U.S. patent application number 09/964944 was filed with the patent office on 2002-01-24 for contactless total charge measurement with corona.
Invention is credited to Horner, Gregory S., Miller, Tom G., Verkuil, Roger L..
Application Number | 20020008536 09/964944 |
Document ID | / |
Family ID | 25432289 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008536 |
Kind Code |
A1 |
Miller, Tom G. ; et
al. |
January 24, 2002 |
Contactless total charge measurement with corona
Abstract
A method of measuring total charge of an insulating layer on a
semiconductor substrate includes applying corona charges to the
insulating layer and measuring a surface photovoltage of the
insulating layer after applying each of the corona charges. The
charge density of each of the corona charges is measured with a
coulombmeter. A total corona charge required to obtain a surface
photovoltage of a predetermined fixed value is determined and used
to calculate the total charge of the insulating layer. The fixed
value corresponds to either a flatband or midband condition.
Inventors: |
Miller, Tom G.; (Solon,
OH) ; Verkuil, Roger L.; (Wappinger Falls, NY)
; Horner, Gregory S.; (Santa Clara, CA) |
Correspondence
Address: |
PEARNE & GORDON LLP
526 SUPERIOR AVENUE EAST
SUITE 1200
CLEVELAND
OH
44114-1484
US
|
Family ID: |
25432289 |
Appl. No.: |
09/964944 |
Filed: |
September 27, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09964944 |
Sep 27, 2001 |
|
|
|
09749485 |
Dec 26, 2000 |
|
|
|
09749485 |
Dec 26, 2000 |
|
|
|
08912697 |
Aug 18, 1997 |
|
|
|
6191605 |
|
|
|
|
Current U.S.
Class: |
324/762.05 |
Current CPC
Class: |
G01R 31/2656 20130101;
G01R 31/2831 20130101; G01R 29/24 20130101 |
Class at
Publication: |
324/767 |
International
Class: |
G01R 031/26 |
Claims
What is claimed is:
1. A method for measuring a total charge of an insulating layer on
a substrate, said method comprising the steps of: (a) depositing
corona charges on the insulating layer; (b) measuring a surface
photovoltage for the insulating layer after depositing each of said
corona charges; (c) determining a total corona charge required to
obtain a surface photovoltage of a predetermined fixed value; and
(d) using said total corona charge to determine the total charge of
the insulating layer.
2. The method according to claim 1, further comprising the step of
measuring a charge density for each of said deposited corona
charges.
3. The method according to claim 1, wherein said fixed value is
associated with a flatband condition.
4. The method according to claim 3, wherein said fixed value is
about 0.0 volts.
5. The method according to claim 1, wherein said fixed value is
associated with a midband condition.
6. The method according to claim 5, wherein said fixed value is
about .+-.0.3 volts.
7. The method according to claim 1, wherein said step of
determining said total corona charge includes continuing to deposit
said corona charges until the surface photovoltage measured is
equal said fixed value and said total corona charge corresponds to
a sum of said corona charges deposited.
8. The method according to claim 7, further comprising the step of
reversing polarity of said corona charges if said surface
photovoltage changes in a direction away from said fixed value.
9. The method according to claim 1, wherein said step of
determining said total corona charge includes using a data set of
discrete points, wherein said discrete points include said surface
photovoltages measured after depositing said corona charges and
corresponding total corona charges deposited to obtain said surface
photovoltages.
10. The method according to claim 9, wherein said step of using
said data set includes interpolating said total corona charge from
said discrete points.
11. The method according to claim 1, further comprising the step of
correcting each surface photovoltage with a Dember Voltage.
12. A method for measuring a total charge of an oxide layer on a
semiconductor wafer, said method comprising the steps of: (a)
measuring a surface photovoltage of the oxide layer; (b) depositing
a corona charge on the oxide layer; (c) remeasuring said surface
photovoltage of the oxide layer; (d) reversing polarity of said
corona charge if said surface photovoltage changed away from a
predetermined fixed value; (e) repeating steps (b) to (d) until
said surface photovoltage is equal to said fixed value; (f)
determining a total corolla charge deposited on the oxide layer
corresponding to said surface photovoltage which is equal to said
fixed value; and (g) using said total corona charge to determine
the total charge of the oxide layer.
