Multi-dimensional high-resolution probe for semiconductor measurements including piezoelectric transducer arrangement for controlling probe position

A probe which is positioned in at least one axis by a piezoelectric transducer is provided. One or more piezoelectric transducers control position of the probe with respect to another probe, with respect to a sample surface, or with respect to a previous position of the probe itself. A method for measuring spreading resistance is provided where the distance between two probes is reproducibly controlled in the range of a few angstroms by measuring tunneling current between the two probes, and electrical contact between the two probes and a sample is reproducibly provided by monitoring current between the probes and the sample.

BACKGROUND OF THE INVENTION 
The present invention relates, in general, to probes, and more 
particularly, to probes positioned using piezoelectric transducers. 
In general, the resistivity of a grown or diffused layer in a 
semiconductor, the resistivity of a metallic film, or the resistivity of 
an electrically conductive film, is an important parameter which 
represents the electrical characteristics of these materials. In 
particular, spreading resistance is often used to measure thickness of 
diffused or epitaxial layers, and for establishing the impurity profile 
for these structures. Spreading resistance is the resistance associated 
with the divergence of the current lines which emanate when a small-tipped 
electrical probe is placed onto a semiconducting or conducting surface. 
Commercially available spreading resistance probe (SRP) apparatus usually 
use two probes. The two probes are carefully machined to provide a 
controlled shape and size. The two probes are mounted in a fixed 
relationship to each other, typically about twenty micrometers apart and 
the probes are in the same plane. Because non-reproducible contact 
resistance between the probes and the sample causes measurement error, the 
probes are usually forced into the sample surface with a gravity load to 
provide a reproducible ohmic contact between the probes and the sample 
surface. 
A known current is applied between the two probes, and the voltage drop is 
measured across these probes to obtain a spreading resistance. A major use 
of SRPs is to determine doping profiles of diffused layers. This is 
accomplished by angle lapping a sample to provide a beveled surface and 
then making spreading resistance measurements along the length of the 
bevelled surface. Previous SRP apparatus use pneumatic drive means to 
position the probes in relation to the sample and to move the probes along 
the length of the bevelled surface. 
One disadvantage of previous SRPs is that spacing between the probes is 
difficult to control, and is quite large by modern device standards. Many 
semiconductor devices have diffused structures which are only a few 
microns or less in size. Thus, an SRP with probe spacing of twenty microns 
could not be used. Prior SRPs do not provide any method to accurately 
monitor the probe tip spacing during calibration and testing. Moreover, 
because the probes are in a fixed position with respect to each other, it 
is very difficult to make spreading resistance measurements in two 
dimensions. Because lateral diffusion in modern small geometry devices is 
as important to device performance as vertical diffusion, it is desirable 
to be able to accurately measure spreading resistance in two dimensions. 
Another disadvantage of previous SRPs is that because the probes are forced 
into the sample surface with a fixed and significant force, the surface of 
the sample is damaged with unpredictable effects on measurement accuracy. 
Because the probes penetrate 0.005-0.01 micrometer into the sample 
surface, only a few measurements can be taken on a junction which is on 
the order of 0.1 micrometer deep. Also, the large force damages 
fine-tipped probes, requiring the use of large diameter probes. Large 
diameter probes, as set out above, require correspondingly large spacing 
between the probes. 
An additional disadvantage of previous SRP apparatus is that the pneumatic 
drives used to position the probes provide limited precision in 
positioning the probes resulting in limited resolution and accuracy of the 
impurity profiles calculated from the spreading resistance measurements. 
Improved depth resolution results from minimal incremental position 
changes when moving the probes along the length of the bevelled surface. 
However, conventional pneumatic drives provide only relatively large 
incremental position changes and correspondingly limited depth resolution. 
Accordingly, it is desirable to have an SRP apparatus that has probes which 
can be multi-dimensionally positioned with high-resolution, that minimizes 
penetration depth into a sample surface, that is compact and does not 
produce vibrations. 
SUMMARY OF THE INVENTION 
Briefly stated, the present invention is achieved by a probe having 
position controlled in at least one axis by a piezoelectric transducer 
(PZT). One or more PZTs control position of the probe with respect to 
another probe, with respect to a sample surface, or with respect to a 
previous position of the probe itself. 
