid unwanted electrical communication between conductors. 
FIG. 5A schematically shows the arrangement of conductors in the area 
modified. For simplicity of illustration, the overlying SiO.sub.2 and 
Si.sub.3 N.sub.4 layers (e.g., layers 115 and 118 of FIG. 1A) are not 
shown. At 500 is shown the M2 conductor to be severed to expose M1 
conductor 505. With the device in operation, M2 conductor 510 carries 
signal "A" , M2 conductor 500 to be severed carries signal "B", M2 
conductor 515 carries signal "C", and M1 conductor 505 and M2 conductor 
520 carry signal "D". Conductors 505 and 520 are joined by a via indicated 
at 525. 
First, passivation was removed by FIB milling from a 4 .mu.m.times.2 .mu.m 
region of conductor 500 outlined by the dashed lines at 530 in FIG. 5A. 
Milling was conducted with a beam energy of 30 key and current of 100 pA, 
the milling rate enhanced by localized injection of 
dielectric-preferential XeF.sub.2 gas during milling. 
A segment of conductor 500 within the depassivated region was then removed 
by FIB milling. Milling was conducted with a beam energy of 30 kev and a 
current of 100 pA, the milling rate enhanced by localized injection of 
metal-preferential CI4 gas during milling. FIG. 5B shows exposed cut-ends 
of severed conductor 500 at 535 and 540. Milling was continued with the 
injection of dielectric-preferential XeF.sub.2 gas to expose M1 conductor 
505 in the area outlined at 545. 
A 10 .mu.m.times.10 .mu.m patch 550 of insulator material was then 
deposited to cover the exposed portions of conductors 500 and 505, as 
shown in FIG. 5C. The insulator material was formed by delivering a 1:1 
mixture of DBTS and CBr.sub.4 gas at 3e-5 torr pressure onto a localized 
region of the device surface through a 10 cm long gas injector having an 
0.8 mm inside diameter. The tip of the injector was maintained at a 
distance of 0.5 mm from the device surface. Other parameters of the 
insulator-deposition were as described above in Example 1.1. A focused 
beam of gallium ions at an energy of 15 kev and with a beam current of 250 
pA was scanned over the region to be covered as the gas was injected. 
After depositing the insulator material, milling was resumed to create a 2 
.mu.m.times.2 .mu.m hole 555 and a 2 .mu.m.times.2 .mu.m hole 560 through 
the passivation to expose conductor 500 near each of its severed ends. 
Milling was performed with a beam energy of 30 kev and with a beam current 
of 20 pA, with localized injection of gas which enhances milling of 
dielectric. A voltage-contrast FIB image of the device acquired after 
milling holes 555 and 560 showed the portion of conductor 500 visible 
through hole 560 substantially darker than the portion of conductor 500 
visible through hole 555; this indicated that the portion of conductor 500 
visible through hole 560 was electrically floating and that good 
electrical isolation between the cut ends of conductor 500 was preserved 
even after deposition of insulator patch 550. Electrical isolation between 
the cut ends of conductor 500 was confirmed by monitoring the 
secondary-electron-detector signal during milling. A plot of the signal 
showed a large amplitude increase as conductor 500 was exposed in hole 
555, and a substantially smaller amplitude increase as conductor 500 was 
exposed in hole 560. The difference between the two amplitude increases 
was about one order of magnitude. 
Conductive material was then deposited to fill holes 555 and 560 and form a 
bridge 565 which extends over insulator pad 550 in electrical contact with 
conductor 500 at each side of the severed portion, as shown in FIG. 5D. 
After depositing the metal bridge, milling was resumed to create a 2 
.mu.m.times.2 .mu.m probe hole 570 through the passivation to expose 
conductor 510, a 2 .mu.m.times.2 .mu.m probe hole 575 through the 
passivation to expose conductor 515, and a 2 .mu.m.times.2 .mu.m probe 
hole 580 through the passivation to expose conductor 520. The probe holes 
were then used to verify electrical properties of the deposited insulator 
material. 