13. The method according to claim 12, wherein said fixed value is
associated with a flatband condition.
14. The method according to claim 13, wherein said fixed value is
about 0.0 volts.
15. The method according to claim 12, wherein said fixed value is
associated with a midband condition.
16. The method according to claim 15, wherein said fixed value is
about .+-.0.3 volts.
17. The method according to claim 12, further comprising the step
of correcting each surface photovoltage with a Dember Voltage.
18. The method according to claim 12, further comprising the step
of selectively adjusting a magnitude of said corona charge prior to
the step of repeating steps (b) to (e).
19. A method for measuring a total charge of an oxide layer on a
semiconductor wafer, said method comprising the steps of: (a)
depositing a corona charge on the oxide layer; (b) measuring a
surface photovoltage of the oxide layer; (c) determining a total
corona charge density associated with said surface photovoltage;
(d) repeating steps (a) to (c) a plurality of times to obtain a
data set of discrete points for the surface photovoltages and the
total corona densities; (e) using said data set to determine a
total corona charge density corresponding to a surface photovoltage
of a predetermined fixed value; and (f) using said total corona
charge to determine the total charge of the oxide layer.
20. The method according to claim 19, wherein said step of using
said data set includes interpolating said fixed value from said
discrete points.
21. The method according to claim 18, further comprising the step
of correcting each surface photovoltage with a Dember Voltage.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention generally relates to testing a
semiconductor wafer and, more particularly, to measuring a total
charge of an insulating layer of the semiconductor wafer using
corona charge.
[0002] The production of insulating layers, particularly, thin
oxide layers, is basic to the fabrication of integrated circuit
devices on semiconductor wafers. A variety of insulating dielectric
layers are used for a wide range of applications. These insulating
layers can be used, for example, to separate gate layers from
underlying silicon gate regions, as storage capacitors in DRAM
circuits, for electrical device isolation and to electrically
isolate multilayer metal layers.
[0003] The devices, however, are very sensitive to induced charges
near the silicon surface. In most cases, device performance depends
strongly on the concentration of free charges in the silicon. As a
result, unwanted variations in device performance can be introduced
by charges in the insulating layer and the insulating layer
interface. The charges can result, for example, from static
charging of the insulating layer surface, poorly forming the
insulating layer, excessive ionic contamination within the
insulating layer, and metallic contamination within the insulating
layer. In addition to degradation of device performance, electrical
isolation of individual devices can be impaired by unwanted surface
channels due to induced charges. A property of increasing interest,
therefore, is total charge Q.sub.tot or sometimes referred to as
net charge Q.sub.net of the insulating layer.
[0004] As illustrated in FIG. 1, there are five principle
components of the total charge Q.sub.tot of an oxide layer: surface
charge Q.sub.s; mobile charge Q.sub.m; oxide trapped charge
Q.sub.ot; fixed charge Q.sub.f; and interface trapped charge
Q.sub.it. The surface charge Q.sub.s is charge on time top surface
of the oxide layer and is frequently static charge or charged
contaminants such as metallics. The mobile charge Q.sub.m is ionic
contamination in the oxide layer such as potassium, lithium, or
sodium trapped near the air/SiO.sub.2 interface or the Si/SiO.sub.2
interface. The oxide trapped charge Q.sub.ot is electrons or holes
trapped in the bulk oxide. The fixed charge Q.sub.f is charge at
the Si/SiO.sub.2 interface. The interface trapped charge Q.sub.it
varies as a function of bias condition.
