Low cost PC based scanning Kelvin probe

REVIEW OF SCIENTIFIC INSTRUMENTS

VOLUME 69, NUMBER 11

NOVEMBER 1998

Low cost PC based scanning Kelvin probe I. D. Baikiea) and P. J. Estrup Department of Physics, Brown University, Providence, Rhode Island 02912

~Received 17 August 1998; accepted for publication 26 August 1998! We have developed a novel, low cost, scanning Kelvin probe ~SKP! system that can measure work function ~wf! and surface potential ~sp! topographies to within 1 meV energy resolution. The control and measurement subcomponents are PC based and incorporate a flexible user interface, permitting software control of major parameters and allowing easy user implementation via automatic setup and scanning procedures. We review the mode of operation and design features of the SKP including the digital oscillator, the compact ambient voice-coil head-stage, and signal processing techniques. This system offers unique tip-to-sample spacing control ~to within 40 nm! which provides a method of simultaneously imaging sample height topographies and is essential to avoid spurious or ‘‘apparent’’ wf changes due to scanning-induced spacing changes. We illustrate SKP operation in generating high resolution wf/sp profiles of metal interfaces ~as a tip characterization procedure! and operational electronic devices. The SKP potentially has a very wide range of applications ranging from semiconductor quality control thin film and surface analyses to corrosion and biopotential imaging. © 1998 American Institute of Physics. @S0034-6748~98!05211-3#

1~c!#. The work function difference F AB between the electrodes is thus equal, and opposite to, the dc potential necessary to produce a zero or ‘‘null’’ output signal, i.e., eV CPD 52DF AB , where e is the electronic charge. The Kelvin method has a high relative accuracy in the meV range, however it does not provide absolute wf determination that would permit easy comparison with literature data on clean surfaces. In Sec. III we demonstrate linescan analysis across various metal interfaces as a method of ~i! determining if the tip wf, F tip , remains constant during surface processing, abrasion due to tip crashes, etc., and ~ii! calibrating the approximate ~60.2 eV! absolute F tip . As suggested by the title of Ritty et al.’s work,20 ‘‘Conditions necessary to get meaningful measurements from the Kelvin method,’’ and Baikie et al.’s,21 ‘‘Noise and the Kelvin method,’’ wf measurements have, in the past, been prone to difficulties caused by parasitic capacity and poor signal to noise ~S/N! levels offered by traditional lock-in amplifier ~LIA! self-nulling detection of the balance point.22,23 The extremely low S/N ratio at balance coupled with phase instabilities and the distinct possibility of driver talkover with piezoelectric actuators24–26 ~which employ relatively high voltages close to the sample! make these detection systems inherently prone to noise.1,21,27 Such designs often require high LIA integrating times which are too long to permit rapid scanning. The modified atomic force microscope ~AFM! design of Nonnenmacher et al.28 and the piezo-tube scanner of Baumgartner and Leiss23 both require a very high degree of control to the tip-to-sample spacing due to the extremely small amplitudes of vibration used. The work function differences measured by the AFM design28 are 10–100 times smaller than expected using macroscopic techniques, probably due to the distributed capacity between tip and sample.29,30 This is

I. INTRODUCTION

The Kelvin probe is a noncontact, nondestructive vibrating capacitor device used to measure the work function ~wf! difference or, for nonmetals, the surface potential ~sp! between a conducting specimen and a vibrating tip. The wf is a multiparameter variable and is an extremely sensitive indicator of surface condition or of surface modification via adsorption,1–4 evaporated layers,5,6 surface roughness,7 surface and bulk contamination,7,8 oxide layer imperfections,9 illumination,10 catalysis,11 etc. The probe measures the average wf of the sample under the tip without the bias toward low wf patches characteristic of photo and field emission;12,13 it can be applied under a range of conditions including ambient,14 vacuum,15,16 and even fluid17 environments. The principle of Kelvin’s method, first demonstrated to the Royal Society in 1898,18 is exquisitely simple: form a capacitor out of two conductors, allow electronic conduction, and detect the charge transfer. In 1932 Zisman19 added the ac method, illustrated in Fig. 1. Based on a Fermi-level equilibrium model, if an ~external! electrical contact is made between the two electrodes their Fermi levels equalize and the resulting flow of electrons from the metal with the smaller wf produces a potential gradient V CPD ~contact potential difference! between the plates @see Fig. 1~b!#. By vibrating the probe, a varying capacitance is produced which causes a current to flow back and forth between the plates. Inclusion of a ‘‘backing’’ potential V b permits biasing of one electrode with respect to the other: at the unique point V b 52V CPD , the electrical field between the plates vanishes @see Fig. a!