A method for measuring spreading resistance is provided where the distance 
between two probes is reproducibly controlled in the range of a few 
angstroms by measuring tunneling current between the two probes, and 
electrical contact between the two probes and a sample surface is 
reproducibly provided by monitoring tunneling current between the probes 
and the sample surface.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a highly simplified perspective view of a portion of a 
multi-dimensional high-resolution probe in accordance with the present 
invention. FIG. 1 illustrates an example of the mechanical arrangement of 
elements of a two-probe spreading resistance probe (SRP). Electrical 
coupling needed for control circuitry and measurement functions are 
illustrated in FIG. 2 and FIG. 3. While the present invention is described 
in terms of a SRP, it should be understood that other similar measurement 
systems such as four-point probes and the like can take advantage of the 
probe positioning features of the present invention, with only minor 
modifications to the present invention. Accordingly, such modifications 
are intended to be within the scope of the present invention. FIG. 1 shows 
a first probe 17 and a second probe 18 which are similar to probes used in 
conventional SRP apparatus or in scanning tunneling microscopy (STM) 
apparatus or atomic force microscopy apparatus. Probes and 18 may be 
ground flat on one side, as illustrated, to allow them to be positioned 
closer to each other. The actual shape and size of probes 17 and 18 can 
vary widely to adapt to the particular needs of the application. Probes 17 
and 18 comprise a durable conductive material such as tungsten. An example 
of probes which are acceptable for use with the present invention is shown 
in M.Fotino, "NANOMETER-SIZED TUNGSTEN TIPS FOR STM AND AFM", published in 
the Proceedings of the 49th Annual Meeting of the Electron Microscopy 
Society of America, pages 386-7, 1991. 
Probe 17 is attached to a piezoelectric transducer (PZT) 23, hereinafter 
referred to as Z-PZT 23 because it moves the probe in a Z-axis. PZTs are 
designed to expand, contract, or otherwise deform in a predetermined 
manner in response to an applied electric field. It is possible for a 
single PZT to control motion in two or three dimensions, as is shown in 
U.S. Pat. No. 5,017,266 entitled "METHOD OF MAKING AN INTEGRATED SCANNING 
TUNNELING MICROSCOPE" issued to M. Zdeblick and T. Albrecht on May 21, 
1991. Such a multi-dimensional PZT is suitable for use in the present 
invention, although the simpler arrangement illustrated in FIG. 1 is 
believed to be sufficient. 
Probe needle 17 is also mechanically coupled to Y-PZT 26, which moves probe 
17 in the Y-axis and X-PZT 24, which moves probe 17 in an X-axis. Because 
X-PZT 24, Y-ZT 26, and Z-PZT 21 function independently, it is only 
important that they each are mechanically coupled to probe 18, and great 
flexibility exists as to the actual mechanical attachment of the elements 
to each other. Together, PZTs 23, 24 and 26 are capable of moving probe 17 
in three dimensions. Probe 18 is attached to Z-PZT 21, Y-PZT 19, and X-PZT 
22 and can move in three dimensions as described in reference to probe 17. 
In accordance with the present invention, all fine movement of probe 
needles 17 and 18 is provided by PZTs. Commercially available PZTs, widely 
used in scanning-tunneling microscopes, have a sensitivity of about ten to 
twenty angstroms per volt. A 50-100 millivolt signal applied to one of the 
PZTs will move either probe 17 or 18 about one angstrom in a desired 
direction. PZTs are available with a total range of 160 micrometer linear 
scan, thus each probe 17 or 18 can be moved a total distance of 160 
micrometers with position accuracy in the range of an angstrom. Because 
the PZTs operate without vibration, as do pneumatic drives, and the PZTs 
have little hysteresis, position accuracy can be maintained and repeated 
for long periods of time. 
Probe movements greater than 160 micrometers, as are required when setting 
up a probe, can be provided by a mechanical coarse positioning apparatus 
(not shown). The PZTs can be mounted on the coarse positioning apparatus 
which will allow probes 17 and 18 to be moved distances which are outside 
the linear scan range of the PZT. Previous SRP apparatus allow only course 
adjustment of the relative position between two probes. By using PZTs, 
fine position control of probes 17 and 18 is provided. Prior SRP apparatus 
use needles which are in a fixed position with respect to each other 
during operation. In accordance with the present invention, each probe 17 
and 18 can be moved independently and finely adjusted in three dimensions 
during operation. Fine position control of probes 17 and 18 allows the 
probes to be spaced only a few tens of angstroms from each other if 
desired. Independent motion also allows one or both of probes 17 and 18 to 
be repositioned during measurement, to provide impurity profiles in 
several dimensions. 
