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Scanning tunneling microscope with large scan range
Ultramicroscopy 42-44 (1992) 1602-1605 North-Holland
Scanning tunneling microscope with large scan range V.K. A d a m c h u k , A . V . E r m a k o v a n d S.I. F e d o s e e n k o Institute of Physics, St. Petersburg State University, St. Petersburg 198904, Russia Received 12 August 1991
A new design is described for a scanning tunneling microscope (STM) intended for the detailed characterization of surfaces of industrial or technological interest. The instrument used was designed specifically for modern technology and precision engineering: large scan range (up to 200 /~m lateral, 12 /zm vertical) at low operating voltage (below 300 V), operation in ambient air, automated and computerized control.
1. Introduction
2. Description of the microscope
While STM [1,2] is still considered mainly a scientific technique, there is an increasing interest in technological applications of STM. STM can be successfully used in quality control of the microtopography of conducting technical surfaces, such as optical elements, microelectronic elements, and details of precision engineering [3]. For many applications, related to surface metrology and microfabrication, a long-scan STM is desirable, having scan ranges of several tens of micrometers [4]. STM is distinguished by the following advantages: measurements are carried out in the air; a sample is not limited to dimensions up to 200 mm, there is no need to break its integrity; a quick replacement of a sample is possible (within several minutes); the device is simple in its operation; three-dimensional image details. In our contribution, the STM system will be briefly characterized, and the results of measurements on highly oriented pyrolitic graphite surface and microelectronic grating will be reported to evaluate the microscope's performance. Work has been done to achieve STM with large-scan imaging.
The system design has been adapted to specific technology needs over large scan ranges with automated and computer control; operation in ambient air with atomic resolution has not been attempted, because the resolution of ~ 1 nm is sufficient to solve many practical profilometry problems [5]. Our device allows one to carry out microtopography measurements of samples of size up to 200 m m in each dimension, operates in the air, and time required for sample and tip replacement is less than a minute. There are two interchangeable piezodrives: a piezodrive of high resolution has maximum scan ranges ~ 10 x 10 ~ m laterally (X, Y), and 1 /zm vertically ( Z ) with lateral resolution 1 nm and vertical resolution 0.1 nm; a piezodrive with large scan range has maximum scan ranges up to 200 tzm laterally (X, Y), and 12 /zm vertically ( Z ) with lateral resolution 10 nm and vertical resolution 1 nm. A scheme of the microscope is shown in fig. 1. The probe tip (1) (made of tungsten wire @ 0.1 m m by electrochemical etching) is mounted in the tip holder epoxy adjusted in the center of piezoelectric scanner (2). The tip holder on the scan-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1603
V.K. Adamchuk et al. / STM with large scan range
Fig. 1. Schematic diagram of the scanning tunneling microscope: (1) tip holder and probe tip, (2) piezoelectric scanner, (3) piston, (4) wedge, (5) micrometer screw, (6) worm-like reduction gear, (7) sample, (8) sample table.
ner has been designed specifically for easy and rapid change of the tip. The scanner, in turn, is attached to the piston (3) of the mechanical feed facility. The piston rises and falls with the aid of a wedge (4), which is moved by micrometer screw (5). The latter is rotated by the handle or by the motor directly for coarse tip-to-sample positioning or through worm-like reduction gear (6) for fine tip-to-sample positioning. The piston movement values are correspondingly 250 and 6.25/xm per turn-over. The sample (7) is situated horizontally on the table surface (8). By the mechanical feed facility the tip is approximated to the sample surface through the hole in the table. The tunneling voltage, Vt, is applied to the table surface and, consequently, to the sample. The Z position feedback loop is generally operated at 0.1-10 nA tunnel current. Our circuit has a low-noise preamplifier which acts as a current-to-voltage converter with voltage-to-current ratio l0 s V / A . This is followed by the first input of the differential integrating amplifier; the reference voltage, Vref, is applied to the second input of the amplifier. The time constant of the integrating amplifier is chosen such that at a tunneling current deviation of 1 nA from the set one, the tip moves along Z at a speed ~ 2 / ~ m / s . We find that the feedback control response is sufficient so that we can mechanically move the tip toward the sample at a speed ~ 10/.Lm/s to establish the tunneling con-
Fig. 2. STM image of a highly oriented pirolytic graphite surface with a high-resolution piezodrive.