FIG. 6 shows a series of signals acquired from the sample device before and 
after the operations described above. A set of reference signals was 
acquired from the sample device in its original state: line 600 is 
reference signal "A" on conductor 510, line 610 is reference signal "B" on 
conductor 500, line 620 is reference signal "C" on line 515, and line 630 
is reference signal "D" on line 520. A similar set of signals was acquired 
from the sample device after severing and reconnecting conductor 500: line 
605 is signal "A" acquired at probe hole 570, line 615 is signal "B" 
acquired at bridge 565, line 625 is signal "C" acquired at probe hole 575, 
and line 635 is signal "D" acquired at probe hole 580. The before and 
after signals agree favorably, indicating that the deposited insulator pad 
550 provides good electrical isolation between conductors 500 and 505, 
with no influence on the adjacent conductors. 
The sample device was sectioned through the length of bridge 565 as 
indicated by line VII--VII in FIG. 5D. A FIB image of the sectioned device 
was acquired, a line drawing of which is shown in FIG. 7. The image is a 
tilted, perspective view. The portions shown in section include conductor 
505, severed conductor 500, insulative regions 700 of native SiO.sub.2 
covered with the deposited insulator material, a region of deposited 
insulator material 710 over conductor 505, and deposited metal bridge 565 
at 720. Bold line 730 divides the sectioned portion of the FIB image from 
the upper portion of the image showing the top surface of the device. 
Visible in the upper portion of the image are the top surface of bridge 
565 at 740, the contour of via 525, and probe hole 580 with an exposed 
portion of conductor 520. Resistance of the deposited insulator material 
was estimated from relative brightness of conductors 505 and 500/520 in 
the FIB image, in which conductor 505 was grounded and conductors 500/520 
were electrically floating. The image was acquired with a FIB energy of 30 
kev and current of 5 pA. While not a precise indicator, the voltage 
contrast between conductors 505 and 500 was conservatively estimated to be 
1 volt. At 5 pA beam current, resistance of the deposited insulator 
material was estimated to be 1 volt/5 pA=200 M.OMEGA.. Even a voltage 
contrast of as little as 0.1 volt would give a resistance of 0.1 volt/5 
pA=20 M.OMEGA.. The actual resistance is believed to be higher than 200 
M.OMEGA. and perhaps as high as 1 G.OMEGA.. Whatever the actual 
resistance, the signals of FIG. 6 show virtually no signal attenuation 
under conditions which one would typically encounter in IC diagnosis. 
The precursor gas mixture used for FIB-assisted insulator deposition can be 
a mixture of ammonium carbonate ((NH.sub.3).sub.2 CO.sub.3) and DBTS in 
accordance with the invention, rather than the precursor gas mixture of 
CBr.sub.4 and DBTS used in Examples 1.1 and 1.2. Examples 2.1-2.6 
illustrate. Material deposited using typical parameters (as in Example 
2.1) was subjected to Auger spectroscopy analysis, which showed the 
deposited material consisted entirely of silicon, gallium and oxygen 
atoms. Referring to FIG. 8, in each of Examples 2.1-2.6 a pad 800 of 
insulative material of specified dimensions was deposited on a test 
structure having aluminum conductors 805 and 810 on a native SiO.sub.2 
layer 815 and having a 1 .mu.m gap 820 between conductors 805 and 810. 
Conductors 805 and 810 are approximately 1 .mu.m thick. The thickness 825 
of the deposited insulator material above the conductors varies from 
example to example. Resistance between conductors 805 and 810 was measured 
before depositing pad 800 and at various voltages after depositing pad 
800. In some examples, the test structure was sectioned to measure 
cross-sectional area of pad 800 and bulk resistivity of the deposited 
insulative material was calculated. 
EXAMPLE 2.1 
A 6 .mu.m.times.6 .mu.m pad 800 of insulator material was deposited to a 
thickness of approximately 1.75 .mu.m by directing a focused gallium (Ga) 
ion beam of 250 pA beam current at 30 keV beam energy for 20 minutes while 
a precursor gas of ammonium carbonate and DBTS mixed at 1:1 ratio was 
delivered onto a localized region of the device surface through a 10 cm 
long gas injector having an 0.8 mm inside diameter. Partial pressure of 
each of the ammonium carbonate and DBTS gases was set to 3e-5 Torr chamber 
pressure (i.e., the crucible for each gas source was held at a temperature 
which would produce 3e-5 Torr chamber pressure if that gas was being 
injected by itself). The tip of the injector was maintained at a distance 
of 0.5 mm from the device surface. Deposition rate of the insulative 
material was approximately 3 .mu.m.sup.3 per minute. 