FIG. 1 illustrates a sample substrate 11 having diffused junctions J1 and 
J2 formed therein by diffused regions 12 and 13. Junctions J1 and J2 
represent metallurgical junctions, not necessarily semiconductor 
junctions, as diffused regions 12 and 13 may be of the same conductivity 
type as substrate 11. Sample substrate 11 has an upper surface 41. A 
beveled surface 42, alternatively referred to as the sample surface, is 
formed as is done in conventional SRP techniques to expose portions of 
junctions J1 and J2. 
Probe 17 is brought into electrical contact with sample surface 42 at 
positions indicated by dashed line 16. Similarly, probe 18 is brought into 
electrical contact with sample surface 42 at positions indicated by dashed 
line 14. Referring now to FIG. 2, an SRP measurement is taken by forcing a 
known current using a current source 32 between probes 17 and 18 while 
they are in electrical contact with sample surface 42, and measuring the 
voltage using a voltmeter 31 between probes 17 and 18. Spreading 
resistance measurements are taken along the length of sample surface 42 
and used to calculate an impurity profile for substrate 11 and diffused 
regions 12 and 13. 
FIG. 2 and FIG. 3 illustrate a method of using a multi-dimensional 
high-resolution probe in accordance with the present invention. The 
elements of FIGS. 2 and 3 that are the same as FIG. 1 have the same 
reference numerals used in FIG. 1. A circuit 30, identified by a dashed 
box in FIG. 2 is used for detecting the position of probe 18 with respect 
to probe 17 and for applying an electric field to Y-PZT 19 to control 
relative position of probes 18 and 19. After probes 17 and 18 are coarsely 
positioned over a sample surface 15 as described hereinbefore, a Y-axis 
control circuit 37 is activated by an external signal provided on control 
input 34 to perform fine positioning. Probe 17 is coupled to ground; and 
probe 18 is coupled to a bias voltage 35. Bias voltage 35 is coupled to 
ground through to Y-axis control circuit 37 so that it can be decoupled to 
allow probe 18 to have a floating potential when not in use. Bias voltage 
35 provides a voltage between probe 17 and probe 18. Y-axis control 
circuit 37 applies an electric field, which can be controllably varied, to 
Y-PZT 19. The electric field causes Y-PZT 19 to move probe 18 toward probe 
17. 
Voltage at output 38 is proportional to the position of probe 18. Output 39 
is used to monitor tunneling current flowing between probe 17 and probe 
18. Tunneling current rises exponentially as probe 18 approaches within a 
few tens of angstroms of probe 17, and so is a very sensitive gauge of the 
separation distance between probes 17 and 18. Probes 17 and 18 are moved 
towards each other until the tunneling current reaches a predetermined 
value, which is correlated to a known separation. Once this reference 
position is found, control circuit 37 can modify the electric field on 
Y-PZT 19 to provide any desired separation between probes 18 and 17 within 
the linear scan range of Y-PZT 19. It is important that Y-PZT 19 is highly 
linear. Once the reference position is determined, Y-axis control circuit 
disconnects bias voltage 35 so it will not interfere with measurements. 
Operation and control of X-PZT 22 and X-PZT 24 is substantially similar to 
that described for Y-PZT 19 in FIG. 2. In other words, control of X-PZT 22 
and X-PZT 24 is achieved by a circuit substantially similar or identical 
to circuit 30 shown in FIG. 2, and so are not separately illustrated. 
X-PZT 22 and X-PZT 24 function to align and move probes 17 and 18 in the 
X-direction, or coming out of the page as shown in FIG. 2. 
After the separation between probes 17 and 18 is performed, the probes are 
moved towards sample surface 15 using a Z-axis control circuit 27 shown in 
FIG. 3. 
A circuit 20, identified by a dashed box in FIG. 3 is used for detecting 
the position of probe 18 with respect to sample surface 15 and for 
applying an electric field to Z-PZT 21 to control vertical position of 
probe 18. A bias voltage is applied to probe 18, and sample surface 15 is 
coupled to ground. Bias voltage 25 is reference to ground potential 
through control circuit 37, allowing bias voltage 25 to be applied between 
probe 18 and sample surface 15, which is also coupled to ground. Z-axis 
control circuit 27 is activated by an external signal provided on control 
input 36 and applies a variable electric field to Z-PZT 21 to move probe 
18 up and down with respect to surface 15. Output 28 is used to monitor 
the electric field on Z-PZT 21 and thus the position of Z-PZT 21. Output 
29 monitors tunneling current between probe 18 and sample surface 15. 