dition without having an accidental contact. Acquired analog data can be conveniently performed on a storage oscilloscope or on an X - Y recorder, with two ramp generators controlling the motion of the X and Y piezodrives which move the tip. Digital data are acquired using an IBM personal computer (IBM P C / A T ) . In order to evaluate the resolution of our STM unit, the image of an exploited surface of highly oriented pirolytic graphite ( H O P G ) is observed in air ambient pressure. As shown in fig. 2, a step having an estimated height of 7 ,~ can be seen in good agreement with the bulk lattice parameter of graphite (2c = 6.7 ,~). The lateral and vertical resolutions are estimated to be of the order 1 and 0.1 nm, respectively. It is to be noted that there has been no attempt to achieve ~Jm lO
0 200
2Uo
o
Fig. 3. Topographic image of line structures etched in silicon wafer which can be obtained with the large-scan-range piezodrive (with nonlinearity correction, but without skew correction).
V.K. Adamchuk et al. / STM with large scan range
1604
atomic-scale resolution for this device, because it would lead to the device's considerable complication. Fig. 3 shows an example of the STM image which can be obtained in the air with the large scan range piezodrive unit. The topographic image is of the line structures etched in silicon wafer with 2 0 / z m periodicity. Maximum scanning field covers a unit cell of about 200 × 2 0 0 / z m 2. Such a large scanning area is achieved due to the application of the originally designed piezoscanner for the tip movement. In our instrument the tip is scanned using a sectionalized PCR-type piezoceramic bar with a cross-shaped section [6], 6.5 m m in width and 90 m m in length. There are independent metallized zones on the ribs. When applying a voltage between two electrodes of one of the ribs, it shrinks (perpendicular to the electric field), leading to bar bending. It is possible to shrink the bar along its axis by applying identical voltages to all ribs. Bar shrinkage along its axis leads to tip motion along the Z coordinate, and bending along the X and Y coordinates. The operation principle of the piezoscanner on the base of the cross-shaped section bar is analogous to that of the piezoscanner on the base of the piezoelectric tube. In our instrument the bar with length L is divided into two parts as schematically shown in fig. 4. Part 1 with length L 1 works only for the bending and serves for the tip movement along the sample surface. Part 2 with length L 2 works only for the compression arid moves the tip along the normal to the sample
t/l AX/2
~ X pm 4-00
200-
O
a
-
/ o'.3
i Lx,,L b
Fig. 4. (a) Schematic view of cross-shaped section piezoelement operation. (b) Lateral achievable displacement versus ratio L 1/L for applied voltage V= 300 V and bar length L = 90 mm.
surface. Such a division permits one to make the movement along X and Y completely independent of the movement along Z. Under the voltage applies to one of the ribs the bar bends by an arc of radius R. The dependence of the A X movement of the part 1 length is given by:
[ L,L AX=2[
~-
L~ 2R)'
(1)
where R is a function of the thickness of the piezoelectric rib, relevant to the piezoelectric coefficient and applied voltages. The dependence of lateral achievable displacement, AX, versus ratio L1/L for the used piezoelement, applied voltages V = 300 V and bar length L = 90 m m is shown in fig. 4b. It is seen that the use of the whole bar for the movement along X and Y axes results in a scanning area about 400 × 400 /zm 2, but that there is no ceramics for the movement along Z. If one uses 30% of the bar length for the lateral movement, then the scanning area will be more than 200 × 200 /zm 2, and the remaining 70% of the bar length allows one to provide a movement along Z of about 12 /zm. Such a ratio between the movements along X, Y and Z has resulted in the optimal situation for the microtopography investigation of the majority of microelectronic structures and optical surfaces.
3. Computer automation
The STM unit is automated as much as possible with a control unit and an IBM P C / A T personal computer. The PC serves as i n p u t / output device to the control unit and for data storage and display. The STM analog-to-digital interface hardware is used to control the microscope directly. The interface between the STM and P C / A T is inserted inside the computer system block and consists of two digital-to-analog ( D / A ) 12-bit converters to drive the microscope piezoelectric translators and a 12-bit analog-todigital converter ( A / D ) to digitize the STM feedback voltage. The ( A / D ) - ( D / A ) converters are connected with the data bus through 12-bit registers. The address decoder recognizes the appeals
V.K. Adamchuk et aL / STM with large scan range
to the interface blocks and works out controlling signals. The voltages to drive the X - Y piezoelectric elements are constructed by a program method and amplified by high-voltage amplifiers. This program enables one to scan with the selected speed of the tip movement along the surface independently of the scanning area, as well as automatically changing the scanning speed, maintaining an accuracy of the tip-to-sample separation of <0.1 nm by controlling the speed change of the Z coordinate. The computer program that performs the control, data acquisition, display and processing has been written in C language. It is arranged in a modular way to allow one to select the necessary steps and to perform some processing for immediate data analysis. The functions available are the following: filtration, plane subtraction, device calibration, piezoscanner's nonlinearity correction, two- and three-dimensional imaging, section net construction, and creation of the effect of surface illumination by a lamp of different size located at the selected point above the surface. The gray-scale or line-scan constructed in the screen can be dumped to a printer attached to the P C / A T or to the hard disk in standard PCX format. The images can be used by such programs as P A I N T B R U S H or V E N T U R A PUBLISHER.
