U.S. patent application number 12/264163 was filed with the patent office on 2009-02-26 for lrl vector calibration to the end of the probe needles for non-standard probe cards for ate rf testers.
Invention is credited to Martin Breinbauer, Steffen Chladek.
Application Number | 20090051380 12/264163 |
Document ID | / |
Family ID | 39050112 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090051380 |
Kind Code |
A1 |
Chladek; Steffen ; et
al. |
February 26, 2009 |
LRL VECTOR CALIBRATION TO THE END OF THE PROBE NEEDLES FOR
NON-STANDARD PROBE CARDS FOR ATE RF TESTERS
Abstract
A method and apparatus for radio frequency vector calibration of
s-parameter measurements to the tips of the wafer probe needles of
an automatic test equipment production tester. The method involves
a modified Line-Reflect-Line (LRL) calibration routine that uses a
Thru-Reflect-Line to LRL shift to eliminate the need for a
precisely characterized reflect standard used during a conventional
LRL calibration. The method further involves de-embedding the
non-ideal effects of the non-zero length thru standard used during
the calibration routine to improve measurement accuracy of the
tester. The apparatus may involve the use of RF relays to allow
multiple wafer probe needles to share RF test ports.
Inventors: |
Chladek; Steffen; (La
Troche, FR) ; Breinbauer; Martin; (Munich,
DE) |
Correspondence
Address: |
NIXON PEABODY, LLP
401 9TH STREET, NW, SUITE 900
WASHINGTON
DC
20004-2128
US
|
Family ID: |
39050112 |
Appl. No.: |
12/264163 |
Filed: |
November 3, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11463174 |
Aug 8, 2006 |
|
|
|
12264163 |
|
|
|
|
Current U.S.
Class: |
324/762.02 |
Current CPC
Class: |
G01R 35/005
20130101 |
Class at
Publication: |
324/754 |
International
Class: |
G01R 35/00 20060101
G01R035/00 |
Claims
1.-7. (canceled)
8. An apparatus for testing an integrated circuit comprising: an
automatic test equipment production tester including a test head
having at least two radio frequency measurement ports and a
plurality of wafer probes, each wafer probe having a plurality of
wafer probe needles; at least one radio frequency relay comprising:
an input port; at least two output ports; and a plurality of relay
states; wherein the input port of the at least one radio frequency
relay is operably connected to one of the at least two measurement
ports and each of the at least two output ports of the relay is
operably connected to one of the plurality of wafer probe
needles.
9. The apparatus of claim 8 further comprising a memory in
communication with the automatic test equipment production tester
to store a set of calibration correction factors for each relay
state.
10. The apparatus of claim 9 wherein a set of calibration
correction factors for each relay state is obtained by performing a
modified Line-Reflect-Line calibration method, wherein a reference
plane is shifted to tips of the wafer probe needles based only on
knowledge of geometric lengths of two line calibration
standards.
11. The apparatus of claim 10 wherein each set of calibration
correction factors is stored in the memory.
12.-14. (canceled)
15. The apparatus of claim 8, wherein the plurality of wafer probes
and the at least one radio frequency relay are implemented on a
load board coupled to the at least two radio frequency measurement
ports.
16. The apparatus of claim 8, wherein each of the radio frequency
measurement ports is configured to be shared by a plurality of
output pins of a device under test (DUT), each output pin being
coupled to a corresponding one of the plurality of states of the
radio frequency relay via a respective wafer probe needle.
17. The apparatus of claim 10, wherein the line calibration
standards are configured to be accommodated without having to
adjust spacing between the wafer probes during calibration.
18. The apparatus of claim 10, wherein the line calibration
standards are implemented on a wafer using microstrip lines.
19. A method for testing an integrated circuit comprising:
providing a relay coupled to a radio frequency measurement port in
an automatic test equipment production tester, the relay having a
plurality of states, each relay state configured to operably
connect the radio frequency measurement port with a corresponding
wafer probe needle included in a wafer probe; obtaining a set of
calibration correction factors corresponding to each relay state;
and storing the respective set of calibration correction factors to
each relay state.
20. The method of claim 19, wherein the operation of obtaining
comprises: performing a modified Line-Reflect-Line (LRL)
calibration method, wherein a reference plane is shifted to the
tips of the wafer probe needles based only on knowledge of
geometric lengths of a non-zero-length through calibration standard
and a second line calibration standard.
21. The method of claim 20, wherein performing the modified LRL
calibration method further comprises: de-embedding effects of the
non-zero-length through calibration standard, resulting in shifting
the reference plane to the tips of the wafer probe needles.
22. The method of claim 19, wherein the method further comprises:
sharing the radio frequency measurement port by a plurality of
output pins of a device under test (DUT), wherein each output pin
is coupled to a corresponding one of the plurality of states of the
radio frequency relay via a respective wafer probe needle.
Description
FIELD OF THE INVENTION
[0001] Aspects of the present invention involve an apparatus and
method for radio frequency vector calibration for S-Parameter
measurements, including an apparatus and method for vector
calibration of an ATE production tester up to the end of the probe
needles using a modified LRL (line-reflect-line) method of
calibration.