The deposited insulative material was calculated to have a bulk resistivity 
across the 1 .mu.m gap of approximately 200 M.OMEGA.-cm. The deposited pad 
was observed to have sheer vertical walls, little overspray and uniform, 
homogeneous deposition. 
EXAMPLE 2.2 
Deposition parameters for this example were the same as for Example 2.1, 
except that the DBTS partial pressure was set to 3e-5 Torr and the 
ammonium carbonate partial pressure was set to 2e-5 Torr, giving a 3:2 
chamber-pressure partial pressure ratio between the two components of the 
gas mixture (more DBTS than ammonium carbonate). 
The deposition rate appeared to be somewhat greater than for Example 2.1. 
The deposited material appeared in a FIB image to be of brighter contrast 
than for Example 2.1, suggesting lower resistivity than the material 
deposited in Example 2.1. The deposited insulative material was calculated 
to have a bulk resistivity across the 1 .mu.m gap of approximately 800 
K.OMEGA.-cm. 
EXAMPLE 2.3 
Deposition parameters for this example were the same as for Example 2.1, 
except that the DBTS partial pressure was set to 2e-5 Torr and the 
ammonium carbonate partial pressure was set to 3e-5 Torr, giving a 2:3 
chamber-pressure partial pressure ratio between the two components of the 
gas mixture (less DBTS than ammonium carbonate). 
The deposition rate appeared to be much less than for Example 2.1. The 
deposited material appeared in a FIB image to be of darker contrast than 
for Example 2.1, suggesting higher resistivity than the material deposited 
in Example 2.1. The deposited insulative material was calculated to have a 
bulk resistivity across the 1 .mu.m gap of not less than 200 M.OMEGA.-cm. 
(Resistance measurements vary with applied bias voltage. With an applied 
bias of one to ten volts, the leakage current was on the order of tens of 
picoAmperes. It is believed that a portion of the leakage current is 
contributed by leakage through paths other than the deposited insulator, 
so the actual bulk resistivity of the deposited insulator is believed to 
be greater than the calculated value. The intrinsic resistance between 
conductors 805 and 810 of the test structure shown in FIG. 8 is typically 
400 G.OMEGA. to 500 G.OMEGA. prior to deposition and without cleaning the 
surface of possible contaminants by FIB etching.) 
EXAMPLE 2.4 
Deposition parameters for this example were the same as for Example 2.1, 
except that the scanning box was 4 .mu.m.times.4 .mu.m and the deposition 
time was 10 minutes. Scanning with 250 pA beam current over the 4 
.mu.m.times.4 .mu.m region, the average beam-current density was 15.6 
pA/.mu.m.sup.2. (Each 500-line scan of the region is performed in 30 msec, 
independent of the specified size of the region. Average beam-current 
density is defined as the ratio of instantaneous beam current to the 
scanning area.) 
Other than a small ridge of material deposited around the periphery of the 
region, the FIB basically etched the surface of the test structure without 
depositing insulative material. 
EXAMPLE 2.5 
Deposition parameters for this example were the same as for Example 2.1, 
except that the scanning box was 10 .mu.m.times.10 .mu.m and the 
deposition time was 10 minutes. Scanning with 250 pA beam current over the 
10 .mu.m.times.10 .mu.m region, the average beam-current density was 2.5 
pA/.mu.m.sup.2. (Each 500-line scan of the region is performed in 30 msec, 
independent of the specified size of the region.) 
Thickness of the 10 .mu.m.times.10 .mu.m pad deposited was estimated at 
approximately 1/4 .mu.m to 1/3 .mu.m. Deposition rate of the insulative 
material was approximately 2.5 .mu.m.sup.3 per minute. 
EXAMPLE 2.6 
A 5 .mu.m.times.5 .mu.m pad 800 of insulator material was deposited to a 
thickness of approximately 2 .mu.m by directing a focused gallium (Ga) ion 
beam of 250 pA beam current at 30 keV beam energy for 10 minutes while a 
precursor gas of DBTS only was delivered onto a localized region of the 
device surface through a 10 cm long gas injector having an 0.8 mm inside 
diameter. Partial pressure of the DBTS gas was set to 2e-5 Torr chamber 
pressure. The tip of the injector was maintained at a distance of 0.5 mm 
from the device surface. Deposition rate of the insulative material was 
approximately 5 .mu.m.sup.3 per minute. 