Probe 18 can be moved arbitrarily close to sample surface 15. As probe 18 
touches surface 15, at first a schottky contact is made between probe 18 
and surface 21. As more force is applied by Z-PZT 21, an ohmic contact is 
made between probe 18 and surface 15. Current between probe 18 and surface 
15 increases exponentially as an ohmic contact is made, so that current 
monitored at output 29 is a sensitive indicator of contact impedance. 
Probe 17 is attached to Z-PZT 23 and is moved in relation to surface 15 in 
a manner similar to probe 18. A circuit 20', identified by a dashed box in 
FIG. 3 is used for detecting the position of probe 17 with respect to 
sample surface 15 and for applying an electric field to Z-PZT 23 to 
control relative vertical position of probe 17. All of the component 
elements of circuit 20' are the same as previously described for circuit 
20, and labeled using the same numbers as circuit 20 but bear a prime 
designation to distinguish circuit 20' from circuit 20. 
An important advantage of an apparatus in accordance with the present 
invention is current between probe 18 and surface 15 can be monitored at 
output 29 to determine precisely when a reproducible ohmic contact is 
made. This avoids the previous method of dropping the probe onto the 
sample surface with a large known force to ensure ohmic contact. Using 
Z-PZT 21 to apply force to probe 18, penetration of probe 18 into surface 
15 is minimized, greatly improving depth resolution and measurement 
distortion caused by large penetration depths. Also, smaller diameter 
probes can be used, and useful life of probes 17 and 18 is expected to be 
much greater. 
Once probes 17 and 18 are in electrical contact with surface 15, spreading 
resistance measurements are taken using the measurement techniques 
described in reference to FIG. 1. Once a measurement is performed, probes 
17 and 18 may be lifted from surface 15 to the coarse position, and 
relocated to a new position using X-PZTs 22 and 24 shown in FIG. 1. 
Alternatively, it may be possible to make spreading resistance measurements 
without physical contact between probes 17 and 18 and the sample surface. 
This may be possible by positioning probes 17 and 18 so close to surface 
15 that a current can be established in sample surface 15 by tunneling 
current from probes 17 and 18. Because such a non-contact spreading 
resistance measurement mandates that probes 17 and 18 be positioned within 
a few angstroms of sample surface 15, it is impossible using conventional 
SRP apparatus. Of course, the contact between probes 17 and 18 will be 
non-ohmic if probes 17 and 18 do not make physical contact. It is believed 
that the effects of the non-ohmic contact can be compensated for because 
Z-PZTs 21 and 23 can provide a highly reproducible non-ohmic contact such 
that the effects of the non-ohmic contact can be minimized by modifying 
the computations used to calculate impurity profiles. One advantage of the 
non-contact SRP measurement is that probes 17 and 18 may be moved in the 
X-axis without lifting, but instead by scanning X-PZTs 22 and 24 while 
taking a series of spreading resistance measurements. Also, a non-contact 
measurement eliminates errors caused by damage to the sample surface. 
Another feature of the apparatus in accordance with the present invention 
is that Y-PZTs 19 and 26 can be used to scan probes 17 and 18 in the 
Y-axis while making a series of spreading resistance measurements. This 
technique allows the separation distance between probes 17 and 18 to be 
controllably varied while taking a series of measurements allowing a 
Y-axis impurity profile to be calculated. By moving probes 17 and 18 in 
both the X-axis and the Y-axis, a two dimensional impurity profile of a 
junction can be calculated. Because the spacing between probes 17 and 18 
is controlled to within a few angstroms, device structures with 
sub-micrometer features can be accurately profiled. 
While the invention is described with specific preferred embodiments, it is 
evident that many alternatives and variations will be apparent to those 
skilled in the semiconductor arts. More specifically the invention has 
been described for a spreading resistance probe apparatus, although the 
method is directly applicable to other probe apparatus. 
By now it should be appreciated that there is provided a probe apparatus 
which can be positioned with high accuracy and moved with high-resolution 
in several dimensions. A spreading resistance probe in accordance with the 
present invention provides impurity profiles of sub-micrometer geometry 
device structures having junction depths of only a few hundred angstroms. 
Compact piezoelectric transducers are used to move probes resulting in a 
spreading resistance probe which is vibration free and extremely small. 
Highly accurate and reproducible probe positioning is achieved by 
monitoring tunneling current between the probes and between the probes and 
a sample surface, providing high-resolution spreading resistance profiles.