BACKGROUND OF THE INVENTION
[0002] Everyday consumer products such as televisions and cellular
telephones often contain integrated circuits that are configured to
perform some type of electrical or processing function. These
integrated circuits are fabricated on semiconductor wafers that may
contain several copies of a particular integrated circuit. The
wafer is processed to separate and produce individual integrated
circuit "die" that then may be packaged into finished integrated
circuits, often referred to as a "chip." Functionality of an
integrated circuit is generally verified by testing it. Such
testing may be performed at the wafer level using a set of probe
needles to contact each device (on-wafer measurements made during
wafer sort) or may be done after each die has been packaged.
[0003] On-wafer testing is becoming increasingly important for
radio frequency (RF) integrated circuit devices, such as monolithic
microwave integrated circuit (MMIC) devices. It is sometimes more
cost effective to test devices at the wafer level to screen out
defective devices rather than perform the testing after the devices
are packaged. For on-wafer testing, the performance of a device can
generally be characterized by measuring certain parameters at the
device terminals (ports) without regard to what is inside the
device. Referring to FIG. 1, a RF device 10 may be modeled as a two
port network having an input port 20 (generally port 1) and an
output port 30 (generally port 2). Such a two port network may be
characterized by any of several parameter sets including
y-parameters (conductance), z-parameters (resistance), h-parameters
(a mixture of conductance and resistance) or s-parameters
(scattering). Each parameter set involves a set of four variables
associated with the two-port model. For each parameter set, two of
the variables represent the excitation of the network and the other
two represent the response of the network to the excitation. Each
of the two-port parameter sets describe the performance of the
network. However, the variables and the parameters describing their
relationships are different for each parameter set. At higher
frequencies such as RF, s-parameters are generally easier to
measure than other kinds of parameters.
[0004] For higher frequencies such as RF, the wavelength is
comparable to the dimensions of the transmission line. For such
frequencies, the representation of a network using a voltage and
current approach like Y, Z, and H parameters becomes dependent on
the point of measurement along the transmission line. This can be
avoided by using S-Parameters to represent the network. A
transmission line can be any pair of wires or conductors used to
transmit the traveling waves from one point to another point,
usually of controlled size and contained in a controlled dielectric
material to create a controlled impedance. Thus, the s-parameters
of a device under test (DUT) can be measured by a measurement
system located at some distance from the DUT provided that the
measurement system is connected to the DUT by coaxial cables, high
quality strip lines or any other suitable low-loss transmission
line.
[0005] FIG. 2 is a diagram of a two-port network 40 showing
incident complex voltage waves 50, 60 (a1, a2) and reflected
complex voltage waves 70, 80 (b1, b2) used in s-parameter
definitions. As shown in FIG. 2, s-parameters are defined by
complex voltage waves 50, 60 (having both a magnitude and phase
component) incident on port 1 and port 2 and complex voltage waves
70, 80 reflected from port 1 and port 2 of the two-port network 40.
That is, the s-parameters 90, 100, 110 and 120 (S11, S22, S21, and
S12 respectively) relate the normalized traveling waves that are
scattered or reflected when a device is inserted into a
transmission line. The traveling waves 50, 60, 70 and 80 are
normalized to the characteristic impedance Z.sub.O of the
transmission line. S-parameters involve measurements with each port
of the DUT stimulated in turn. For a two port DUT the microwave
source 130 power is applied to each port. This is usually
accomplished by using a microwave transfer switch to connect the
source 130 to each port in turn. Due to the non-ideal nature of the
switch, its effect is generally included in the measurement path
during calibration. S-parameter testing generally involves the
measurement of the DUT's four s-parameters 90, 100, 110 and 120 to
verify that they are within design tolerances.
[0006] S-parameters are typically defined with the port not being
stimulated terminated in a perfect load, Z.sub.O. For example,
s.sub.11 90 (the input reflection coefficient) is equal to the
ratio of the reflected wave 70 on port 1 to the input wave 50 on
port 1 (b.sub.1/a.sub.1) with a perfect load 140 on port 2
(Z.sub.L=Z.sub.O). Use of a perfect load 140 makes the incident
wave 60, a.sub.2, on port 2 zero. Thus, the accuracy of s-parameter
measurements generally depends on how good a termination is applied
to the port not being stimulated.
[0007] When a DUT is connected to the test ports of the measurement
system, the measured s-parameters are only accurate when the
measurement system is calibrated to minimize the effects of source
and load impedance mismatch. This "systematic error" often does not
vary over time and can be characterized during the calibration
process and removed during the measurement process through a
mathematical process called error correction. Measurement system
calibration may also reduce other repeatable systematic errors
caused by imperfections in the test equipment, cabling, load boards
and RF probe cards including directivity and crosstalk errors
related to signal leakage.
[0008] FIG. 3 is a block diagram illustrating a typical s-parameter
wafer testing system 150. The measurement system test ports 160,
170 are connected to microstrip lines 180, 190 on a load board
using coaxial cables 200, 210. The microstrip lines 180, 190 on the
load board are used to connect the coaxial cables 200, 210 to wafer
probe needles 220, 230 which in turn are used to connect to the DUT
240. A challenge in s-parameter measurements is to define where the
measurement system 150 ends and the DUT 240 begins (see FIG. 3).
