Scanning Capacitance Microscopy Profiles Semiconductor Carriers

The scanning capacitance microscope (SCM) may not be a household term, but it is gaining followers in the inspection industry. Semiconductor manufacturers are discovering that this tool can help bring previously obscure images into sharp focus.

The SCM is a new analytical adjunct to the more familiar scanning probe microscope. It helps resolve some of the more complex semiconductor issues such as imaging movement of carrier materials inspecting junction locations and measuring dimensions critical to semiconductor operation.

The SCM performs high-resolution topographical profiling and measuring and provides information on hardness, work function, electric- and magnetic-field strength and capacitance. It also measures carrier concentration in semiconductor material.

Scanning capacitance microscopy extends the 1-D information available today from other inspection techniques to the second dimension. The capability of the SCM to image and measure the carriers makes it useful for developing, manufacturing and testing semiconductor devices.

How the SCM Works

An ultra-high-frequency resonant-capacitance sensor is the basis of SCM detection. The resonator connects to the cantilever/tip assembly via a transmission line (Figure 1). When the resonating probe tip is put in contact with a semiconductor, the sensor, transmission line, probe and carrier near the tip of the sample become part of the resonator. This means tip-sample capacitance and variations in it load the end of the transmission line and change the resonant frequency of the system.

Small changes in resonant frequency create enormous changes in the resonant sensor output signal. The SCM can detect these changes because it has a sensitivity to capacitance variations in the attofarad range (10^-18 F).

SCMs generate the necessary capacitance variations in the sample near the tip by applying an AC field between the scanning contact atomic-force microscope tip and the sample. The microscope simultaneously measures the resulting change in capacitance using a kilohertz AC bias voltage applied to the semiconductor.

Prompted by the alternating electric field, the free carriers near ground potential and beneath the tip in the semiconductor are alternately attracted and repulsed by the tip. This alternating depletion and accumulation of carriers under the tip can be modeled as an equivalent moving capacitor plate (Figure 2). Since capacitance is inversely proportional to the distance between the plates, this movement varies the capacitive load on the resonant sensor system and the system resonance frequency.

The depth of depletion and the equivalent capacitor plate movement are determined by:

The strength of the applied field.

The quality and thickness of the dielectric between the conductive probe and the semiconductor.

The free carrier concentration.

The stronger the field or the fewer the carriers, the deeper the field depletes the semiconductor beneath the dielectric. However, if the field is weak or the carrier concentration is high, the depletion depth will be very shallow—between 10 Å and 100 Å beneath the surface.

If a semiconductor sample has uniform doping but different thicknesses of overlaying dielectric film, the depletion will be deeper under the thinner film. In a sample having high- and low-carrier concentration regions, the depletion in the low-concentration region is deeper for the same applied voltage. The amplitude of the capacitance change generated by the AC bias is higher at that location.

The SCM measures the movement of the carriers in the semiconductor. The signal for the SCM can be considered as the change in capacitance due to depletion (dC) for a unit change in the applied voltage (dV).

Because an AC voltage waveform is applied, the dV is considered the applied peak-to-peak voltage. The dC is proportional to the change in depletion depth of the semiconductor under the probe.

The relationship between capacitance and voltage for a semiconductor is typically plotted on a capacitance-voltage (C-V) curve similar to Figure 3. For SCM imaging, a constant amplitude sine-wave voltage is applied to the dV sample and then an image is constructed from the amplitude of the capacitance modulation (dC).

C-V Relationship

The C-V relationship, around for more than 35 years, measures material characteristics in semiconductors. A typical curve shows a high-frequency capacitance vs voltage relationship for both high- and low-concentration n-type materials.

For example, as the conductor becomes positive relative to the semiconductor, the electron carriers in the semiconductor are attracted to the semiconductor-dielectric interface and accumulate there (Figure 2). As the voltage on the conductor swings negative relative to the semiconductor, the electrons move away from the dielectric. They deplete the semiconductor near the dielectric of carriers and increase the spacing between the equivalent capacitor plates which lowers the capacitance.

Lower carrier concentration semiconductors deplete to a deeper level and the capacitance decreases faster with respect to voltage. As in Figure 3, the slope of the C-V curve is larger for the low-carrier concentration material. The SCM detects the slope of this C-V relationship.

2-D Dopant Profiling

Two-dimensional profiling is expected to become a technology for next- generation IC manufacturing. Hopefully, SCM will provide 2-D dopant profiling.

For example, the Technology Computer Aided Design (TCAD) simulators offer the potential for making and testing new technologies in a virtual fabrication house. TCAD is used to develop new semiconductor devices and processes. It can help design an IC with a shorter gate length and predetermined properties and functions without fabricating the device

However, the new models must be calibrated with empirical data. Many aspects of the model depend on dopant diffusion in two dimensions and resultant carrier- concentration profiles. The SCM is a possible tool for these designs because it can measure the second dimension by extending the 1-D data.

Applications

Semiconductor Memory

The increased computing speed and calculations demanded by today’s complex software programs have created a parallel need for enhanced speed and density of memory components. To increase the density of memory cells, or bits, manufacturers have moved from planar silicon processing to forming trench capacitors similar to Figure 4.

The trench capacitor helps maintain a constant capacitance in each cell while decreasing the planar cross-sectional area. The blue region in Figure 4 represents the silicon wafer, the white band indicates the insulating layer between the trench side wall implant (red) and the polysilicon filling the trench (brown). The trench implant is charged by opening the transistor switch controlled by the voltage placed on the word line. The purple area is the channel where charge flows when the switch opens. The red arrow at the top indicates the implant angle, the gray arrow shows the wafer rotation during the trench implant. The trench effectively raises the surface area by taking advantage of the third dimension of the wafer depth.