The deposited pad was observed to grow more quickly than in Examples 
2.1-2.5 (i.e., more quickly than with a mixture of DBTS and ammonium 
carbonate gases). The deposited material appeared in a FIB image to be of 
brighter contrast than for Examples 2.1-2.5, suggesting lower resistivity 
than the material deposited in Examples 2.1-2.5. The deposited insulative 
material was calculated to have a bulk resistivity across the 1 .mu.m gap 
of approximately 100 K.OMEGA.-cm to 200 K.OMEGA.-cm. 
Experiments using a mixture of DBTS and ammonium carbonate with each gas of 
the mixture set to a partial pressure of 1.5e-5 Torr (rather than 3e-5 
Torr as in Example 2.1) suggest that only the effective deposition rate is 
varied, therefore limiting the maximum beam current that could be used for 
a given insulator deposition operation. 
Trials using a mixture of DBTS and ammonium carbonate were conducted to 
create a probe pad for passing a signal conductor through a power plane, 
as illustrated in FIGS. 1A-1D and 2A-2D. In one such trial, the insulative 
material (125) was deposited using a Ga-ion FIB and a 1:1 ratio of DBTS to 
ammonium carbonate, an opening (130) was milled, and conductive material 
(135) containing metal and carbon was deposited. A defect in the deposited 
conductive material (135) caused it to violently explode when voltage was 
applied, leaving the deposited insulative material (125) in place. 
Subsequent examination revealed no damage to the deposited insulative 
material, suggesting that the deposited insulative material is quite hard 
and capable of withstanding physical abuse. Other evidence suggests 
likewise. 
Trials were also conducted in which a mixture of DBTS and ammonium 
carbonate was used with a Ga-ion FIB to deposit insulative material over 
conductive material previously deposited using the Ga-ion FIB. Such 
conductive material typically contains metal and carbon, with less than 
50% metal content, and responds to an applied FIB as if soft and amorphous 
and by demonstrating increased electrical resistance. To minimize damage 
to such conductive material during the deposition of insulator material, 
it is useful to first deposit over the conductive material a thin layer of 
insulative material at low average beam-current density (e.g., as in 
Example 2.5) before depositing insulative material at a higher average 
beam-current density (e.g., as in Example 2.1). 
Parameters affecting insulator deposition process with a mixture of DBTS 
and ammonium carbonate are summarized as follows: 
Average Beam-Current Density. Material deposition appears to be optimized, 
in terms of insulating quality and deposition rate, with an average 
beam-current density between 3 pA/.mu.m.sup.2 and 5 pA/.mu.m.sup.2, with 
an acceptable range between 1 pA/.mu.m.sup.2 and 15 pA/.mu.m.sup.2. When 
exceeding 15 pA/.mu.m.sup.2 there appears to be no net deposition but 
instead etching by the FIB. Below 1 pA/.mu.m.sup.2, the deposition process 
is prohibitively slow and also results in more implanted Ga ions 
per/.mu.m.sup.3, thus lowering the effective resistance of the deposited 
material. 
Absolute and Relative Gas Pressures. Material deposition appears to be 
optimized with the partial pressure of each of the DBTS and ammonium 
carbonate gases set to 3e-5 Torr, providing a 1:1 partial pressure ratio 
of DBTS to ammonium carbonate. 
Deposition using a DBTS to ammonium carbonate partial-pressure ratio of 3:2 
produced a sample with qualities that approached those of a sample 
deposited with DBTS alone, i.e., appearing lighter in relative contrast in 
a FIB image and having poorer insulating characteristics. Deposition using 
a DBTS to ammonium carbonate partial-pressure ratio of 2:3 produced a 
sample having low relative contract in a FIB image and having good 
insulating quality, but the overall deposition rate was proportionally 
slower than with a 1:1 partial-pressure ratio. A 1:1 partial-pressure 
ratio offers an optimal deposition rate with good quality insulator. 