This location is called the "measurement reference plane." As shown
in FIG. 3 there are multiple choices for where the measurement
reference plane may be located in measurement system 150. For
example, the measurement reference plane could be the defined as
the Measurement System Test Port Reference Plane 250 located at the
measurement system test ports 160, 170, the Coaxial Reference Plane
260 located at the ends of the coaxial cables 200, 210 or the
On-Wafer Reference Plane 270 located at the ends of the probe
needle tips 220, 230. However, choice of where the measurement
reference plane is located is dependent on the availability of
known reference standards used to calibrate the measurement system
150 that can be physically connected or inserted, preferably
without the use of adaptors, at the measurement reference plane
during the calibration process. This is because the calibration
process involves measuring certain calibration standards of known
characteristics and using these measurements to establish the
measurement reference plane.
[0009] When known reference standards are available for insertion
at the measurement reference plane, error contributions up to the
measurement reference plane will be calibrated out. But any error
contributions between the measurement reference plane and the DUT
240 become part of the measured DUT response. Ideally, the
measurement reference plane should be the On-Wafer Reference Plane
270 located at the probe needle tips 220, 230 for on-wafer
measurements so that just the DUT response is measured by the test
system 150.
[0010] As discussed above, known reference standards are connected
at the measurement reference plane during the calibration process.
If adaptors are used to insert the reference standards at the
measurement reference plane, the accuracy of the calibration may be
degraded. This is a result of the calibration process using known
calibration standards, i.e., standards that have been previously
characterized, to determine the error correction as discussed
below. Because adaptors are not ideal, use of them introduces
additional errors that are not removed during the calibration
process. For example, referring back to FIG. 3, choosing the
Coaxial Reference Plane 260 as the measurement reference plane
involves a set of standards that can be physically connected
directly to the coaxial cables. Choosing the On-Wafer Reference
Plane 270 as the measurement reference plane involves calibration
standards that can be physically connected to the probe needle tips
220, 230.
[0011] There are two basic types of error correction: response
calibration and vector error correction. Response calibration is a
reduced error correction method, which is only used to de-embed the
scalar transmission parameters |s.sub.12| and |s.sub.21| of the
DUT. This is achieved by inserting a reference trace instead of the
DUT 240. While response calibration is simple to perform, it
removes only a few of the possible errors. Vector error correction
is a more thorough method of error correction, but involves
measuring phase as well as magnitude, and a set of calibration
standards with known, precise electrical characteristics. The
vector correction process characterizes the systematic errors by
measuring known calibration standards. The difference between the
measured and known responses of the standards is used to calculate
an error model which is then used to remove the systematic errors
from subsequent measurements.
[0012] There are several calibration methods available to do vector
error correction when measuring the s-parameters of a two-port
network. These include, but are not limited to Short-Open-Load-Thru
(SOLT), Thru-Reflect-Line (TRL) and Line-Reflect-Line (LRL). For
each of these calibration methods, specific, accurately know
standards are measured during the calibration process. These
calibration methods derive their names from the standards used
during the calibration process.
[0013] For example, a calibration can be done at the coaxial ports
200, 210 of the measurement system 150 to remove the effects of the
measurement system and any cables or adaptors that are a part of
the calibration path. One of the most commonly used calibration
methods for calibrating to the coaxial ports 200, 210 is the SOLT
method because the characterized calibration standards are readily
available. FIG. 4 shows a typical sequence of connection events for
a two-port SOLT calibration. The SOLT calibration is done by making
full S-Parameter measurements of the Open 280, Short 290, Load 300
and Thru 310 connected to port 1 and port 2. These measurements
along with the known characteristics of the calibration standards
allow the error correction for the forward direction, the source
connected to port 1 with port 2 terminated, to be calculated. The
error correction for the reverse direction, the source connected to
port 2 with port 1 terminated, is calculated in a similar
fashion.
[0014] The SOLT calibration method works well when the DUT 240 can
be attached to the measurement system RF ports using the same
connector types for which a precision calibration kit is available.
However, if DUT 240 has non-standard connectors involving the use
of adaptors or if non-standard probe cards are used to probe a
device on a wafer, then it becomes more difficult to remove the
effects of the measurement path from the device characteristics.
This is a result of the measurement reference plane being
established at the Coaxial Reference Plane 260 during the SOLT
calibration procedure as shown in FIG. 3. Thus, any measurement
errors caused by non-standard connectors, adaptors or probe cards
inserted between the measurement reference plane and the DUT 240
are measured as part of the DUT response. That is, the measurement
includes the effects (loss, phase shift, and mismatch) of the test
fixture as well as the DUT response.
[0015] Additionally, the SOLT calibration method is not readily
suited to calibrating s-parameter measurements made by automatic
test equipment (ATE) testers during wafer sort because the
calibration method involves a set of impedance standards that are
not easily fabricated on the wafer. It can be difficult and costly
to fabricate high quality SOLT standards on the wafer. FIG. 5 shows
an example of SOLT calibration structures for calibrating
ground-signal-ground (GSG) probes 320, 330 for on-wafer
measurements. FIG. 5A shows the GSG probes connected to the Open
structure 340 (probes in the air). FIG. 5B shows the GSG probes
connected to the 50 ohm Load structure 350. FIG. 5C shows the GSG
probes connected to the Short structure 360. FIG. 5D shows the GSG
probes connected to the Thru structure 370.
