23 Scanning Capacitance Microscope (SCM)

Dr. Anchal Srivastava

Contents of this Unit

1.  Introduction

1.1 Historical Background

1.2 Scanning Capacitance Microscope (SCM)

2. Principle Operation of SCM

3. Different Modes of SCM

4. Sample Preparation

5. Summary

Learning Outcomes

• After studying this module, you shall be able to learn
• A short review on the history and current state of the SCM
• The working principle of SCM and it is explained how the capacitance measurement can be performed. Standard procedure for the sample preparation.
• An evaluation of the spatial resolution.

1. INTRODUCTION

Physical characterization of semiconductor device has always been a challenging task for device engineers and researchers. Up to now, standard methods for characterizing semiconductors did not provide an effective means for determining two-dimensional quantities of sub-device scale. These methods include Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Secondary Ion Mass Spectroscopy (SIMS), Spreading Sheet Resistance Profiling (SRP), and one-dimensional Capacitance Voltage (C-V), etc. With the advent of smaller device geometry and high reliability requirements, new characterization tools are needed. As alternative approaching tools, various types of scanning probe microscopy (SPM) have been applied to not only characterizing semiconductor devices but also monitoring semiconductor device processes. Scanning Capacitance Microscopy (SCM) combined with Atomic Force Microscopy (AFM) is one of the powerful methods for the characterization of semiconductor devices due to its non-destructive technique and high spatial resolution.

1.1 Development of Scanning Capacitance Microscope (SCM)

Over the last 20 years almost the same amount of derived AFM techniques have entered the world. Benefiting from the splendid capabilities of AFM to measure topography with high spatial resolution in two dimensions, additional features were included in the basic AFM technique to enable the analysis of different characteristics for different materials. Due to the continuous scaling of the semiconductor devices (at that time 0.5 µm technology was a fact), the operation of a semiconductor device started to be dominated by its 2D dopant distribution. This implied the need for measurement techniques with the capability to determine the 2D dopant distribution (simultaneously with the topographical information) with high resolution. A first tool for such was invented in 1984 by Matey and Blanc. The technique was called scanning capacitance microscopy (SCaM). The principle of SCaM, also used in the RCA (Radio Corporation of America ) video disc playback system. A capacitance probe is scanned relative to the sample surface by a scan generator. Capacitance variations between the probe and the sample are sensed by a capacitance to voltage converter, which applies a voltage signal (Vc) to the recording medium that varies with the capacitance variations. The recorded signal is applied to a display to provide an observable map of variations in capacitance. Matey showed a lateral resolution of 0.1 µm by 2.5 µm in a system where the tip is scanned in the tracks of a pregrooved disc. A few years later, in 1988, SCaM was combined with AFM (nowadays known as SCM) and a resolution approaching the 100 nm scale was demonstrated by Martin et al. Martin describes one of the first SCM measurements done on a pn junction. Since 1992, the National Institute of Standards and Technology (NIST) has had a SCM program with the goal of making SCM a quantitative 2D carrier profiling technique. The NIST program has developed models of the SCM measurement based on 3D finite element solutions of Poisson’s equation and Windows-based software for rapid carrier profile extraction from SCM images. Researchers at the University of Utah and at the ETH Zurich have also developed quantitative SCM image to dopant profile conversion software. From this point on SCM has been evolving a lot and it is in the scope of this thesis to be part of this evolution. However before going into more detail in the mysterious world of SCM, it is important to discuss some of the other doping profiling techniques also derived from AFM.

1.2 Scanning Capacitance Microscope (SCM)

The most common implementation of a scanning capacitance microscope (SCM), combines an atomic force microscope (AFM) with a high-frequency capacitance sensor. A capacitance sensor that can detect very small (≈10−21 F) changes in capacitance is used. A conducting AFM tip is electrically connected to the capacitance sensor. The AFM is used to acquire an image of topography and to control tip position, while the sensor is used to measure the capacitance between the tip and the sample under test simultaneously. Commonly, an ac voltage (around 1 Vpeak-to-peak at 10 kHz) is used to induce and modulate a depletion region in a semiconductor sample. The depletion region changes in response to the ac voltage and produces a varying, or differential, SCM tip-to-sample capacitance. The capacitance sensor detects the varying capacitance and produces a proportional output voltage. A lock-in amplifier that is referenced to the modulation frequency is used to measure the magnitude and phase of the sensor output voltage. The signal measured by the SCM is the output voltage of the lock-in amplifier, which is proportional to the induced tip-to-sample differential capacitance. As the SCM/AFM tip is scanned over the sample surface, simultaneous images of topography and differential capacitance are obtained.