4. Conclusion The scanning tunneling microscope with two exchangeable piezoscanners has been designed and built specifically with industrial application in mind: large scanning ranges up to ~ 200 x 200 x
1605
12 /zm 3 at low operating voltage (below 300 V), operation in ambient air, computerized and automated control. The software for the IBM P C / A T computer has been developed with capabilities of real-time imaging, data acquisition, and image processing. To evaluate the instrument's performance, we have applied STM to the three-dimensional topographic measurements of highly oriented pirolytic graphite surface and of line structures etched in silicon wafer. The estimated lateral and vertical resolutions are 1 and 0.1 nm for the high-resolution piezodrive and 10 and 1 nm for the large-scan-range piezodrive, respectively. The STM designed can be useful in quality control and detailed characterization of the microtopography of technical surfaces.
Acknowledgment One of the authors (V.K.A.) would like to thank Dr. J. Behm for stimulating discussions and technical advice.
References [1] G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 49 (1982) 57. [2] G. Binnig and H. Rohrer, Helv. Phys. Acta 55 (1982) 726. [3] M. Gehrtz, H. Strecker and H. Grimm, J. Vac. Sci. Technol. A 6 (1988) 432. [4] R. Garcia Cantu and M.A. Huerta Garnica, J. Vac. Sci. Technol. A 8 (1990) 354. [5] V.K. Adamchuk, V.M. Alexandrov, A.V. Ermakov and I.V. Lubinetsky, Lett. J. Exp. Theor. Phys. (USSR) 14 N3 (1988) 256. [6] V.K. Adamchuk, A.V. Ermakov, I.V. Lubinetsky, G.A. Jitomirsky and A.E. Panich, Rev. Sci. Instr. (USSR) N5 (1989) 182.
Scanning tunneling microscope with large scan range V.K. A d a m c h u k , A . V . E r m a k o v a n d S.I. F e d o s e e n k o Institute of Physics, St. Petersburg State University, St. Petersburg 198904, Russia Received 12 August 1991
A new design is described for a scanning tunneling microscope (STM) intended for the detailed characterization of surfaces of industrial or technological interest. The instrument used was designed specifically for modern technology and precision engineering: large scan range (up to 200 /~m lateral, 12 /zm vertical) at low operating voltage (below 300 V), operation in ambient air, automated and computerized control.
1. Introduction
2. Description of the microscope
While STM [1,2] is still considered mainly a scientific technique, there is an increasing interest in technological applications of STM. STM can be successfully used in quality control of the microtopography of conducting technical surfaces, such as optical elements, microelectronic elements, and details of precision engineering [3]. For many applications, related to surface metrology and microfabrication, a long-scan STM is desirable, having scan ranges of several tens of micrometers [4]. STM is distinguished by the following advantages: measurements are carried out in the air; a sample is not limited to dimensions up to 200 mm, there is no need to break its integrity; a quick replacement of a sample is possible (within several minutes); the device is simple in its operation; three-dimensional image details. In our contribution, the STM system will be briefly characterized, and the results of measurements on highly oriented pyrolitic graphite surface and microelectronic grating will be reported to evaluate the microscope's performance. Work has been done to achieve STM with large-scan imaging.
The system design has been adapted to specific technology needs over large scan ranges with automated and computer control; operation in ambient air with atomic resolution has not been attempted, because the resolution of ~ 1 nm is sufficient to solve many practical profilometry problems [5]. Our device allows one to carry out microtopography measurements of samples of size up to 200 m m in each dimension, operates in the air, and time required for sample and tip replacement is less than a minute. There are two interchangeable piezodrives: a piezodrive of high resolution has maximum scan ranges ~ 10 x 10 ~ m laterally (X, Y), and 1 /zm vertically ( Z ) with lateral resolution 1 nm and vertical resolution 0.1 nm; a piezodrive with large scan range has maximum scan ranges up to 200 tzm laterally (X, Y), and 12 /zm vertically ( Z ) with lateral resolution 10 nm and vertical resolution 1 nm. A scheme of the microscope is shown in fig. 1. The probe tip (1) (made of tungsten wire @ 0.1 m m by electrochemical etching) is mounted in the tip holder epoxy adjusted in the center of piezoelectric scanner (2). The tip holder on the scan-
0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
1603
V.K. Adamchuk et al. / STM with large scan range
Fig. 1. Schematic diagram of the scanning tunneling microscope: (1) tip holder and probe tip, (2) piezoelectric scanner, (3) piston, (4) wedge, (5) micrometer screw, (6) worm-like reduction gear, (7) sample, (8) sample table.
