Scanning Capacitance

Overview

Since its commercialization Scanning Capacitance Microscopy (SCM) has become the preferred technique for imaging dopant variations in semiconductor devices. SCM is used to measure implant profiles and to develop semiconductor processes. In contrast with PicoCurrent, which is effective in measuring resistivity, the SCM module is more sensitive to dopant and charge in the sample.

What makes MultiProbe's SCM solution unique is that it can be configured right into the MultiProbe nanoprobing system, providing insitu probing as well as capacitance imaging without having to withdraw the probe or remove the sensor, and is compatible with PicoCurrent imaging and parametric probing.

Dopant detection range: 10e15 to 10e20 atoms/cm3
Sensitivity: 10e-18 Farads

Features

How SCM Works: When the SCM tip is brought into close proximity with the sample surface, a Metal/Oxide/Semiconductor (MOS) capacitor is formed between them, where: M is the metal probe, S is the semiconductor material and O is a thin dielectric formed formed on the semiconductor surface. Free carriers within the sample are able to move under the influence of an AC electric field applied by the conductive probe (tip).; The capacitance measured by the SCM sensor varies as the carriers move towards (accumulation) and away from (depletion) the probe. This is shown schematically above. When the sample is fully depleted the measured capacitance is that of the oxide plus the depletion layer. When carriers are accumulated at the surface, the measured capacitance is that of the oxide layer.; This capacitance variation in response to the tip-applied field forms the basis of the SCM measurement. If we consider the system to be an MOS capacitor, then it is helpful to think about the variation in capacitance as being the result of a change in the separation of the plates of a parallel-plate capacitor. While this is a simplified view, it is still useful. However, since the interaction is three dimensiona,l parallel plates are only an approximation.; The capacitance between two plates is given by;; C = εA/t; Where:
ε is the dielectric constant
A is the area
t is the spacing between the plates; Therefore capacitance is high when the plates are closest.; Under accumulation, the charge is attracted towards the surface. This is analogous to the bottom plate moving upwards. The plate separation t shrinks and the capacitance increases. For n-type material the measured capacitance is therefore highest when the applied voltage is positive. The capacitance decreases as the bias is shifted negative as a result free carriers being pushed away from the surface, analogous to an increase in the plate separation.; Movement of free carriers, and hence the amplitude of the capacitance variation, is a function of the dopant level of the sample directly beneath the probe. For heavily doped materials the carriers do not move far. Hence, the measured capacitance variation between accumulation and depletion is small. The opposite is true for lightly doped semiconductors which yield a large capacitance change.
What SCM is used for: The AC bias, dV, applied via the probe relative to the sample, moves carriers resulting in a change in capacitance, dC. The capacitance variation, dC, measured by the sensor is amplified with the aid of a lock-in amplifier. Output from the lock-in amplifier in the form of dC/dV amplitude and/or phase is then displayed as an image. The contrast mechanism for the image is the change in the measured capacitance and hence contained within the image is information on both the dopant level as well as the dopant species.; The sensitivity of the SCM sensor is of the order of attoFarads (10^-18 F), enabling the possibility of scanning samples at the silicon or contact level. For these two cases the capacitance change measured will vary over several orders of magnitude.
At The Silicon Level: Samples may be imaged at the silicon level in one of two geometries. The simplest of these is a top-down scan with samples prepared by de-processing back to the silicon level whilst taking care not to introduce too much topography. Sample preparation is usually accomplished with a combination of mechanical lapping and etch chemistry chosen specifically for the device architecture in question. Upon completion the sample will have topography variations of the order of a few nanometers.; This figure shows SCM images of SRAM support circuitry taken at the silicon level. It demonstrates the high spatial resolution available with the MultiProbe SCM sensor. The transition at the STI/Silicon interface is measured at approximately 3nm wide. This represents a significant improvement over existing SCM techniques.; By cross-sectioning the sample, the implant profile as a function of depth is exposed to the SCM probe and sensor. Sample preparation for this kind of analysis represents a significant challenge with today’s device technologies, especially with the need to hit a single transistor. Sample preparation aside, the cross-sectional analysis of samples at the silicon level provides the user with information that is unattainable with any other method. After years of SCM imaging this orientation has become the most important for device visualization and failure analysis.
At The Contact Level: At the contact level, SCM is used to image the sample in much the same way as current measurements are done. Samples are lapped back to the contact level using standard lapping techniques. A short chemical etch is performed leaving the metal contacts a few nanometers proud of the silicon surface. Imaging a sample at the contact level measures the capacitance variations of whatever structure the contact is connected to. Data obtained by this technique is shown below. The image shows topography on the left and SCM data on the right.; When imaged at the silicon level, the measured capacitance variation is due to the movement of majority carriers as they undergo accumulation/depletion under the influence of the applied tip bias. A typical tip/sample capacitance is on the order of 10^-16 Farads (100 attoFarads). The change induced by the accumulation/depletion of carriers during the imaging process will be much smaller than the total tip/sample capacitance ~10aF.; Data obtained in this configuration is primarily used for the visualization of implant regions of a device. This is critical information for the failure analysis process that cannot be obtained with any other technique.; In contrast, capacitance variation seen by the SCM sensor at the contact level is no longer determined by the probe/silicon interaction. In this imaging orientation, the contrast mechanism is defined by what is connected to the contact in question. Differences between source/drain implant, well connection, and gate poly, etc. are easily measured. These structures have a much higher capacitance associated with them than the tip/silicon capacitance discussed earlier. A gate capacitance for a minimum geometry device, for instance, is about 300aF. The capacitance of a source or drain diffusion is of the order of femptoFarads. For example, a typical DRAM trench capacitor has a capacitance of around 30fF.
SCM from MultiProbe: A Significant disadvantage of existing SCM sensor designs is their inability to allow other measurements at the same time. The sensors themselves are designed to be modular and only used when SCM imaging is required. You cannot, for instance, make a pico-current image with the SCM sensor in place. Design limitations of the electronics prohibit this, resulting in a huge reduction in productivity. Prior to removal of the sensor, the tip must be withdrawn and hence, the image or probing location is lost. Each type of measurement, SCM, current or voltage etc., requires that a different sensor be mounted on the imaging platform and the site of interest relocated.; The SCM sensor from MultiProbe represents the next generation in SCM imaging tools. The electronics are specifically designed to allow SCM, DC, AC and parametric measurements without the need to remove the SCM sensor or probe. This directly translates to an increase in productivity and throughput. With the high SCM spacial resolution, MultiProbe SCM represents a significant improvement over existing SCM systems.; Additional details about Multiprobe’s SCM can be found in this application note