Address correspondence to Bio-currents Research Centre, Marine Biological Laboratory, 7 Water Street, Woods Hole, MA 02543; electronic mail: [email protected]

0034-6748/98/69(11)/3902/6/$15.00

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© 1998 American Institute of Physics

Rev. Sci. Instrum., Vol. 69, No. 11, November 1998

I. D. Baikie and P. J. Estrup

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FIG. 1. Electron energy level diagrams of two different metals ~a! without contact, ~b! with external electrical contact, and ~c! with inclusion of the backing potential V b . e A , e B , and f A , f B refer to the Fermi levels and work functions of plates A and B, respectively; the other symbols are as described in the text. In ~b! the surface charge is related to the contact potential difference through Q5V CPDC KP where C KP is the Kelvin probe capacity. If the plates are connected by the external emf, V b , and vibrated, then the current i5 d q/ d t5(V CPD2V b ) d C KP / d t.

in marked contrast to the macroscopic method reported here ~see Sec. III!. An elegant solution for the reduction in AFMKelvin probe ~KP! tip capacity using an integrated metalized layer to shield the tip has been recently reported.31,32 This scanning KP ~SKP! system utilizes a combination of high signal levels and ‘‘off-null’’ detection; the first amplification stage (3107 ) is integrated into the tip electrode and the balance point is determined by linear extrapolation rather than nulling. Further, the head stage actuator is ex-

tremely well shielded from the tip and driver talkover is negligible. The vibration geometry is ideal, i.e., plane parallel, and, by applying a dc offset to the voice coil, the probe spacing can be controlled to within 40 nm, facilitating both sample tracking and automatic setup procedures. This design thus avoids the relatively expensive LIA; indeed the base cost of the system comprising, digital oscillator, head stage, three-dimensional ~3D! microtranslator, data acquisition system, and amplifier is under $6000.

FIG. 2. Schematic of the scanning Kelvin probe arrangement.

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II. SCANNING KELVIN PROBE A. SKP overview

The principal objective of this study was to construct a low cost PC based scanning probe which could be utilized as a test bed to study a range of materials and surface/interface phenomena under ambient conditions. The flexible user interface33 allows software control of probe, measurement, and scan parameters, with additional on-line 3D plotting, postscan filtering, and tip deconvolution procedures. Autosetup, where the probe spacing is varied ~at constant amplitude of vibration! in order to determine the optimum probe height for the prevailing noise conditions, and tracking features provide an integrated automated platform accessible to nonspecialists thus opening this technology to a wider user base than has previously been the case. Based upon the information gained in developing this prototype we have also constructed an ultrahigh vacuum ~UHV! SKP version34 and a multihead bio-Kelvin probe35 for measuring phototropism in plants. An overview of the scanning probe arrangement is shown in Fig. 2. The host PC communicates with the three subsystems via the XT/AT bus: digital oscillator ~DO! which sets voice-coil frequency, amplitude, and probe trigger; data acquisition system ~DAS! based upon the National Instruments PCI-1200 board, which measures the peak-to-peak ~ptp! output signal V ptp as a function of the backing potential V b , and sample translation ~x, y and course z! via the parallel-port interface. The backing voltage can be connected to either sample or tip so the sample is therefore mounted on an insulator to accommodate either configuration. The trigger signal is used to synchronize data acquisition so that the DAS can measure, at relatively high frequencies ~30–40 kHz!, the portion of the signal corresponding to the peak-to-peak height. This is accomplished by setting a variable delay derived from the digital oscillator. Application of a dc offset, via a 12-bit digital-to-analog converter ~DAC!, permits high accuracy ~40 nm! probe vertical positioning. The three-axis microstepper positioner ~AMSI Corp. 6006!, coupled with linear translation stages ~Newport 460XYZ! permits macroscopic sample positioning ~0.4 mm/step!. The scanning system can perform linescans or topographies in the range of 200 mm–2 cm on a side. For larger samples, e.g., semiconductors wafers, we have implemented a (r w ) system which is more cost efficient than a large format xy stage. B. Digital oscillator

This subsystem, seen in Fig. 3~a!, allows digital control of oscillation, amplitude, dc offset, and DAS triggering. The PC bus, denoted by the double arrow, communicates with the DO card using our own driver interface and the PCI-1200 card, which has two on-board 12-bit DACs. The PC bus interface performs address decoding and data latching, and dual inline package ~DIP! switches set a user installable base address. Each line emanating from this block can be addressed by writing a hex value to a PC port address: ~a! oscillator frequency, ~b! trigger output, ~c! oscil-