[0016] None of these standards are ideal. For example the short
structure 360 is not an ideal short, but rather behaves as an
inductor at high frequencies. The open structure 340 is not an
ideal open but rather behaves as a capacitor at high frequencies.
In particular it is difficult to obtain a precise 50 ohm load
structure 350 at high frequencies. Thus, such SOLT calibration
standards are characterized prior to use. When a calibration is
done using the characterized calibration standards, deviations from
these known characteristics are treated as measurement system
errors to be calibrated out.
[0017] Sometimes the SOLT calibration method is used during wafer
sort by having the calibration standards fabricated on a separate
wafer. This allows the set of reference standards to be
characterized and the resistive load standard to be trimmed to its
desired value, usually 50 ohms, prior to use of the standards.
Prior to testing the DUT 240, the known standards are probed to
calibrate the measurement system 150. This approach works well when
the calibration standards are collocated with the test wafer
containing the DUTs. However, this method becomes less desirable
when space constraints involve swapping the test wafers and
calibration standards during measurements. The SOLT calibration
method is also impractical when the calibration standards are
fabricated on the test wafer containing the DUTs. Here the
calibration standards on each wafer need characterization to remove
variations in the calibration standards from wafer to wafer. As
discussed above, the precise 50 ohm load standard generally is
trimmed to its desired value before use, which is impractical when
the standards are fabricated on the same wafer as the DUTs.
[0018] Another method for calibrating s-parameter measurements of a
two-port network is TRL. This calibration method uses thru, reflect
and line calibration standards that can be implemented using
transmission lines. The TRL calibration procedure involves making
measurements with a Thru standard 390 connected to the test ports
400, 410, a Line standard 420 of unknown propagation constant but
of known Z.sub.O connected to the test ports, and unknown high
Reflect standards 430, 440 (open or short) connected to each of the
test ports as shown in FIG. 6. The primary constraints when using
this calibration technique are that the system impedance be equal
to the characteristic impedance of the Line standard 420 and the
reflect standards 430, 440 need to be the same on both test ports
400, 410. When a TRL calibration is performed, the reference plane
is established at the middle of the Thru, which for a zero length
Thru, is the DUT reference plane. However, in a wafer probing
situation, the probe needles generally cannot be moved making it
impossible to realize a zero length Thru 390 when the TRL standards
are implemented on a wafer.
[0019] The TRL reference standards are more suitable than SOLT
standards for fabrication on a non-coaxial media such as a
semiconductor wafer because the TRL standards can be implemented
with microstrips. FIG. 7 shows an example of a microstrip 450. The
microstrip can be fabricated on a semiconductor wafer 460 by
depositing a metal layer 470 on the surface of the wafer which is
then etched to define the width of the microstrip. The impedance of
the microstrip is determined by its geometry factor (w/t) and the
relative permittivity constant of the semiconductor wafer
(.epsilon.) as is known to those skilled in the art.
[0020] FIG. 8 provides an example of TRL calibration standards
implemented with microstrips. FIG. 8A shows a Thru structure 480
connected to the probes 490, 500. FIG. 8B shows a Reflect structure
510 connected to the probes 490, 500. FIG. 8C shows a Line
structure 520 connected to the probes 490, 500. As shown in FIG.
8A, one of the standards used during the TRL calibration process is
a Thru standard 480. This standard is generally implemented as a
zero length line. Such a standard does not exist for on-wafer
measurements because wafer probe needles 490, 500 often are rigid
with a fixed spacing between the needles. As such, they cannot be
directly connected to each other using a zero length thru standard.
Rather, as shown in FIG. 8A, the Thru structure 480 can be
implemented using microstrips with a non-zero length.
[0021] LRL is an alternative calibration method related to TRL. The
calibration standards needed for LRL are two different line
lengths, Line1 and Line2 standards 530, 540, and a Reflect standard
545 (usually an open or a short) as shown in FIG. 9. When the LRL
calibration method is used, the reference plane is established by
the reflect standard 545. Because the reflect standard 545 is used
to establish the reference plane, it needs to be precisely
characterized. This again presents problems when the standards are
implemented on the same wafer as the DUT instead of on a fully
characterized separate calibration wafer. Additionally, LRL is not
suitable for calibrating probe cards with fixed spacing probe
needles because LRL involves an undesirable change of probe-probe
spacing during calibration to measure the longer line, usually
Line2 standard 540.
[0022] What is needed is a calibration method that accounts for all
the errors up to the device under test. That is, a method that
establishes the reference plane at the DUT using calibration
standards that are not precisely characterized. What is further
needed is a calibration method that utilizes standards easily
fabricated on a wafer and which can be used to calibrate fixed
spacing probe cards. What is also needed is a method of calibration
that supports the assignment of several calibration correction sets
to a single RF measurement port to allow accurate measurement of
several RF DUT pins that are connected to the RF port of the
measurement system using a RF relay.