ner has been designed specifically for easy and rapid change of the tip. The scanner, in turn, is attached to the piston (3) of the mechanical feed facility. The piston rises and falls with the aid of a wedge (4), which is moved by micrometer screw (5). The latter is rotated by the handle or by the motor directly for coarse tip-to-sample positioning or through worm-like reduction gear (6) for fine tip-to-sample positioning. The piston movement values are correspondingly 250 and 6.25/xm per turn-over. The sample (7) is situated horizontally on the table surface (8). By the mechanical feed facility the tip is approximated to the sample surface through the hole in the table. The tunneling voltage, Vt, is applied to the table surface and, consequently, to the sample. The Z position feedback loop is generally operated at 0.1-10 nA tunnel current. Our circuit has a low-noise preamplifier which acts as a current-to-voltage converter with voltage-to-current ratio l0 s V / A . This is followed by the first input of the differential integrating amplifier; the reference voltage, Vref, is applied to the second input of the amplifier. The time constant of the integrating amplifier is chosen such that at a tunneling current deviation of 1 nA from the set one, the tip moves along Z at a speed ~ 2 / ~ m / s . We find that the feedback control response is sufficient so that we can mechanically move the tip toward the sample at a speed ~ 10/.Lm/s to establish the tunneling con-
Fig. 2. STM image of a highly oriented pirolytic graphite surface with a high-resolution piezodrive.
dition without having an accidental contact. Acquired analog data can be conveniently performed on a storage oscilloscope or on an X - Y recorder, with two ramp generators controlling the motion of the X and Y piezodrives which move the tip. Digital data are acquired using an IBM personal computer (IBM P C / A T ) . In order to evaluate the resolution of our STM unit, the image of an exploited surface of highly oriented pirolytic graphite ( H O P G ) is observed in air ambient pressure. As shown in fig. 2, a step having an estimated height of 7 ,~ can be seen in good agreement with the bulk lattice parameter of graphite (2c = 6.7 ,~). The lateral and vertical resolutions are estimated to be of the order 1 and 0.1 nm, respectively. It is to be noted that there has been no attempt to achieve ~Jm lO
0 200
2Uo
o
Fig. 3. Topographic image of line structures etched in silicon wafer which can be obtained with the large-scan-range piezodrive (with nonlinearity correction, but without skew correction).
V.K. Adamchuk et al. / STM with large scan range
1604
atomic-scale resolution for this device, because it would lead to the device's considerable complication. Fig. 3 shows an example of the STM image which can be obtained in the air with the large scan range piezodrive unit. The topographic image is of the line structures etched in silicon wafer with 2 0 / z m periodicity. Maximum scanning field covers a unit cell of about 200 × 2 0 0 / z m 2. Such a large scanning area is achieved due to the application of the originally designed piezoscanner for the tip movement. In our instrument the tip is scanned using a sectionalized PCR-type piezoceramic bar with a cross-shaped section [6], 6.5 m m in width and 90 m m in length. There are independent metallized zones on the ribs. When applying a voltage between two electrodes of one of the ribs, it shrinks (perpendicular to the electric field), leading to bar bending. It is possible to shrink the bar along its axis by applying identical voltages to all ribs. Bar shrinkage along its axis leads to tip motion along the Z coordinate, and bending along the X and Y coordinates. The operation principle of the piezoscanner on the base of the cross-shaped section bar is analogous to that of the piezoscanner on the base of the piezoelectric tube. In our instrument the bar with length L is divided into two parts as schematically shown in fig. 4. Part 1 with length L 1 works only for the bending and serves for the tip movement along the sample surface. Part 2 with length L 2 works only for the compression arid moves the tip along the normal to the sample
t/l AX/2
~ X pm 4-00
200-
O
a
-
/ o'.3
i Lx,,L b
Fig. 4. (a) Schematic view of cross-shaped section piezoelement operation. (b) Lateral achievable displacement versus ratio L 1/L for applied voltage V= 300 V and bar length L = 90 mm.