[0005] Conventional methods of determining the total charge
Q.sub.tot of an oxide layer include capacitance-voltage (CV),
surface photovoltage (SPV) with biasing, and SPV analysis. The CV
method typically measures each of the individual component charges,
except the surface charge Q.sub.s which can be measured by the CV
method, with a metal contact formed on the surface of the oxide
layer and then obtains the total charge Q.sub.tot by summing up the
individual component charges. The SPV with biasing method uses a
contacting probe separated from the oxide layer with a Mylar
insulator to bias the semiconductor. The total charge Q.sub.tot is
determined by measuring the required bias of the probe to force a
certain SPV. The SPV analysis method takes SPV measurements and
infers the total charge Q.sub.tot via theoretical modeling.
[0006] While these methods may obtain the total charge Q.sub.tot,
they each have drawbacks. The CV method requires expensive and time
consuming sample preparation. The SPV with biasing method requires
a contacting probe which can allow charge transfer from the oxide
layer to the probe. The SPV analysis method relies on theoretical
modeling and may not be extremely accurate. Additionally, the SPV
methods only work over a narrow range of total charge Q.sub.tot,
when the semiconductor is in depletion. Accordingly, there is a
need in the art for an improved method of measuring the total
charge of an insulating layer which is contactless, is a direct
measurement with no theoretical modeling, is sensitive over a wide
range of total charge, and is extremely accurate.
BRIEF SUMMARY OF THE INVENTION
[0007] The present invention provides a method for measuring a
total charge of an insulating layer on a substrate which overcomes
at least some of the disadvantages of the above-noted related art.
According to the present invention, the method includes depositing
corona charges on the insulating layer and measuring a surface
photovoltage for the insulating layer after depositing each of the
corona charges. The method further includes determining a total
corona charge required to obtain a surface photovoltage of a
predetermined fixed value and using the total corona charge to
determine the total charge.
[0008] According to one variation of the method according to the
present invention, the total corona charge is determined by
continuing to deposit the corona charges until the surface
photovoltage measured is equal the fixed value. The total corona
charge then corresponds to a sum of the corona charges deposited.
According to another variation of the method according to the
present invention, the total corona charge is determined using a
data set of discrete points, preferably by interpolation. The
discrete points include the surface photovoltages measured after
each of the corona charges and corresponding total corona charges
deposited to obtain each of the surface photovoltages.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0009] These and further features of the present invention will be
apparent with reference to the following description and drawings,
wherein:
[0010] FIG. 1 is a diagrammatic view of a semiconductor wafer
illustrating principle components of a total charge of an
insulating layer;
[0011] FIG. 2 is a schematic diagram of an apparatus for measuring
a total charge of an insulating layer according the present
invention;
[0012] FIG. 3 is an exemplary graph illustrating how the total
charge can be determined by incrementally depositing a corona
charge until obtaining a surface photovoltage (SPV) equal to a
fixed value; and
[0013] FIG. 4 is an exemplary graph illustrating how the total
charge can be determined by interpolating a data set of measured
surface photovoltages (SPV) and associated total corona charge
densities.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0014] FIG. 1 illustrates an apparatus 10 for testing a
semiconductor wafer 12 according to the present invention. The
semiconductor wafer 12 includes a semiconductor substrate 14 and a
dielectric or insulating layer 16 disposed on the substrate 14. The
substrate 14 is typically a silicon substrate and the insulating
layer 16 is typically an oxide layer. However, it should be
understood that the method of the present invention is applicable
to a variety of insulating layers grown and/or deposited on
substrates of semiconductor materials or metals. An air/dielectric
interface 18 is formed at the top surface of the insulating layer
16 and a dielectric/substrate interface 20 is formed between the
insulating layer 16 and the substrate 14. A measurement region 22
of the insulating layer 16 is selected to be tested by the
apparatus 10.
[0015] The illustrated apparatus includes a wafer chuck 24 for
holding the wafer 12 during testing, a contactless calibrated
corona discharge source or gun 26 for depositing corona charges, a
coulombmeter 28 for measuring deposited corona charges, an SPV
device 30 for measuring surface photovoltages, a position actuator
34 for locating various components over the wafer 12, and a
controller 36 for operating the apparatus 10. The wafer chuck 24
holds the wafer 12 during the measurement process and the wafer 12
is preferably secured to the wafer chuck 24 with a vacuum.