SUMMARY
[0023] One aspect of the present invention involves a method for
radio frequency vector calibration of an ATE production tester
including a plurality of wafer probe needles, each wafer probe
needle including a tip. The calibration method comprises obtaining
at least two calibration standards, each having an initially
unspecified complex propagation constant and each having a
different geometric length. The method further involves measuring a
delay value and a loss value of each calibration standard and
determining the complex propagation constant of each calibration
standard. Finally, the method involves establishing a reference
plane at the tips of the wafer probe needles. The method may
further involve performing a Thru-Reflect-Line calibration at the
wafer probe needles using a non-zero length thru calibration
standard having a middle region and a line calibration standard to
set the reference plane to the middle region of the non-zero length
thru. The method may then involve determining an attenuation
constant and phase constant of the non-zero length thru and
shifting the reference plane to the tips of the wafer probe needles
based only on the knowledge of the geometrical lengths of the
calibration standards used.
[0024] Another aspect of the present invention involves an
apparatus for sharing RF measurement ports among multiple wafer
probe needles. The apparatus includes an ATE production tester
including a test head having at least two RF measurement ports and
a plurality of wafer probe needles. The apparatus also includes at
least one RF relay with an input port, at least two output ports
and a plurality of relay states. The input port of the at least one
RF relay is operably connected to one of the at least two
measurement ports and each of the at least two output ports of the
relay is operably connected to one of the plurality of wafer probe
needles.
[0025] Another aspect of the present invention involves an
apparatus for performing calibrated DUT measurements on an ATE
production tester comprising a test head having at least two RF
measurement ports and a load board having a plurality of wafer
probe needles, each probe needle having a tip, operably connected
to the test head and the DUT. The apparatus comprises the tester
and a set of calibration standards. The tester is calibrated to the
tips of the wafer probe needles by way of obtaining at least two
calibration standards, each having an initially unspecified complex
propagation constant and each having a different geometric length.
The calibration further involves measuring a delay value and a loss
value of each calibration standard and determining the complex
propagation constant of each calibration standard. And finally, the
calibration involves establishing a reference plane at the tips of
the wafer probe needles.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a block diagram of a device modeled as a two-port
network.
[0027] FIG. 2 is a block diagram of a two-port network showing the
s-parameters of the two-port network.
[0028] FIG. 3 is a block diagram of a wafer test system.
[0029] FIG. 4 depicts the sequence of connection events for a
two-port SOLT calibration.
[0030] FIG. 5A is a diagram of an SOLT OPEN structure to calibrate
GSM wafer probes.
[0031] FIG. 5B is a diagram of an SOLT 50 Ohm LOAD structure to
calibrate GSM wafer probes.
[0032] FIG. 5C is a diagram of an SOLT SHORT structure to calibrate
GSM wafer probes.
[0033] FIG. 5D is a diagram of an SOLT THRU structure to calibrate
GSM wafer probes.
[0034] FIG. 6 depicts the sequence of connection events for a TRL
calibration.
[0035] FIG. 7 is a diagram of a microstrip line implemented on a
semiconductor wafer.
[0036] FIG. 8A is a diagram of a TRL THRU structure to calibrate
GSM probes.
[0037] FIG. 8B is a diagram of a TRL REFLECT structure to calibrate
GSM probes.
[0038] FIG. 8C is a diagram of a TRL LINE structure to calibrate
GSM probes.
[0039] FIG. 9 depicts the sequence of connection events for a LRL
calibration.
[0040] FIG. 10 is a block diagram of an RF ATE production
tester.
[0041] FIG. 11A depicts the LRL LINE1 structure for calibrating GSM
probes.
[0042] FIG. 11B depicts the LRL REFLECT structure for calibrating
GSM probes.
[0043] FIG. 11C depicts the LRL LINE2 structure for calibrating GSM
probes.
[0044] FIG. 12 is a flowchart illustrating the operations of an RF
vector calibration of an ATE tester, in accordance with one
embodiment of the present invention.
[0045] FIG. 13 depicts where the reference plane is established for
a TRL calibration.
[0046] FIG. 14 depicts where the reference plane is established for
a TRL calibration using a non-zero length LINE1 standard instead of
a THRU standard.
[0047] FIG. 15 is a block diagram of a measurement system to verify
the modified LRL calibration procedure of one embodiment of the
present invention.
[0048] FIG. 16 is the measured S21 magnitude response of the
measurement system of FIG. 15 after SOLT calibration to the RF
ports, after SOLT calibration to the DUT and after LRL calibration
to the DUT.
[0049] FIG. 17 is the measured S21 phase response of the
measurement system of FIG. 15 after SOLT calibration to the RF
ports, after SOLT calibration to the DUT and after LRL calibration
to the DUT.
[0050] FIG. 18 is the measured s.sub.11 magnitude response of the
measurement system of FIG. 15 after SOLT calibration to the RF
ports, after SOLT calibration to the DUT and after LRL calibration
to the DUT.
[0051] FIG. 19 is the measured s.sub.11 phase response of the
measurement system of FIG. 15 after SOLT calibration to the RF
ports, after SOLT calibration to the DUT and after LRL calibration
to the DUT.
[0052] FIG. 20 is a block diagram of an ATE production tester with
RF relays to connect multiple wafer probe needles to each RF test
port used by one embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0053] One aspect of the present invention involves a method for
calibrating automatic test equipment having non-standard probe
needles to optimize measurement of the s-parameters of the device
under test (DUT). In one particular arrangement, the method
involves a vector calibration using calibration standards
fabricated on the semiconductor wafer containing the devices to be
tested. The method further involves a Thru-Reflect-Line (TRL) to
Line-Reflect-Line (LRL) shift to eliminate precise characterization
of a reflect standard as is generally needed by conventional LRL
calibration methods.