Figure1. Schematic diagram of SCM

Differential capacitance imaging of this sort is entirely dependent on the presence of a semiconductor sample. If the surface of the semiconductor sample is electrically passivated with a thin insulating layer, the charge on the surface of the semiconductor can be modulated with an externally applied electric field. In this case, the metallized SCM tip forms a metal-oxide-semiconductor (MOS) capacitor with the semiconductor sample under test. The differential capacitance signal arises from the depletion region capacitance generated in the semiconductor by the AC voltage applied to the tip. If insulators or conductors are imaged in this mode, no differential capacitance signal will be produced. The MOS capacitor is a critical part of a MOS field effect transistor (MOSFET) and the physics of such a device are well understood. As will be discussed later, the three dimensional (3D) nature of the SCM tip necessitates a 3D model to predict the capacitance between the tip and the sample adequately.

2.   PRINCIPLE OPREATION OF SCM

Figure 2. Block diagram of a scanning capacitance microscope configured for constant voltage mode operation

Figure 2 shows a block diagram of a typical scanning capacitance microscope operated in the constant delta voltage (V) mode. The SCM consists of four major components: (1) AFM, (2) conducting tip, (3) capacitance sensor, and (4) signal detection electronics. The AFM controls and reports the x-, y-, and z-positions of the SCM tip. The conducting tip is similar to ordinary AFM tips, except that it needs to be of high electrical conductivity and in electrical contact with both the capacitance sensor and the sample. The radius of the tip determines the ultimate spatial resolution of the SCM image. Commercially available metal-coated silicon tips, tips made completely of very highly doped silicon or diamond, or solid metal tips have been used.

Figure 3. Capacitance detection circuit used in the RCA VideoDisc sensor and later modified For use in scanning capacitance microscopy

The capacitance sensor enables the scanning capacitance microscope operation. The archetypical SCM uses a capacitance sensor similar to that used in the RCA Video Disc player. Commercial SCMs use sensors that are similar in concept, though unique and proprietary in design. The sensor measures capacitance using a driven LCR resonant circuit. The capacitance to be measured is incorporated in series as part of the LCR circuit. The amplitude of the oscillations in this resonant circuit provides a measure of the capacitance. Figure 3 shows a simplified schematic of the capacitance sensor circuitry. The circuit consists of three inductively coupled circuits: an ultra-high frequency (UHF) oscillator, the central LCR resonant circuit, and a peak detection circuit.

The UHF (ωhf ≈ 915 MHz for the VideoDisc) oscillator is used to drive the LCR resonator at a constant frequency. The oscillator circuit contains a mechanically tunable potentiometer and capacitor to allow some tuning of the UHF signal frequency and amplitude. The magnitude of the UHF signal voltage, Vhf , which

is applied between the tip and sample, is an important variable for SCM image interpretation.

The capacitance between the SCM tip and the sample, Capacitance of tip/sample (CTS), is incorporated into the central LCR circuit. As shown in Figure 4, the LCR circuit displays a characteristic bell-shaped response versus frequency, with the peak response at the resonant frequency. Changes in CTS cause a shift in the resonant frequency of the response curve, in turn causing the amplitude of the oscillations in the resonant circuit to change. The total capacitance in the sensor LCR circuit includes the tip-sample capacitance, the stray capacitance between the sample and sensor, and a variable tuning capacitance. In an RCA style sensor, a varactor diode is used to provide a voltage-variable capacitance. The varactor capacitance can be used to adjust the total capacitance in the LCR circuit, which in turn changes the circuit’s resonant frequency. In this way, the sensor can be “tuned” to produce a similar response for a range of tip-sample capacitances.

Figure 4. Capacitance sensor LCR circuit response illustrating how the shift in resonant frequency is converted into an output voltage.

SCM image contrast is due to the voltage-dependent capacitance between the conducting SCM tip and a semiconductor sample under test. When imaging silicon with the SCM, this capacitor is usually considered a metal-oxide-semiconductor (MOS) capacitor, though a metal-semiconductor Schottky contact may also be formed. The quality of the SCM measurement of silicon depends on the formation of a thin oxide and establishment of an unpinned semiconductor surface. A MOS capacitor displays a characteristic voltage dependent capacitance due to the “field effect.” The field effect is also the basis of the technologically important metal-oxide-semiconductor field effect transistor (MOSFET).

3. DIFFERENT MODES OF SCM

• The SCM detection electronics are used to measure the difference capacitance, ∆C, between the tip and sample.
• SCMs are commonly operated in one of two different modes, the constant ∆V mode or the constant ∆C mode.