[0016] The corona gun 26 includes a non-contact corona-charge
depositing structure such as one or more needles 38 and an
electrode housing 40 which, along with the needles 38, focuses the
corona discharge onto the measurement region 22 of the insulating
layer 16. The needles 38 are preferably disposed a distance above
the top surface 18 of the insulating layer 16 to minimize fringing
effects and other causes of charge deposition non-uniformity. U.S.
Pat. No. 5,498,974, expressly incorporated herein in its entirety
by reference, discloses a suitable corona gun for depositing corona
charge on an insulating layer and a suitable Kelvin probe for
measuring the voltage on the surface of the layer.
[0017] The needles 38 are connected to a charge biasing means such
as a high-voltage power supply 42 via a suitable line. The power
supply 42 provides a desired high voltage output (e.g., .+-.6-12
Kv) to the corona gun 26 to produce positive or negative corona
charges depending on the polarity of the supply. The power supply
42 is suitably connected to the controller 36 via an appropriate
signal line for feedback control of the power supply 42 during
operation of the apparatus 10 as described in more detail
hereinafter.
[0018] The coulombmeter 28 is used to measure the deposited corona
charge and preferably includes a first operational amplifier or
current-to-voltage converter 44 and a second operational amplifier
or charge integrator 46. The input of current-to-voltage converter
44 is connected via a suitable signal line to the substrate 14 and
the wafer chuck 24. A corona current I.sub.c flows from the corona
gun 26 and through the wafer 12 to the current-to-voltage converter
44. This current I.sub.c is converted by the current-to-voltage
converter 44 to a voltage and then integrated by the charge
integrator 46 to generate a voltage proportional to the charge
Q.sub.c deposited onto the insulating layer 16 by the corona gun
26. The outputs of the current-to-voltage converter 44 and the
charge integrator 46 are each connected to the controller 36 via
suitable signal lines to feed the current I.sub.c and the deposited
corona charge Q.sub.c information to the controller 36 during
operation of the apparatus 10 as described in more detail
hereinafter. Note that an electrical contact between the wafer 12
and the chuck 24 because the regulating displacement currents are
sufficient to perform the measurement.
[0019] The SPV device 30 is used to measure surface photovoltages
of the insulating layer 16 and preferably includes a very high
intensity light source 48 such as, for example, a xenon flash tube.
It is noted, however, that other types of SPV devices can be used
such as, for example, LED, laser, or AC with lock-in.
[0020] The position actuator 34 is used to locate the corona gun
26, and the SPV device 30, over the measurement region 22 of the
wafer 12. The position actuator is preferably a high-speed linear
translator including a mobile carriage which selectively moves
along a track disposed above the wafer chuck 24. The corona gun 26
and the SPV device 30, are each suitably spaced apart and attached
to the carriage. A control unit is suitably connected to the
controller 36 via an appropriate signal line for feed-back control
during operation of the apparatus 10 as described in more detail
hereinafter.
[0021] The controller 36 is used to control the operation of the
apparatus 10 and preferably includes an input device 62 connected
via a suitable line. The controller 36 controls the high-voltage
power supply 42, the SPV device 30, the Kelvin control 54, and the
position actuator control unit 60 and receives information from the
current-to-voltage converter 44 and the current integrator 46.
Based on the method set forth hereinbelow, the controller 36 can
provide a measurement of total charge Q.sub.tot of the insulating
layer 16. The controller 36 may be, for example, a dedicated
microprocessor-based controller or a general purpose computer.
[0022] To obtain a total charge Q.sub.tot measurement for an
insulating layer 16 of a semiconductor wafer 12 according to a
first method of the present invention, the actuator preferably
first locates the SPV device 30 over the measuring region 22 of the
wafer 12 to obtain an initial SPV measurement V.sub.SPV of the
insulating layer 16. The lamp 48 is flashed and a recording of a
peak intensity of the SPV transient is captured by an A/D card of
the controller 36. Because of the high intensity output of the lamp
48, a measurable SPV can be obtained in both in accumulation and in
depletion or inversion. Note that other types of SPV devices such
as, for example, LED, laser, or AC lock-in amplifier can be
used.