[0054] FIG. 10 depicts an automatic test equipment (ATE) production
tester 550 with a test head 560 having radio frequency (RF)
measurement ports 565, 570 in communication with vector network
analyzer (VNA) ports 575, 580. The RF ports 565, 570 are connected
to wafer probe needles 590, 600 via microstrip lines 610, 620
fabricated on a load board 630. The probe needles make contact with
the DUT 640 during testing or with the calibration standards during
calibration. The DUT 640 may be one of several contained on a
silicon wafer. The wafer may also have calibration standards
fabricated on it.
[0055] Unlike FIGS. 5 and 8, FIG. 11 shows one way to fabricate a
set of LRL calibration structures on a silicon wafer that are used
by one embodiment to calibrate ATE production tester 550 to the
probe needles 590, 600 that does not involve a change in probe
needle spacing when the calibration structures are probed. FIG. 11A
shows a Line1 Structure 650. FIG. 11B shows a Reflect standard 660
and FIG. 11C shows a Line2 standard 670 of different length than
Line1 structure 650. In one particular arrangement, the tester 550
is calibrated to the tips of the probe needles, i.e., to the point
of contact with the wafer. Because the probe needles 590, 600 are
of fixed spacing, the Line1 calibration standard 650, as shown in
FIG. 11A, is fabricated with a microstrip of length equal to the
spacing between the probe needles while the Line2 calibration
standard 670, as shown in FIG. 11C, is fabricated with a longer
microstrip that also has the spacing of its endpoints equal to the
spacing between the probe needles. This is accomplished by
fabricating Line2 with bends rather than as a straight line. The
Reflect standard structure 660, as shown in FIG. 11B, is
implemented using shorts.
[0056] As previously noted, the measurement system 550 and the
Line1 and Line2 calibration standards, 650 and 670, respectively,
should have the same characteristic impedance. Most RF measurement
systems generally have a characteristic impedance of 50 ohms. The
following equation gives the characteristic impedance of a
microstrip:
Z.sub.0=60*ln(8h/w+w/(4h))/SQRT(.epsilon..sub.eff)if w/h<1,
otherwise
Z.sub.0=120*.pi./((w/h+1.393+0.677*ln(w/h+1.444))*SQRT(.epsilon..sub.eff-
))
where
.epsilon..sub.eff=((.epsilon..sub.r+1)/2+(.epsilon..sub.r-1)/2)/SQRT(1+(-
12*h)/w)
[0057] h=substrate thickness
[0058] w=width of the microstrip
[0059] .epsilon..sub.r=relative permittivity of the substrate.
[0060] This enables the width of the microstrip to be chosen to
provide a 50 ohm characteristic impedance for a given thickness and
relative permittivity of the wafer substrate on which the standards
are fabricated.
[0061] As indicated above, the Line2 standard 670 is fabricated
with bends. Such bends can affect the electrical length of Line2
and also its characteristic impedance, which defines the reference
impedance of the calibration procedure. As previously indicated,
this characteristic impedance should be 50 ohms. If the
characteristic impedance of Line2 is not about 50 ohms, a simple
calculation after the de-embedding procedure can be done to adjust
the characteristic impedance of the calibration matrices:
S ' = P - 1 ( S - .gamma. ) ( I - .gamma. ) - 1 P ##EQU00001## with
##EQU00001.2## P ii = Re ( A ii ) Re ( B ii ) B ii A ii 2 A ii A ii
+ B ii ##EQU00001.3## and ##EQU00001.4## .gamma. ii = B ii - A ii B
ii + A ii ##EQU00001.5##
[0062] A.sub.ii and B.sub.ii are the reference impedances of the
single ports of S [A.sub.ii] and S' [B.sub.ii]. I is the unity
matrix.
[0063] In typical applications, the effects of bending the line can
be controlled through design such that their impact on measurement
results can generally be ignored.
[0064] The phase shift of a lossless microstrip is a function of
its length, l, and measurement frequency, f, as given by the
following equation:
Phase Shift=(2*.pi.*f/c)*l where c=speed of light on the microstrip
line.
[0065] The frequency dependence of the phase shift of the
microstrip involves a difference in length of the Line1 and Line2
standards, 650 and 670. The target is to provide a phase shift
difference of between 20 degrees and 160 degrees. Such a condition
is imposed to ensure a unique solution to the mathematical
equations that compute the calibration correction factors used to
de-embed the effects of Line1 standard 650, as discussed below.
Having to satisfy this condition limits the frequency range over
which the LRL calibration method can be used. That is, LRL is a
narrowband calibration method. For most applications, this does not
present problems because the device being tested also has a limited
frequency range over which it works. Thus, knowing the frequency
range of interest allows the lengths of the Line1 and Line2
standards, 650 and 670, respectively, to be properly chosen to
calibrate the tester over the range of frequencies of interest.
[0066] FIG. 12 is a flowchart illustrating the operations of a
vector calibration performed in accordance with one embodiment of
the invention to calibrate the ATE production tester to the tips of
the probe needles. The method illustrated in FIG. 12 flowchart may
also be implemented as executable software code. The code may be
adapted to run on a workstation connected to the ATE production
tester, run on a server connected to a network accessible by one or
more processing devices, and on a standalone processing device
(such as a personal computer, workstation, or the like). The code
may also be recorded on a computer readable medium, such as a
floppy disk, CD-ROM, RAM, ROM, and the like.
