Electricity: measuring and testing – Electrostatic field – Using modulation-type electrometer
Reexamination Certificate
1997-07-15
2001-01-09
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Electrostatic field
Using modulation-type electrometer
C324S662000, C073S105000, C250S307000
Reexamination Certificate
active
06172506
ABSTRACT:
The present invention relates to new atomic force microscopes (AFMs), and to methods for operating such AFMs in intermittent contact mode to scan samples for electrical properties such as dopant profiles or film thickness.
AFMs are high-resolution, surface-measuring and surface-modifying instruments. There are three general modes of operation for AFMs: the contact mode (repulsive mode), the non-contact (attractive mode), and the intermittent contact mode.
In the intermittent contact or “tapping” mode of AFM operation, as described in detail in U.S. Pat. Nos. 4,966,801; 5,412,980; and 5,415,027 by Elings et al., a probe tip on a cantilever is oscillated and scanned across the surface of samples in intermittent contact with the samples. In this mode of operation, the amplitude of probe oscillation may be kept constant through feedback which servos the relative vertical position of the cantilever mount to the sample, or vice versa, so that the probe follows the topography of the sample surface. The probe's oscillation amplitude is preferably greater than 20 nm to maintain the energy in the lever arm much higher than the energy lost when the probe touches sample surfaces. Oscillation at such amplitudes also minimizes the sticking of the probe tip to sample surfaces. Sample height data is obtained from the Z actuator control signal that maintains the established amplitude setpoint, or from a vertical motion sensor.
The methods of this invention comprise operating AFMs in intermittent contact mode to measure, scan or modify the topography of sample surfaces wherein an error signal for tracking sample surfaces is the difference between an amplitude setpoint and a signal corresponding to the oscillation amplitude of the AFM probe as the probe tip makes intermittent contact with, or taps, the sample surfaces.
Oscillation of the AFM probe modulates the capacitance of the tip-sample system, at the frequency of the tapping. The amplitude of the capacitance modulation signal results from the electrical series combination of the modulated air gap capacitance, i.e. the capacitance between the tip and sample, and the substantially unmodulated capacitance of the sample surface, i.e. the sample capacitance. As the air gap between the tip and sample becomes larger, the air gap capacitance approaches zero, making the series capacitance approach zero. As the distance between the tip and sample varies, the detected capacitance also varies. This capacitance varies between the air gap capacitance (“C
A
”), when the tip is far off the surface, and the tip-sample capacitance, when the tip touches the surface of the sample.
In these AFMs, the probe and the probe tip are conductive, and are electrically connected to a capacitance sensing circuit. As the probe tip is scanned over the sample surface, the capacitance sensing circuit generates a signal corresponding to the capacitance of the tip-air gap-sample system.
Preferred embodiments of these capacitance sensing circuits include an RCA®-style capacitance sensor. In such sensors, a UHF oscillator inductively drives a resonant circuit near its resonant frequency. The conductive AFM probe is electrically connected to this resonant circuit by way of a transmission line. Changes in tip-sample capacitance modify the resonant frequency of this circuit, and thereby change the amplitude of the oscillator signal induced in the circuit. The amplitude of the signal is detected and outputted from the capacitance sensing circuit as a signal corresponding to the tip-sample capacitance, plus any parasitic capacitance. Other types of capacitive sensors, e.g. capacitive bridge circuits and impedance transformers, may also be used.
The signal amplitudes are demodulated, preferably at the frequency of the probe tip oscillation, or tapping, producing signals corresponding to the oscillation amplitude of the tip-sample capacitance. Alternatively, modulation at harmonics of the oscillation, or mixing of multiple oscillation frequencies, may be used. The demodulated capacitance signals may be stored, and may also be displayed as an image representative of the tip-sample capacitance as it varies across the sample surfaces. Such images may, for example, represent variations in carrier or impurity concentrations across semiconductor samples, or variations in the capacitance across a dielectric layer on the surface of a semiconductor sample or a conductor sample. Variations in dielectric capacitance may correspond to intrinsic properties such as trapped charge or dielectric constant or variation in thickness.
These new AFMs have many advantages. These AFMs measure capacitance (C) of samples directly. Their intermittent contact mode of operation provides better performance in topographical measurement than contact or non-contact modes. The intermittent contact mode of operation reduces friction between the tip and sample, facilitating capacitance measurements on samples with minimal damage to samples or probe tip. Capacitance of thin films may be measured over conductive and semiconductive samples. No bias need be applied between the probe tip and sample, so the tip does not short out to conducting surfaces, although a bias voltage can be used to make measurements at various bias voltages.
REFERENCES:
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“Quantitative Two-Dimensional Dopant Profile Measurement and Inverse Modeling by Scanning Capacitance Microscopy,” Article by Y. Huang, C.C. Williams; publ. in Appl. Phys. Lett. 6(3), Jan. 16, 1995, pp. 344-346.
“Characterization of Two-Dimensional Dopant Profiles: Status and Review,” Article by A.C. Diebold/M.R. Kump; publ. in J. Vac. Sci. Technol. B 14(1), Jan./Feb. '96, pp. 196-201.
“Tapping Mode Capacitance Microscopy,” Article by Kazuya Goto/Kazuhiro Hane; publ. in Rev. Sci. Instrum. 68(1), Jan. '97, pp. 120-123.
Adderton Dennis M.
Elings Virgil B.
Bright Patrick F.
Metjahic Safet
Nguyen Vincent Q
Veeco Instruments Inc.
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