[0023] The position actuator 34 next locates the corona gun 26 over
the measuring region 22 of the wafer 12 to deposit a corona charge
Q.sub.c on the measurement region 22 of the insulating layer 16.
The controller 36 provides appropriate control signals for the
corona gun 26 to deposit a corona charge Q.sub.c. The corona charge
Q.sub.c deposited on the insulating layer 16 is measured by the
coulombmeter 28 and recorded by the controller 36.
[0024] The position actuator then locates the SPV device 30 over
the measuring region 22 of the wafer 12 to again measure the SPV
V.sub.SPV of the insulating layer 16. The SPV measurement V.sub.SPV
is preferably recorded by the controller 36 and compared to a
predetermined target value V.sub.SPVtarget stored in the controller
36. Preferably, the target value V.sub.SPVtarget is equal to a
fixed value (0 volts) which indicates a "flatband condition". At
flatband, no net charge is present on the insulating layer 16 and
no space charge imaging is in the silicon substrate 14. It should
be understood that the target value V.sub.SPVtarget can be equal to
fixed values other than zero. For example, the target value
V.sub.SPVtarget can be equal to a fixed value (typically about
.+-.0.300 V) which indicates a "Midband condition". At midband, the
SPV V.sub.SPV is equal to the fixed value which depends on the
doping of the particular substrate 14.
[0025] If the SPV measurement V.sub.SPV is not substantially equal
to the target value V.sub.SPVtarget, the above described steps of
depositing the corona charge Q.sub.c and remeasuring the SPV are
repeated. If the new SPV measurement V.sub.SPV changes beyond the
target value V.sub.SPVtarget from the previous SPV measurement
V.sub.SPV, the controller 36 provides appropriate control signals
for the corona gun 26 to reverse the polarity of the next deposited
corona charge Q.sub.c. Note that for a target value V.sub.SPVtarget
of zero volts, a change in polarity from the previous SPV
measurement to new SPV measurement indicates that the polarity of
the next deposited corona charge Q.sub.c should be reversed. As
required, the controller 36 can adjust the magnitude of the next
deposited corona charge Q.sub.c to obtain an SPV measurement
V.sub.SPV equal to the target value V.sub.SPVtarget.
[0026] When the SPV measurement V.sub.SPV is substantially equal to
the target value V.sub.SPVtarget, the controller 36 sums each of
the individual corona charge increments Q.sub.c to obtain a total
corona charge Q.sub.applied@target applied to the insulating layer
16 to obtain the SPV measurement V.sub.SPV equal to the target
value V.sub.SPVtarget. The controller 36 then determines the total
charge Q.sub.tot of the insulating layer 16 from the total applied
corona charge Q.sub.applied@target wherein the total charge
Q.sub.tot is the negative of the total applied corona charge
Q.sub.applied@target, i.e. Q.sub.tot=-Q.sub.applied@target.