[0067] Referring again to FIG. 12, when a calibration is performed,
the ATE production tester measures the s-parameters with the Line1
calibration standard 650 (see FIG. 11A) connected to the probe
needles (operation 680). The tester then measures the s-parameters
with the Line2 standard 670 (see FIG. 11C) connected to the probe
needles (operation 690). Next, the tester measures the s-parameters
with the Reflect standard 660 (see FIG. 11B) connected to the probe
needles (operation 700). Once the s-parameters have been measured
using the three calibration standards, calibration correction
factors are computed (operation 710) and the effects of Line1 are
de-embedded (operation 720), both as described in more detail
below.
[0068] When the calibration correction factors are computed, the
correction routine initially assumes that a TRL vector calibration
is performed even though a set of LRL calibration standards are
measured. A TRL calibration is assumed rather than performing a
conventional LRL calibration to avoid the need for characterized
reflect standards used during such a calibration. During a TRL
calibration, a Thru 740 is used to establish the measurement
reference plane 730 at the middle of the Thru 740 as depicted in
FIG. 13. For a zero-length Thru, this results in the measurement
reference plane at the DUT as desired because no phase shift or
magnitude loss is introduced into the measurement path by the
zero-length Thru. However, because a Line1 calibration standard 770
of finite length is used instead of a zero length Thru, the
measurement reference plane 750 is established at the middle of
Line1 as shown in FIG. 14 rather than at the edges of Line1, the
desired reference plane 760, where the DUT is connected.
[0069] During the computation of the calibration correction
factors, there are two possible solutions to the equations. To
determine the correct solution, a trial de-embedding is performed
to determine which solution leads to feasible results. Only this
solution is used in subsequent calculations.
[0070] The non-ideal behavior of Line1 standard 770 becomes part of
the DUT response unless its effects are removed by a process called
de-embedding. De-embedding the non-ideal behavior of Line1 standard
770 results in the measurement reference plane being shifted to the
edges of Line1, the desired DUT reference plane 760. In the
de-embedding calculation for Line1 standard 770, the geometrical
lengths of Line1 and Line 2 need to be known. Other information to
de-embed Line1 is obtained from the calibration measurements
performed using the standards. This means that characterized
standards are not needed for this calibration technique. The
geometrical lengths of Line1 and Line2 can be controlled using good
design techniques and manufacturing processes.
[0071] Referring back to FIG. 11, the measured s-parameters of
Line1 and Line2 standards 650, 670 provide sufficient information
to shift the measurement reference plane to the edges of the Line1
standard 650 which is the desired DUT reference plane. Recalling
that in one embodiment of the invention the calibration standards
are implemented using microstrips, the phase constant .beta. is
given by the following equation:
.beta.=2*.pi.*f/c where
[0072] f=frequency of the measurement and
[0073] c=speed of light on the microstrip line.
[0074] Alternatively, because the phase constant defines the phase
shift per geometrical length in degrees per meter, .beta. can also
be calculated as follows:
.beta.=.DELTA.phase/.DELTA.length,
[0075] where .DELTA. phase is difference in the measured phase
responses of Line1 standard 650 and Line2 standard 670 and A length
is the difference in the geometrical lengths of Line1 and Line2
standards which is known from the design of Line1 and Line2
standards on the wafer.
[0076] The attenuation constant, .alpha., defines the attenuation
of the microstrip per geometrical length in dB per meter and can be
calculated as follows:
.alpha.=.DELTA.magnitude/.DELTA.length,
[0077] where .DELTA. magnitude is the difference in the measured
magnitude responses of Line1 standard 650 and Line2 standard 670
and A length is the difference in the geometrical lengths of Line1
and Line2 standards.
[0078] The above calculations provide the information needed to
determine the complex propagation constant, k, of the microstrip as
follows:
k=.alpha.+j*.beta..
[0079] The geometric design and material parameters of the
microstrip Line1 and Line2 standards 650, 670 determine the
propagation constant which is assumed to be the same for both the
shorter and the longer line standards, Line1 and Line2,
respectively.
[0080] Given that the geometrical length of Line1 standard 650 is
known from design and the propagation constant k has been
determined by measurements, the electrical length in degrees and
the loss in dB of Line1 standard can be calculated as follows:
Phase=.beta.*geometrical length of Line1 standard
Loss=.alpha.*geometrical length of Line1 standard
[0081] Half of this phase and loss are used to shift the reference
plane from the middle of Line1 standard 650 to its edges. This
de-embeds the effects of Line1 standard on DUT measurements by
locating the reference plane at the probe needle tips where the DUT
is attached during measurements.
[0082] FIG. 15 shows a measurement setup 780 used to validate the
modified LRL method of one aspect of the present invention.
Standard 50 ohm coaxial cables 785, 790, 795, 800, for which a SOLT
calibration kit is available, are used to enable SOLT calibrations
at the RF Port calibration plane 810 as well as at the DUT
calibration plane 820. A LRL calibration kit is also available to
enable the modified LRL calibration to be performed at the DUT
calibration plane. Mismatches in the measurement path have been
introduced by the insertion of 150 ohm resistors 830, 840 to
ground. These mismatches are representative of the systematic
errors that are to be removed during the vector calibration
process. A vector network analyzer (VNA) is connected to the DUT
850, a 6 dB attenuator, via RF Port1 860 and RF Port2 870.