3.1 Constant ∆V(open loop) mode:

In the constant delta voltage (∆V) mode of SCM, an ac sinusoidal voltage with a magnitude of Vac at a frequency of ωac is applied between the sample and the SCM tip, which is a “virtual” ground; i.e., the tip is inductively coupled to ground. The inductance is such that it passes the ωac signal to ground, but does not ground the ωhf signal, which is used to detect the capacitance. Vac is typically around 1 V at a frequency of 10 kHz to 100 kHz.

Figure 5 illustrates the voltage dependence of capacitance for an MOS structure and the mechanism of SCM contrast generation. The Vac signal induces a change in tip-to-sample capacitance, ∆C(ωac), that depends on the slope of the C-V curve between the tip and the semiconductor. When the tip is over a semiconductor with high dopant concentration, the C-V curve changes slowly with voltage and Vac produces a relatively small ∆C. When the tip is over a semiconductor with low doping concentration, the C-V curve changes rapidly with voltage and Vac produces a relatively large ∆C. When Vac is applied between the tip and the sample, the capacitance sensor responds with a voltage output signal that varies at ωac in proportion to ∆C.

Figureh 5. Schematic illustration of how SCM image contrast is generated for two different dopant densities of silicon.

As shown in Figure 5, the magnitude of the sensor output is detected using a lock-in amplifier that is referenced to ωac. Since n-type and p-type semiconductors produce C-V characteristics that are mirror images of each other, they produce distinctly different responses to the SCM. In general, n-type silicon produces a response that is in-phase with the drive signal and p-type silicon produces a response that is 180◦ out-of-phase with the drive signal. When using a lock-in amplifier in x/y mode, n-type silicon produces a positive signal and p-type silicon produces a negative signal.

The capacitance sensor output provides a relative measure of differential capacitance—larger values of ∆C produce larger sensor output voltages. Because the tip shape and the tuning of the capacitance sensor are variable, the absolute value of capacitance (in farads) measured by the SCM is difficult to determine. When measuring an unknown, the absolute capacitance is determined by comparison to measurements of a reference whose capacitance can be calculated based on its known properties.

Several operational factors strongly influence the SCM signal:

1. AC bias voltage, Vac—larger values of Vac result in larger values of ∆C. The larger change in capacitance is

due to larger number of carriers responding to Vac. Larger values of Vac also result in larger depletion

volumes and lower spatial resolution when the SCM is used to measure quantitative carrier profiles.

2. DC bias voltage, Vdc—the dc voltage, Vdc, applied between the sample and the tip determines where on

the C-V curve that ∆C is measured. The effect of Vdc is relative to the flatband voltage of the MOS capacitor

formed by the tip and the sample.Vfb will vary from sample to sample. The peak SCM response occurs when

Vdc ≈ Vfb. In a sample containing a dopant gradient, Vfb varies with dopant concentration. In practice, Vdc

is usually set equal to the voltage that produces the maximum SCM response in the region with the lowest

dopant concentration.

3. Sensor high-frequency voltage, Vhf —The magnitude of the high-frequency voltage, Vhf , in the LCR loop

of the capacitance sensor is proportional to the magnitude of the output signal. Increasing Vhf increases the

capacitance sensor output and the signal-to-noise of the capacitance measurement. However, similar to the

case for Vac, the measured SCM signal is averaged over its values at the voltages spanned by Vhf . Large

values of Vhf generate large signal at the cost of spatial resolution.

4. Sensor-to-sample coupling—The capacitance sensor forms a grounded LCR loop that includes the tip-

sample capacitance. The capacitance between the sample and the grounded outer shield of the capacitance

sensor completes the loop. Subtle variations in the sample geometry can influence this coupling

capacitance.

Constant ∆C (or closed loop) mode :

For carrier profiling applications, the SCM can also be operated in a constant ∆C mode to control the volume depleted of carriers better. A feedback loop (hence, closed loop mode) is added to the SCM to adjust the magnitude of Vac automatically to keep ∆C constant in response to changes in dopant concentration as the SCM tip is scanned over a dopant gradient. The lower the doping concentration beneath the tip, the smaller the Vac needed to induce the same ∆C. Since ∆C remains constant, the volume depleted of carriers, and therefore the spatial resolution of the image, is less dependent on the dopant concentration in the semiconductor. In the constant ∆C mode of SCM, the SCM signal is the value of Vac that the feedback loop establishes to maintain a constant ∆C. The feedback loop of the constant ∆C mode produces a signal that changes monotonically with carrier concentration only for dopant gradients in like-type semiconductors. When a p-n junction is present, the changes in sign of the SCM signal when transitioning between n-type to p-type material cause the feedback loop to saturate near the junction.