FIG. 3. ~a! Schematic of the digital oscillator circuit: ~a!–~e! are discussed in the text; ~b! demonstration of the variable frequency synthesis procedure using a divided by 4 example, ~i! output of programmable divider, ~ii! output wave form showing the relationship to the user-specified ampere and dc-offset parameters, ~iii! trigger generation. The double arrow denotes the PC bus.

lator amplitude, ~d! oscillator enable, and ~e! dc offset. The oscillation frequency is synthesized from a 4 MHz clock signal which is divided by a 16-bit user selectable integer. Figure 3~b! illustrates a divided by 4 example ~actual division values are much higher!. The output of the programmable divider is fed into a counter which counts to 256, then repeats, producing the desired variable frequency output clock, having a frequency increment of 0.2 Hz in the vicinity of probe resonance. A comparitor, armed with a user selectable integer in the range 0 . . . 255, generates an output pulse at a predefined point in the wave form. This will be used as the DAS’s external trigger input, in the measurement mode, effectively allowing signal averaging via ‘‘boxcar’’ integration.36 An erasable programmable read-only memory ~EPROM!

FIG. 4. Cross-sectional view of the voice-coil head unit: ~A! Pt/Cu tip, ~B! I/V converter, ~C! shaft, ~D! SmCo magnet, ~E! coil, ~F! diaphragm spring, ~G! spring clamp, ~H! housing.



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FIG. 5. ~a! Example of the simplified Delphi user interface ‘‘KelLite’’ showing probe control and measurement parameters and the preview screen; ~b! typical Kelvin output signal.

containing a 256 value digitized sine wave is read at the clock frequency of the counter, and this output is converted to an analog signal by DAC1. The oscillator amplitude, also in the range 0.255, is user selectable. A high-pass filter is employed to eliminate any dc component, then the external DAC2 is used to provide a dc offset to the ac wave form @see Fig. 3~b!#, moving the probe tip unit either toward or away from the sample. Last, a power amplifier outputs the analog wave form to the voice coil. C. Head stage

The head stage ~see Fig. 4! is constructed from an aluminum housing containing the voice-coil driver element, SmCo magnets ~Dexter Magnetic Corp.!, and two stainless steel springs ( f 525.4 mm). The springs have a laser cut ‘‘meander’’ pattern that permits a large amplitude of vibration but they are extremely resistant to off-axis displacements. This suspension system offers a useable frequency range of 30–300 Hz. Amplitude at resonance ( f r 558 Hz) is some 2 mm ptp even with tip loadings of up to 200 g. The integral tip/amplifier arrangement consists of an integrated circuit ~IC! socket attached to a low noise I/V con-

verter ~Analog Devices 549!. The socket permits rapid tip exchange: we utilize Cu ( f 5800 m m) and Pt ( f 550 m m) wires for large and small resolution scans, respectively. D. Signal processing and tracking

Under conditions of low modulation index37 V ptp is given by V ptp5 ~ DF2V b ! RC 0 v e sin~ v t1 u ! ,

~1!

where DF represents the work function difference between probe and sample, V b is the external emf, R is the I/V converter feedback resistance, C 0 is the mean Kelvin probe capacitance, v the angular frequency of vibration, e the modulation index, and u the phase angle. A screenshot of the Delphi program and representative Kelvin wave form ( e 50.4) is shown in Fig. 5. V b is derived from a 12-bit DAC, which is set to a range of potentials ‘‘n’’ about the balance point. The data set @V ptp(n), V b (n)# is thus a straight line. Linear extrapolation produces the intersection with the V b axis ~2DF!, and its gradient provides the mean sample spacing. The latter parameter is utilized to maintain a pre-set probe-to-sample spacing via a voice-coil dc offset. As the

FIG. 6. ~a! 636 mm2 scan of an Al substrate using a Cu tip ( f 50.8 mm) where the Z axis represents the wf difference ~in eV! with respect to the gold surround. ~b! Linescan of Au/Al and Au/Zn interfaces.

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FIG. 7. Examples of ~a! high resolution SKP wf and ~b! associated sample height topographies of a reversed biased pn junction, of side 0.25 mm, and ~c! a SEM scan of the same area.

DAC settling time is ,1 ms, the above measurement procedure can be accomplished at high speed. Under optimum conditions the current maximum rate of work function determination is approximately 5 Hz so that a (30330) topography can be performed in under 3 min. III. SKP LINESCANS AND TOPOGRAPHIES

Figure 6 shows a SKP scan of an aluminum–gold interface performed with a Cu tip. The sample has been made by masking the aluminum substrate with the ‘‘Al’’ symbol, then plasma depositing a thin layer ~2 nm! of gold. The wf scan clearly shows the bare Al substrate due to the relatively large difference in wfs. The wf variation visible on the Au background is