Bulk Resistivity Measurements. FIG. 9 shows the relationship between bulk 
resistivity (M.OMEGA.-cm) and applied bias voltage (volts/.mu.m) for two 
insulator samples of 6 .mu.m.times.6 .mu.m deposited as in Example 2.1 
across a 1 micron (.mu.m) gap between two aluminum traces to a thickness 
of approximately 1.5 .mu.m. For each sample, a first set of measurements 
was taken (curve 900 for the first sample, curve 910 for the second 
sample), the bias voltage was held at 20 volts for 2 minutes, and then a 
second set of measurements was taken (curve 905 for the first sample, 
curve 915 for the second sample). Bulk resistivity values of 200 
M.OMEGA.-cm are typical. Values obtained With these and other samples 
range from about 100 M.OMEGA.-cm to about 300 M.OMEGA.-cm. The absolute 
value of resistance measured for the geometry of the samples deposited was 
approximately 200 GigOhms (G.OMEGA.). Since the test IC exhibits an 
intrinsic resistance of approximately 400 G.OMEGA. to 500 G.OMEGA. between 
the conductors prior to deposition of the insulative material, the 
calculated bulk resistivity values are believed to be lower than the real 
bulk resistivity of the deposited insulative material. It appears that for 
specific applications the quality of deposited insulator can be traded off 
against deposition rate by adjusting the ratio of DBTS to ammonium 
carbonate and/or the beam current. Heating the deposited insulative 
material for a period of time (see below) significantly improves its 
insulating capabilities, affording another method to increase the quality 
of the insulator while maintaining a maximum deposition rate. 
Maximum Breakdown Field. Dozens of samples deposited across a 1 .mu.m gap 
have been tested with up to a 40 volt potential applied across them, 
without any samples breaking down. Two samples were tested to breakdown, 
one of which was baked (see below) and the other of which was not baked. 
Both samples were stable up to an applied voltage of 90 volts, and 
breakdown of both samples occurred between 90 volts and 100 volts. This 
corresponds to a breakdown field of 90 million volts per meter. 
Effective Volumetric Deposition Rates. An average beam-current density of 1 
pA/.mu.m.sup.2 to 10 pA/.mu.m.sup.2 and a 1:1 gas pressure ratio with a 
system pressure of 3e-5 Torr results in a deposition rate of about 2 
.mu.m.sup.3 /minute to more than 15 .mu.m.sup.3 /minute. This has been 
verified over sample sizes ranging from 6 .mu.m.times.6 .mu.m at a beam 
current of 250 pA, to 70 .mu.m.times.70 .mu.m at a beam current of 6,000 
pA. Samples have been grown with thicknesses from 0.5 .mu.m to 4 .mu.m 
with no indication that thicker samples could not be grown with more time. 
A typical deposition for electrical analysis might be to use a 250 pA beam 
current for 20 minutes over a 6 .mu.m.times.24 .mu.m sample size, 
resulting in a deposited-material thickness of approximately 1.7 .mu.m. 
Effect of Baking a Deposited Insulator Sample. After a sample has been 
deposited and its characteristic resistance measured the effects of baking 
the sample in an atmospheric oven have been investigated. It has been 
demonstrated repeatedly that heating the sample to a temperature in excess 
of 100.degree. C. results in an increase in the samples resistivity of 
from 2 to 5 times that before baking. FIG. 10 shows the resistance of two 
identical 6 .mu.m.times.12 .mu.m samples in G.OMEGA. vs. volts/.mu.m 
measured before baking and after baking at 150.degree. C. for 5 hours. 
Curves 1000 and 1005 show the measured resistances of the first and second 
samples, respectively, before baking. Curves 1010 and 1015 show the 
measured resistances of the first and second samples, respectively, after 
baking. The resistances improved slightly more when the baking temperature 
is increased to 150.degree. C. rather than to some lower temperature above 
100.degree. C. Baking for much more than 5 hours does not appear to 
further increase the resistance. It is believed that substantially the 
same results may be achieved with less than a 5 hour baking time. Baking 
is expected to assist in making a more durable sample that can better 
withstand further processing in conventional semiconductor industry 
testing and manufacturing equipment. 
Proof of Concept of Insulator Quality. As a functional test of the 
insulator material we used the insulator process in conjunction with other 
FIB processes of depositing metal-containing conductive material and 
halogen-enhanced etching to allow access to a metal trace lying beneath a 
power-plane, as illustrated in FIGS. 1A-1D and 2A-2D. The resistance 
measured between the signal conductor and the power plane was determined 
to be from 80 G.OMEGA. at 1-volt applied potential to 1 G.OMEGA. at a 
12-volts applied potential, as shown in FIG. 11. The sample did not break 
down at 12 Volts. It was necessary to protect against stray-current paths 
over the surface of the device in order to accurately measure the 
resistance of the deposited insulator material. To this end, it was useful 
to "cap" (locally re-passivate with insulator material) any exposed areas 
near this operation that could serve to sink the applied measuring 
voltage. 