[0027] FIG. 3 illustrates an example of this first method wherein
the target value V.sub.SPVtarget is zero volts, or flatband
condition. A first corona charge Q.sub.c of -0.20E.sup.-07
C/cm.sub.2 is applied on the insulating layer and an SPV
measurement V.sub.SPV of about 0.090 volts is obtained. A second
corona charge Q.sub.c of -0.20E.sup.-07 C/cm.sub.2 is then applied
on the insulating layer 16 such that the total corona charge
Q.sub.applied is -0.40E.sup.-07 C/cm.sub.2. The second SPV
measurement V.sub.SPV is about 0.100 volts. A third corona charge
Q.sub.c of +0.40E.sup.-07 C/cm.sub.2 is applied on the insulating
layer 16 such that the total corona charge Q.sub.applied is
0.00E.sup.-07 C/cm.sub.2. The third SPV measurement V.sub.SPV is
about 0.060 volts. Note that the polarity of the third deposited
corona charge Q.sub.c was changed, because the SPV measurements
V.sub.SPV were going away from the target value (zero) and the
magnitude of the third deposited corona charge Q.sub.c was changed,
specifically increased or doubled, to avoid duplicating the first
measurement. A fourth corona charge Q.sub.c of +0.20E.sup.-07
C/cm.sub.2 is applied on the insulating layer 16 such that the
total corona charge Q.sub.applied is +0.20E.sup.-07 C/cm.sub.2. The
fourth SPV measurement V.sub.SPV is about -0.100 volts. A fifth
corona charge Q.sub.c of -0.10E.sup.-07 C/cm.sub.2 is applied on
the insulating layer 16 such that the total corona charge
Q.sub.applied is +0.10E.sup.-07 C/cm.sub.2. The fifth SPV
measurement V.sub.SPV is about 0.000 volts and substantially equal
to the target value V.sub.SPVtarget. Note that the polarity of the
fifth deposited corona charge Q.sub.c was changed because the
fourth SPV measurement V.sub.SPV went past the target value (zero)
V.sub.SPVtarget and the magnitude of the fifth deposited corona
charge Q.sub.c was changed, specifically reduced by half, to avoid
duplicating the third measurement. Therefore, the total applied
corona charge Q.sub.applied@target to obtain the target value
V.sub.SPVtarget is +0.10E.sup.-07 C/cm.sub.2. The controller 36
then determines the total charge Q.sub.tot of the insulating layer
is +0.10E.sup.-07 C/cm.sub.2.
[0028] In a second method of measuring the total charge Q.sub.tot
of the insulating layer 16 according to the present invention, the
position actuator 34 alternately locates the corona gun 26 and the
SPV device 30 over the measuring region 22 of the wafer 12 to
deposit increments of corona charge Q.sub.c on the insulating layer
16 and to obtain SPV measurements V.sub.SPV of the insulating layer
16. The controller 36 records each SPV measurement V.sub.SPV and
determines and records the total corona charge Q.sub.applied
applied to the insulating layer 16 to obtain that SPV measurement
V.sub.SPV. Therefore, a data set is obtained containing the
plurality of SPV measurements V.sub.SPV along with the
corresponding total applied corona charges Q.sub.applied. The
controller 36 then determines the total applied corona charge
Q.sub.applied@target required for the SPV measurement V.sub.SPV to
be substantially equal to the target value V.sub.SPVtarget from the
data set. The value Q.sub.applied@target is preferably interpolated
from the data set of discrete points. The controller 36 then
determines the total charge Q.sub.tot of the insulating layer 16
from the total applied corona charge Q.sub.applied@target wherein
the total charge Q.sub.tot is again the negative of the total
applied corona charge Q.sub.applied@target,
Q.sub.tot=-Q.sub.applied@target. FIG. 4 illustrates an example of
this second method wherein the target value V.sub.SPVtarget is zero
volts, or flatband condition. A data set is obtained by
incrementally depositing a plurality of corona charges Q.sub.c on
the insulating layer and obtaining a SPV measurement V.sub.SPV for
each incremental deposition. The illustrated data set contains 19
discrete points containing the SPV measurements V.sub.SPV and the
corresponding total applied corona charges Q.sub.applied. The
controller 36 interpolates the discrete points to determine that
the total applied corona charge Q.sub.applied@target at the target
value V.sub.SPVtarget is about +0.10E.sup.-07 C/cm.sub.2. The
controller 36 then determines the total charge Q.sub.tot of the
insulating layer is +0.10E.sup.-07 C/cm.sub.2.
[0029] When the target value V.sub.SPVtarget is zero volts, each of
the SPV measurements V.sub.SPV are preferably corrected with a
small Dember Voltage correction in either of the methods. The
Dember Voltage correction is a small "second order" correction
which can be applied via well known equations.
[0030] It should be evident that this disclosure is by way of
example and that various changes may be made by adding, modifying
or eliminating details without departing from the fair scope of the
teaching contained in this disclosure. The invention is therefore
not limited to particular details of this disclosure except to the
extent that the following claims are necessarily so limited.
* * * * *