surface. Such a division permits one to make the movement along X and Y completely independent of the movement along Z. Under the voltage applies to one of the ribs the bar bends by an arc of radius R. The dependence of the A X movement of the part 1 length is given by:
[ L,L AX=2[
~-
L~ 2R)'
(1)
where R is a function of the thickness of the piezoelectric rib, relevant to the piezoelectric coefficient and applied voltages. The dependence of lateral achievable displacement, AX, versus ratio L1/L for the used piezoelement, applied voltages V = 300 V and bar length L = 90 m m is shown in fig. 4b. It is seen that the use of the whole bar for the movement along X and Y axes results in a scanning area about 400 × 400 /zm 2, but that there is no ceramics for the movement along Z. If one uses 30% of the bar length for the lateral movement, then the scanning area will be more than 200 × 200 /zm 2, and the remaining 70% of the bar length allows one to provide a movement along Z of about 12 /zm. Such a ratio between the movements along X, Y and Z has resulted in the optimal situation for the microtopography investigation of the majority of microelectronic structures and optical surfaces.
3. Computer automation
The STM unit is automated as much as possible with a control unit and an IBM P C / A T personal computer. The PC serves as i n p u t / output device to the control unit and for data storage and display. The STM analog-to-digital interface hardware is used to control the microscope directly. The interface between the STM and P C / A T is inserted inside the computer system block and consists of two digital-to-analog ( D / A ) 12-bit converters to drive the microscope piezoelectric translators and a 12-bit analog-todigital converter ( A / D ) to digitize the STM feedback voltage. The ( A / D ) - ( D / A ) converters are connected with the data bus through 12-bit registers. The address decoder recognizes the appeals
V.K. Adamchuk et aL / STM with large scan range
to the interface blocks and works out controlling signals. The voltages to drive the X - Y piezoelectric elements are constructed by a program method and amplified by high-voltage amplifiers. This program enables one to scan with the selected speed of the tip movement along the surface independently of the scanning area, as well as automatically changing the scanning speed, maintaining an accuracy of the tip-to-sample separation of <0.1 nm by controlling the speed change of the Z coordinate. The computer program that performs the control, data acquisition, display and processing has been written in C language. It is arranged in a modular way to allow one to select the necessary steps and to perform some processing for immediate data analysis. The functions available are the following: filtration, plane subtraction, device calibration, piezoscanner's nonlinearity correction, two- and three-dimensional imaging, section net construction, and creation of the effect of surface illumination by a lamp of different size located at the selected point above the surface. The gray-scale or line-scan constructed in the screen can be dumped to a printer attached to the P C / A T or to the hard disk in standard PCX format. The images can be used by such programs as P A I N T B R U S H or V E N T U R A PUBLISHER.
4. Conclusion The scanning tunneling microscope with two exchangeable piezoscanners has been designed and built specifically with industrial application in mind: large scanning ranges up to ~ 200 x 200 x
1605
12 /zm 3 at low operating voltage (below 300 V), operation in ambient air, computerized and automated control. The software for the IBM P C / A T computer has been developed with capabilities of real-time imaging, data acquisition, and image processing. To evaluate the instrument's performance, we have applied STM to the three-dimensional topographic measurements of highly oriented pirolytic graphite surface and of line structures etched in silicon wafer. The estimated lateral and vertical resolutions are 1 and 0.1 nm for the high-resolution piezodrive and 10 and 1 nm for the large-scan-range piezodrive, respectively. The STM designed can be useful in quality control and detailed characterization of the microtopography of technical surfaces.
Acknowledgment One of the authors (V.K.A.) would like to thank Dr. J. Behm for stimulating discussions and technical advice.
References [1] G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 49 (1982) 57. [2] G. Binnig and H. Rohrer, Helv. Phys. Acta 55 (1982) 726. [3] M. Gehrtz, H. Strecker and H. Grimm, J. Vac. Sci. Technol. A 6 (1988) 432. [4] R. Garcia Cantu and M.A. Huerta Garnica, J. Vac. Sci. Technol. A 8 (1990) 354. [5] V.K. Adamchuk, V.M. Alexandrov, A.V. Ermakov and I.V. Lubinetsky, Lett. J. Exp. Theor. Phys. (USSR) 14 N3 (1988) 256. [6] V.K. Adamchuk, A.V. Ermakov, I.V. Lubinetsky, G.A. Jitomirsky and A.E. Panich, Rev. Sci. Instr. (USSR) N5 (1989) 182.