Table 1 is a summary of useful ranges and optimum/typical values for 
FIB-assisted deposition of insulative material using an injected precursor 
gas mixture of DBTS and ammonium carbonate. 
TABLE 1 
______________________________________ 
Parameter Range of Values 
Optimum/Typical Value 
______________________________________ 
Average 1-15 pA/.mu.m.sup.2 
3-5 pA/.mu.m.sup.2 
Beam-Current Density 
Total Gas Pressure 
1.5-3.5 .times. 10.sup.-5 Torr 
3.0 .times. 10.sup.-5 Torr 
Gas Partial-Pressure 
2:3-3:2 (DBTS:(NH.sub.3).sub.2 CO.sub.3) 1:1 
Ratios (as measured by chamber pressure -- see text) 
Bulk Resistivity 
more than 
(with optimum values) 
100-300 M.OMEGA.-cm 
200 M.OMEGA.-cm 
Maximum Electrostatic 
Approximately 90 Volts per micron 
Breakdown Field 
Deposition Rates 
2-15 .mu.m.sup.3 /minute 
10 .mu.m.sup.3 /minute 
Signal Through 
80 G.OMEGA. @ 1 volt 
Resistance between 
Power Plane 1 G.OMEGA. @ 12 Volts 
power-plane and signal 
______________________________________ 
Other Process Modifications 
The insulator deposition techniques describe above can be modified in a 
variety of ways. The modifications described below are not mutually 
exclusive. 
The device can be baked in an oxygen-rich environment (e.g., in air) to 
improve the resistivity of the deposited insulator material. The improved 
resistivity is believed to be due to capture of free gallium ions in the 
deposited material with oxygen atoms and/or halogen atoms. Baiting is 
performed at a temperature above room temperature, e.g., above 25.degree. 
C. 
The ratio of gases in the injected mixture can be changed. For example, the 
ratio of CBr.sub.4 to DBTS of between about 1:1 and about 1:2 has been 
found to work well, and a range of from 1:10 to 10:1 is expected to be 
useful. 
The gases need not be mixed in a manifold and injected as a mixture from a 
single needle as in the examples described above, but may be injected 
independently through separate needles aimed so as to produce a gas 
mixture on or near the surface in the localized region over which the FIB 
is scanned. 
The average beam-current density can be changed. An advantage of using 
lower average beam-current density to deposit insulator material is that 
the ratio of the amount of gas to the number of incoming gallium ions is 
higher. Fewer gallium ions per volume of gas is believed to result in a 
lower number of gallium ions in the deposited material which are not bound 
(electrically neutralized) by the reactive gas, and therefore to produce 
deposited insulator material of higher resistivity. The only apparent 
disadvantage of using lower average beam-current is that the rate of 
deposition is lower than for average higher beam-current density. Average 
beam-current densities in the range of about 1 pA/.mu.m.sup.2 to about 1 
pA/.mu.m.sup.2 are contemplated within the present invention. 
The beam energy can be changed. It has been demonstrated that high beam 
energy results in low resistivity of deposited material. Beam energies in 
the range of 5 kev to 40 kev are contemplated within the present 
invention. 
It appears from inspection of FIB cross-section images that deposition of 
insulator material using a mixture of DBTS gas and CBr.sub.4 gas with a 
gallium-ion beam may be more effective if the deposition is begun over a 
region of native SiO.sub.2 than over a region of native Si.sub.3 N.sub.4. 
This is believed to be because the native SiO.sub.2 acts as a seed for 
further SiOx deposition. The deposition process can thus be enhanced by 
removing the native Si.sub.3 N.sub.4 passivation layer to expose the 
native SiO.sub.2 in a region on which insulator material is to be 
deposited, before beginning insulator deposition. The insulative quality 
of material deposited using ammonium carbonate and DBTS does not appear to 
be substrate dependent. 
Those of skill in the art will recognize that these and other modifications 
can be made within the spirit and scope of the invention as defined in the 
claims which follow.