Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-05-08
2004-01-20
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S762010, C324S455000, C324S071100
Reexamination Certificate
active
06680621
ABSTRACT:
BACKGROUND
The invention relates to semiconductor wafer testing and more particularly to characterizing the thickness and capacitance of a dielectric layer on a semiconductor wafer in the presence of substantial leakage current.
As is known in the art, semiconductor devices often contain dielectric layers (e.g., silicon dioxide) grown/or deposited on a semiconductor substrate (e.g., silicon). Semiconductor wafers including dielectric layers are used in manufacturing microelectronic devices such as metal-oxide-semiconductor (MOS) capacitors, MOS-field effect transistors (MOSFET), and related integrated circuits.
During manufacture of microelectronic devices, the thickness of the dielectric layer must be frequently monitored with high precision. Typically, the dielectric thickness is measured by optical or electrical methods. Optical ellipsometric methods for determining a dielectric thickness are described for example in U.S. Pat. No. 5,343,293 and the references therein. An electrical method for determining the thickness of a dielectric utilizes capacitance measurements of MOS capacitors fabricated, for test purposes, on the dielectric layer. Once measured, the capacitance can be used to calculate an “equivalent oxide thickness” (EOT), i.e. the thickness of a SiO
2
layer that would produce the same measured capacitance. In other electrical methods, the dielectric thickness can be determined without fabricating MOS test capacitors by charging the surface of the dielectric layer with a corona discharge and measuring the resulting surface potential of the charged dielectric layer with a Kelvin or a Monroe probe. These techniques are discussed, for example, in U.S. Pat. No. 6,037,797, to J. Lagowski et.al. In this method a known electric charge &Dgr;Q
C
is placed on the surface of a dielectric layer (for example, on the surface of a SiO
2
layer on a silicon wafer) by a precisely calibrated corona discharge source. The dielectric layer thickness can be determined from the value of &Dgr;Q
C
/&Dgr;V where &Dgr;V is the change of the dielectric surface potential caused by the charge &Dgr;Q
C
.
Due to substantial leakage of current across the dielectric layer, via tunneling, the method described in U.S. Pat. No. 6,037,797 is ineffective for determining the thickness of ultra-thin dielectrics films, i.e. dielectric films having an EOT equal to or less than about 40 Å. When leakage current tunnels through the dielectric layer, the charge &Dgr;Q
C
is reduced by the amount of charge transported across a dielectric layer during the time of charging and measuring &Dgr;V. In thicker dielectric layers, such as SiO
2
, the leakage current is typically below 10
−12
A/cm
2
for a corona charge, &Dgr;Q
C
, of about 2×10
−7
C/cm
2
. During a typical measuring time for these techniques, e.g., 100 seconds, the leakage of current via tunneling through the thicker dielectric layer reduces &Dgr;Q
C
by about 10
−10
C/cm
2
. The value 10
−10
C/cm
2
, however, is practically insignificant relative to &Dgr;Q
C
, i.e., 2×10
−7
C/cm
2
. For ultra-thin dielectric layers, the leakage currents are orders of magnitude larger than the leakage currents for thicker dielectric layers, e.g., often exceeding 10
−9
A/cm
2
. As a result, leakage of current over a measuring time of 100 seconds will reduce the value of &Dgr;Q
C
by about 10
−7
C/cm
2
and will cause significant errors in calculating the thickness of the dielectric layer. Additionally, in high accuracy measurements on thin dielectric layers, the value of &Dgr;V is typically corrected to account for a voltage drop, &Dgr;V
SB
, across the semiconductor surface space charge layer by replacing &Dgr;V with the expression &Dgr;V-&Dgr;V
SB
. Errors in determining the dielectric layer thickness are more severe in this instance because leakage current not only corrupts the value of &Dgr;Q
C
but also the calculation of &Dgr;V
SB
.
SUMMARY
In general, the invention relates to an apparatus and method for producing electrical measurements of capacitance and thickness of ultra-thin dielectric layers on semiconductor substrates (wafers). The apparatus and method produces effective and accurate measurements of the dielectric layer thickness despite substantial leakage of current across the layer and no apriori knowledge of the relationship between the leakage current characteristics, i.e., measured electrical properties such as voltage and current, and thickness of the dielectric layer. Ultra-thin dielectric layers have an equivalent oxide thickness equal to or less than about 40 Å. As used herein, the term “dielectric” includes but is not limited to oxides, e.g., SiO
2
, Ta
2
O
5
, Al
2
O
3
, nitrides, e.g. Si
3
N
4
, and barium strontium titianate (BST The non-contact electrical technique can be used to record multiple, repeatable measurements of ultra-thin dielectric capacitance and thickness at the same location on the wafer under highly reproducible conditions.
In one aspect of the invention, the method of determining the thickness of a dielectric layer on a semiconductor wafer includes depositing an electric charge sufficient to cause substantially a steady state condition in which charge current is substantially equal to the leakage current; measuring the potential of the dielectric surface; and comparing the measured parameters to calibrated parameters to derive the dielectric layer thickness. The dielectric layer can be a patterned dielectric layer including thick and thin regions of dielectric material. The dielectric material of the thin region can include a different material than the dielectric material of the thick region and the thin region can include more than one dielectric material.
In another aspect of the invention, the method of determining the thickness of a dielectric layer deposited on a semiconducting wafer includes depositing an ionic charge onto a surface of the dielectric layer deposited on the semiconducting wafer with an ionic current sufficient to substantially cause a steady state condition; measuring a voltage decay on the dielectric surface as a function of time; and determining the thickness of the dielectric layer based upon the measured voltage decay. The method can further include measuring the voltage decay after terminating the deposition of ionic charge.
In another aspect of the invention, the method of determining the thickness of a dielectric layer deposited on a semiconducting wafer includes depositing an ionic charge onto a surface of the dielectric layer with an ionic current sufficient to cause substantially a steady state condition; ceasing deposition of ionic charge after establishing the steady state condition; measuring a voltage decay on the semiconducting wafer as a function of time after ceasing the ionic charging; analyzing the voltage decay to determine a characteristic of the measured voltage decay, the characteristic of the measured voltage decay being selected from the group consisting of an initial surface potential, V
0
, a surface potential at a time greater than t=0, V
D
, and an initial rate of voltage decay, dV/dt|
t=0
; and determining the thickness of the dielectric layer based upon the characteristic of the measured voltage decay.
Embodiments of the invention can include one or more the following. Measuring the voltage decay includes using a non-contact probe. The dielectric layer has a thickness of about 40 Å or less. The dielectric layer can be uniform in thickness or patterned. The steady state condition results when the ionic current substantially equals a leakage current flowing from the semiconducting wafer and across the dielectric layer. The step of determining the thickness of the dielectric layer includes determining the initial surface potential, V
0
, on the dielectric layer from the measured voltage decay. The initial surface potential, V
0
, is determined by extrapolating the measured voltage decay back to t=0. The step of determining the thickness of the dielectric layer further
D'amico John
Jastrzebski Lubomir L.
Lagowski Jacek
Savtchouk Alexander
Wilson Marshall D.
Cuneo Kamand
Fish & Richardson P.C.
Patel Paresh
Semiconductor Diagnostics, Inc.
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