[0083] Shown in FIGS. 16 and 17 are the magnitude and phase,
respectively, of the S21 of the DUT over a frequency range of 600
MHz to 1.8 GHz. Graphs 880 and 890 depict the magnitude and phase
of S2, after a standard SOLT calibration has been performed at the
RF ports. Graphs 900 and 910 depict the magnitude and phase of S21
after a SOLT calibration has been performed at the DUT. Graphs 920
and 930 depict the magnitude and phase of S21 after the modified
LRL calibration routine of one embodiment of the invention has been
performed at the DUT. As can be seen by comparing Graphs 920 and
930 with 880 and 890, the measured magnitude and phase of S2, after
the modified LRL calibration routine has been performed at the DUT
compares very favorably to the measured magnitude and phase of S2,
after a SOLT calibration at the DUT has been performed.
[0084] Shown in FIGS. 18 and 19 are the magnitude and phase,
respectively, of the s.sub.11 of the DUT over a frequency range of
600 MHz to 1.8 GHz. Graphs 940 and 950 depict the magnitude and
phase of s.sub.11 after a standard SOLT calibration has been
performed at the RF ports. Graphs 960 and 970 depict the magnitude
and phase of s.sub.11 after a SOLT calibration has been performed
at the DUT. Graphs 980 and 990 depict the magnitude and phase of
s.sub.11 after the modified LRL calibration routine of one
embodiment of the invention has been performed at the DUT. As can
be seen by comparing Graphs 980 and 990 with 940 and 950, the
measured magnitude and phase of s.sub.11 after the modified LRL
calibration routine has been performed at the DUT compares very
favorably to the measured magnitude and phase of s.sub.11 after a
SOLT calibration at the DUT has been performed.
[0085] Another aspect of the present invention involves a
calibration method that allows calibration to the end of the probe
needles when multiple probe needles share a RF port. FIG. 20 shows
an ATE test head 1000 with RF relays 1010, 1020 on the load board
1030. Each RF relay connects an RF port to any one of multiple
probe needles, each of which are connected to the DUT 1040.
Calibrated measurements of s-parameters using any probe needle
connected to RF port1 1050 and any probe needle connected to RF
port2 1060 can be made.
[0086] During a calibration routine, a set of calibration factors
are stored for each RF Port, one set for each position of the RF
relay that connects a probe needle to the RF port. This enables the
non-ideal behavior of the RF relay to be removed from the
measurement path. When a s-parameter measurement is performed, the
appropriate sets of correction factors are used to deembed the
measurement to the ends of the two probe needles and define the
measurement reference plane at the DUT pins being measured.
[0087] While the disclosed embodiments are described in specific
terms, other embodiments encompassing principles of the invention
are also possible. Further, operations may be set forth in a
particular order. The order, however, is but one example of the way
that operations may be provided. Operations may be rearranged,
modified, or eliminated in any particular implementation while
still conforming to aspects of the invention. Embodiments within
the scope of the present invention also include computer readable
media for carrying or having computer-executable instructions or
data structures stored thereon. Such computer-readable media may be
any available media that can be accessed by a general purpose or
special purpose computer. By way of example, and not limitation,
such computer-readable media can comprise RAM, ROM, EEPROM, DVD, CD
ROM or other optical disk storage, magnetic disk storage or other
magnetic storage devices, or any other medium which can be used to
carry or store desired program code means in the form of
computer-executable instructions or data structures and which can
be accessed by a general purpose or special purpose computer. When
information is transferred or provided over a network or another
communications link or connection (either hardwired, wireless, or a
combination of hardwired or wireless) to a computer, the computer
properly views the connection as a computer-readable medium. Thus,
any such a connection is properly termed a computer-readable
medium. Combinations of the above should also be included within
the scope of computer-readable media. Computer-executable
instructions comprise, for example, instructions and data which
cause a general purpose computer, special purpose computer, or
special purpose processing device to perform a certain function or
group of functions.
[0088] Those skilled in the art will appreciate that aspects of the
invention may be practiced in network computing environments with
many types of computer system configurations, including personal
computers, hand-held devices, multi-processor systems,
microprocessor based or programmable consumer electronics, network
PCs, minicomputers, mainframe computers, and the like. Further,
wirelessly connected cell phones, a type of hand-held device, are
considered as within a network computing environment. For example,
cell phones include a processor, memory, display, and some form of
wireless connection, whether digital or analog, and some form of
input medium, such as a keyboards, touch screens, etc. Examples of
wireless connection technologies applicable in various mobile
embodiments include, but are not limited to, radio frequency, AM,
FM, cellular, television, satellite, microwave, WiFi, blue-tooth,
infrared, and the like. Hand-held computing platforms do not
necessarily require a wireless connection. Aspects of the invention
may also be practiced in distributed computing environments where
tasks are performed by local and remote processing devices that are
linked (either by hardwired links, wireless links, or by a
combination of hardwired or wireless links) through a
communications network. In a distributed computing environment,
program modules may be located in both local and remote memory
storage devices.
* * * * *