Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2002-09-24
2004-08-03
Zarneke, David A. (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S750010, C324S762010, C250S492200
Reexamination Certificate
active
06771091
ABSTRACT:
TECHNICAL FIELD
This invention relates to measuring contact potential differences.
BACKGROUND
A contact potential difference (CPD) is a difference between an electrostatic potential at a surface of a sample and a contact potential of a metal electrode of a CPD probe, which is determined by the probe electrode's work function. CPD measurements are non-contact measurements that are particularly useful for characterizing numerous structures extensively used in semiconductor electronics, such as dielectric layers disposed on semiconductor substrates. Examples of these applications are described by J. Lagowski and P. Edelman in “Contact Potential Difference Methods for Full Wafer Characterization of Oxidized Silicon,” Inst. Phys. Conf., Ser. No. 160, p. 133-144 (1997), and by D. K. Schroder in “Contactless Surface Charge Semiconductor Characterization,” Material Science and Engineering, B91-92, p. 196-210 (2002). In cases where the sample is a dielectric film on a semiconductor substrate the contact potential difference of the sample, V
CPDS
, can be expressed as:
V
CPDS
=V
S
−&phgr;
el
,
where &phgr;
el
is the contact potential of the CPD probe electrode and V
S
is the sample surface potential:
V
S
=V
diel
+V
SB
+&phgr;
s
.
Here, V
diel
is the potential drop across the dielectric layer, V
SB
is the semiconductor surface barrier, and &phgr;
s
is the contact potential corresponding to the semiconductor work function at the flatband condition (i.e., when V
SB
=0).
Electrical charge residing in a dielectric layer, on the surface of the dielectric layer, or at the interface between the dielectric layer and the semiconductor substrate can be monitored by measuring a change in a V
CPDS
in response to an electric charge, &Dgr;Q, intentionally placed on the dielectric layer's surface, for example, by a corona discharge in air. This change in V
CPDS
, can be expressed as:
&Dgr;
V
CPDS
=&Dgr;V
diel
+&Dgr;V
SB
,
where &Dgr;V
diel
=&Dgr;Q/C
diel
, C
diel
being the dielectric layer capacitance. &Dgr;V
SB
=&Dgr;Q/(C
SC
+C
it
),
where C
SC
and C
it
are the capacitance of the semiconductor space charge and interface traps, respectively.
Electrical current in the dielectric layer can also be monitored by measuring a rate of change of V
CPDS
, after corona charging of dielectric, dV
CPDS
/dt. In this type of measurement, V
CPDS
is recorded as a function of time. A current, J, is obtained from the rate of change of the voltage across the dielectric layer, V
diel
:
J
=
C
diel
ⅆ
V
diel
ⅆ
t
≈
C
diel
ⅆ
V
CPDS
ⅆ
t
.
Key properties of dielectrics (e.g., electrical conductance, charge trapping) important for semiconductor device functioning are temperature dependent. Therefore, characterization of dielectrics would clearly benefit if CPD could be measured over wide temperature range including elevated temperature as high as 400° C. or even 500° C.
SUMMARY
Typical CPD probes incorporate elements such as a measuring electrode, an operational FET preamplifier, an electromagnetic or piezoelectric vibrator, soldered electric wires, elements connected with glue or epoxy. These elements can be affected, or even destroyed, by elevated temperature. For example, currently manufactured probes would generally fail at temperatures in excess of 400° C.
Measurement systems can be designed to avoid overheating of the probe during measurement of samples at elevated temperature, without any modification of the probe assembly and without a need for probe cooling devices that would stabilize the probe temperature during a measurement of hot samples. To avoid overheating, the probe is cycled between two positions: a room temperature position in the proximity of a reference plate; and a “hot” position in the proximity of sample at elevated temperature. The cycle can be asymmetric in time. For example, for the majority of a typical cycle lasting about 15 seconds, the probe is in a room temperature position. The time the probe spends in this position (e.g., about 10 seconds) is referred to as resting time &Dgr;t
rest
. For a short portion of the cycle (e.g., about 2 seconds), referred to as measuring time, &Dgr;t
measure
, the probe is in the proximity of the sample at elevated temperature. This cycle limits the heating of the probe while it measures the sample and helps to cool the probe back to a room temperature while the probe is in the proximity of a reference plate.
When the sample is at high temperature, such as 400° C. or 500° C., a noticeable heating of the probe can take place even during a short 2-second sample measuring time. This can alter the contact potential, &phgr;
el
, of the probe electrode, and change the probe reading of the sample V
CPDS
=V
S
−&phgr;
el
. The present method makes the sample measurements substantially independent of changes in &phgr;
el
. In other words, the described method provides compensation for any changes in &phgr;
el
that may occur due to probe heating. This is done using two measurements: the contact potential of the sample V
CPDS
=V
S
−&phgr;
el
, which is measured with the probe positions in the proximity of the sample; and, the measurement of a reference plate that is done immediately after returning of the probe to position in the proximity of a reference plate. The second measurement provides V
CPDR
=&phgr;
ref
−&phgr;
el
, where &phgr;
ref
is the contact potential of the reference plate. From these two measurements a difference is obtained &Dgr;V
CPD
=V
CPDS
−V
CPDR
=V
S
−&phgr;
ref
, &phgr;
ref
is constant because the reference plate is kept at a constant reference temperature (e.g., typically room temperature). Thus, &Dgr;V
CPD
provides an accurate measure of the sample contact potential, V
S
, that is not substantially affected by any changes in &phgr;
el
.
While the described methods and systems focus on elevated temperature measurement done with CPD probes, it shall be pointed out that the methods can be applied to measurement with any non-contact probe that may be affected by exposure to elevated temperature. Such probes may include, for example, photovoltaic probes for the surface photovoltage measurement, or optical probes for probing light reflected from the sample or emitted by the sample.
In general, in a first aspect, the invention features a method for elevated sample temperature measurement. The method includes heating a sample to a sample temperature, T, and moving a probe from a first position to a second position, wherein the first position is proximate to a reference plate held at constant temperature, T
0
, and the second position is proximate to the sample, and T is greater than T
0
. The method further includes measuring a contact potential difference of the sample, V
CPDS
, with the probe being held in the second position for a measuring time, &Dgr;t
measure
, sufficiently short to prevent substantial heating of the probe. The method also includes returning the probe to the first position, measuring a contact potential difference of the reference plate, V
CPDR
, and determining a difference &Dgr;V
CPD
=V
CPDS
−V
CPDR
as a measure of a sample contact potential at T.
Implementations of the method can include one or more of the following features.
The probe need not be actively cooled while in the second position. &Dgr;t
measure
can be 2 seconds or less.
The method can further include holding the probe in the first position for a probe resting period, &Dgr;t
rest
, of 5 seconds or more after returning the probe to the first position.
T
0
can be less than 100° C. (e.g., less than 80° C., less than 50° C., less than 30° C., less than 25° C., such as 23° C.). During the measurement of V
CPDS
and V
CPDR
, the temperature of the probe can be kept within 5° C. of T
0
(e.g., within 3° C. of T
0
, within 2° C. of T
0
, within 1° C. of T
0
). The sample temperature T can be between T
0
and 500° C.
The sample can include a dielectric layer. The reference pl
Edelman Piotr
Gossett Frank
Kochey Nick
Lagowski Jacek J.
Savtchouk Alexandre
Fish & Richardson P.C.
Patel Paresh
Semiconductor Diagnostics, Inc.
Zarneke David A.
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