Capacitance-voltage curve measurement

The measurement of capacitance versus dc voltage (i.e., the C-V curve) is the usual method of characterizing any MOS capacitor. Measurement of the SCM tip-sample C-V characteristics reveals the suitability of the sample preparation for quantitative carrier profiling. SCM tip-sample C-V curves can be measured with a boxcar average or a digital oscilloscope operated in x-y mode. An AC voltage at around 1 kHz and scanning the range of the desired voltage of the C-V curve is applied between the tip and the sample. The AC voltage is displayed on the x-channel, while the capacitance sensor output is displayed on the y-channel. The averaging feature of the digital oscilloscope is used to improve the signal-to-noise. To be suitable for quantitative carrier profiling, the C-V curve must display clear accumulation and depletion regions, with little hysteresis between the forward and reverse sweep, and with no increase in capacitance in the depletion region. The ∆C versus V curve can also be measured directly with the SCM. In this case, a SCM image is acquired while slowly changing the value of Vdc so that the desired range for the ∆C-V curve is swept out once in the time it takes to acquire the image. Sections of such an image taken parallel to the slow scan direction reveal the ∆C-V curve. A high-quality surface should yield a single peak (a positive peak for n-type and negative peak for p-type) at the flatband voltage.

4 SAMPLE PREPARATIONS

SCM cross-sectional sample preparation is derived from techniques developed for scanning electron microscope (SEM) and transmission electron microscopy (TEM). Sample preparation usually involves four steps: (1) cross-section exposure, (2) backside metallization, (3) mechanical polishing, and (4) insulating layer formation. Prior to cross sectioning, the region of interest is usually glued face-to-face with a glass cover slide or silicon dummy. This cover prevents rounding and chipping of the surface of interest during mechanical polishing. Thin (a few mm)  cross-sections are cut from the sample-cover sandwich with a mechanical dicing saw. To improve the electrical contact between the back of the sample and the microscope sample holder, the backside is usually coated with a Cr layer (for adhesion), followed by an Au layer (for low resistance and corrosion resistance). The sample is attached to a metal sample holder using a conducting silver-filled epoxy. Front-toback sample resistance of no more than a few ohms indicates a good backside contact. An additional aspect of cross-section preparation is the creation of a sample geometry that minimizes the capacitive coupling between the active parts of the sensor and ground, and that also reduces the stray light from the AFM laser that may impinge on the semiconductor under test. Here the sample is wide enough so that the active parts of the SCM tip, cantilever, and body are physically separated from ground by several millimeters. This reduces the coupling capacitance and prevents excessive loading of the sensor. Because the sample is terminated by an insulating glass cover slide, most stray light from the AFM laser does not impinge on the semiconductor sample, which would generate excess carriers and distort the SCM image.

The mechanical polishing step must achieve both a highly polished surface and a surface with low residual crystalline damage. Sample preparation optimized for SEM image may produce a smooth surface, but leave behind substantial crystalline damage. The final step of silicon sample preparation is the formation of a thin, passivating, insulation layer on the surface to be imaged. This layer is necessary to reduce the number of surface states and to serve as the oxide of the MOS capacitor that the SCM tip forms with the sample. Several processes have been used to form this layer including a short polish with colloidal silicon, a 300◦C anneal for 60 minutes with UV light or without, oxidation in an UV-ozone photoreactor, and a 200◦C anneal in 5% ozone/ 95% O₂. Regardless of the process to form the surface-insulating layer, the quality of the insulating layer for SCM is determined by measuring the C-V with the SCM.

5. SUMMARY

In this module you study

The SCM is a versatile instrument for the electrical characterization of semiconductors and dielectric films with spatial resolution on the order of 10 nm. SCM has already proven useful for qualitative characterization for integrated circuit failure analysis, quantitative dopant profiling of the source drain regions in silicon MOSFETs, and the characterization of silicon dioxide and high-k dielectrics on silicon. SCM’s sensitivity to capacitance changes in the attoFarad range means that it can detect charges due to just a few electrons. Since SCM uses a low contact force AFM to make an electrical contact, it is a most promising tool for the electrical characterization of emerging nanostructures, such as carbon nanotubes, semiconductor nanowires, and nanoparticles.

Suggested Reading

• A.R. Hambley, Electronics, 2nd ed. (Prentice Hall, Upper Saddle River, NJ, 2000), pp. 809–811.
• Hantschel, S. Slesazeck, N. Duhayon, M. Xu, W. Vandervorst, \The peel- of probe : a cost-effective probe for electrical force microscopy”, Proceedings SPIE Vol 4175, p. 50 (2000)
• Anand. Another dimension in device characterization: Scanning capacitance microscopy of InP-based laser structures. Circuit & Devices, 16(2):13, 2000.