Microfabrication of these structures is difficult and evaluation of the process is even more complicated. SCM evaluates the trench side wall implant in the DRAM trench-capacitor process. The implant into the trench is done at an angle to the trenches while the wafer is rotated about the trench axis. This step creates a uniform implant down the trench depth, but the rotation and aspect ratio of the structure make it difficult to maintain the symmetry and uniformity of the implant.

Compound Semiconductor

Compound semiconductors also present difficulties for processing and inspection devices. LEDs, photodetectors, solar cells and diode lasers are examples of compound semiconductors.

The SCM is appropriate for inspecting compound semiconductors because it supplies very-high-resolution carrier-density profiling. It provides details of the electrical structures on a diode laser section with little more sample preparation than cleaving. Sample preparation with other inspection equipment such as a transverse electron microscope takes hours and does not provide the electrical detail needed.

The silicon carbide compound semiconductor used in high-power, high-temperature applications is another good match for SCM inspection. Growth and doping of this compound are difficult and often result in defects such as pin-holing. The dopants are segregated from the crystal and become inactive as a result of this problem. Only the 100-nm-size pin holes and the normal surface of the silicon carbide wafer are visible.

Fortunately, the SCM images the defect by measuring the capacitance. Using the semiconductor’s capacitance, the SCM shows the relative activity of the background region as well as the depleted and segregated carriers.

Acknowledgment

This article is based on the application paper “Scanning Capacitance Microscopy for Carrier Profiling in Semiconductors” by A. Erickson, D. Adderton, Y. Strausser and R. Tench, Digital Instruments, Santa Barbara, CA, Sept. 1996.

Microscopy Products

Scanning Probe Microscope

Measures to Nanometer Level

The Dimension™ Metrology Scanning Probe Microscopy Head measures topography to the nanometer level in the X, Y and Z axes. It is used for critical dimension measurements for line-width, pitch and depth, side wall angles and chemical mechanical polishing. The X and Y flatness is £ 5 nm over the scanner range of ~75 microns and the Z axis is perpendicular to the X and Y plane to <0.1°. The head supports most operating modes including TappingMode™, contact mode, phase imaging, lateral force, magnetic force and electric force. Digital Instruments, (800) 873-9750.

Inspection Stations Provide

In-Line Process Monitoring

The DualBeam 820i and 825i FIB/SEM Workstations are clean-room-compatible and provide in-line process monitoring and defect characterization. The systems also have characterization capabilities in the Z direction to help identify the source and the sequence of a defect. The 825i features cassette-to-cassette wafer loading; the 820i uses manual wafer loading. The FIB sample stage is automatically moved to defects on the inspection-tool wafer maps through an integrated interface. Additional features include auto-wafer sensing, noncontact pre-alignment and 200-mm wafer coverage. FEI, (503) 640-7500.

Stereomicroscope Has

Magnification Range to 180×

The LEICA GZ4 Stereomicroscope features a 4:1 zoom with precentered optical mounting, a 35-mm to 243-mm working distance, a 2.1× to 180× magnification range and a protective material to dissipate static charges. A variety of stands is offered to accommodate large specimens, and large knobs provide even torque while moving throughout the zoom function. Leica, (847) 405-0123.

Microscope Offers Magnifications

Ranging from 5× to 3000×

The Eclipse E800 Microscope with the company’s CFI60 Optical System provides a magnification range from 5× to 3000×. An objective parfocal length of 60 mm, an objective barrel diameter of 25 mm and a 200-mm focal-length tube lens accommodate a working distance to 210 microns. The objectives correct chromatic aberrations from the center to the edge of an image. More than 40 objective lenses are available for the E800 system. Nikon, (800) 52-NIKON, ext. P674.

System Screens and Provides

Quantitative Data for ICs

FACTS², the Fast Automated C-SAM Tray Scanner System, automatically screens plastic IC packages for internal defects at throughput rates of 1 device/s. The acoustic microscopy system provides quantitative data for more than 20,000 IC packages per 8-hour shift. It checks for die face delamination, popcorn cracks and die-attach voiding simultaneously. The system features histograms of defects per lot, per tray and per stack. Sonoscan, (630) 766-7088.

Scanning Electron Microscope

Automatically Measures Wafers

The MI-3080 CD Scanning Electron Microscope automatically measures and inspects critical dimensions of submicron features on semiconductor wafers. The patented Retarding Field Column provides 3-nm resolution at 800 V with greater than 3-nm repeatability. The resolution capabilities are coupled with six measuring algorithms. Throughput is 25 wafers/h. Topcon Technologies, (201) 261-5410.

Scanning Probe Microscope

Has Closed-Loop Scan System

The Explorer™ Scanning Probe Microscope is a modular instrument that features the company’s TrueMetrix™ scan linearization system for eliminating piezo hysteresis. TrueMetrix enables distortion-free zoom, pan and scan rotation of images. The microscope also provides Scanning Tip Technology™ for viewing samples of any diameter, thickness or mass. It supports noncontact and contact atomic force microscopy inspection. Viewing is performed with an integrated 100× optics and CCD camera with a user-selectable scan size from atomic resolution to 100 µm. TopoMetrix, (408) 982-9700.

3-D Imaging Analyzer Combines

Measurement and Surface Detail

The NewView 200 3-D Imaging Surface Structure Analyzer combines the capabilities of a microscope and a surface profiler. It provides the surface details and measurements to characterize surface structure details. The analyzer is based on the company’s NewView 100 Data Acquisition and Analysis System and Nikon’s infinite conjugate optics. You can image from <1-nm to 5,000-µm depths at up to 4.0-µm/s speeds with 0.1-nm height resolution. It uses noncontact scanning white light interferometry to build Z-resolution images. Zygo, (860) 347-8506.

Copyright 1997 Nelson Publishing Inc.

April 1997