Silicon germanium challenges metrology

To enable continued migration to smaller design rules in advanced CMOS processes, the semiconductor industry must deal with a proliferation of materials-from 20 materials options at the 130-nanometer node to nearly double that at the 65-nm node. The new materials are more reliable, switch faster and consume less power than conventional CMOS materials. At the same time, most of these materials are still immature, so they require extensive monitoring to ensure that their deposition processes remain under control in production. In addition, many of these materials require new metrics, such as composition, to verify their performance characteristics. This, in turn, creates new metrology challenges that demand action from the industry through new or existing measurement technologies.

Silicon germanium (SiGe) is one of those proliferating materials. Compared with silicon, SiGe offers greater carrier mobility, an important property for achieving high device-switching speeds and low power consumption. Recent successes in incorporating SiGe into conventional CMOS production have resulted in its increased use in advanced processes.

Along with its benefits, however, SiGe presents many measurement challenges. Film thickness, germanium concentration, defects, lattice strain and surface roughness-parameters not typically monitored in production for Si-must be controlled for SiGe. Some of these measurements must be made on ultrathin multilayer stacks, which exacerbates the challenges. And all of the measurements should be done in a nondestructive manner, at high throughput and on production wafers, to minimize costs-a critical consideration for manufacturing on 300-mm wafers.

This article will review the primary applications of SiGe in advanced CMOS production, as well as the parametric control challenges associated with each application. The article will conclude with an examination of the viability of current metrology technologies in addressing the various challenges.

One primary application for SiGe is the heterojunction bipolar transistor (HBT), which offers significant advantages over conventional Si MOSFETs for use in high-speed, high-frequency communications devices. The SiGe HBT lets IC designers integrate analog, digital and RF functions onto a single chip with minimal additions to existing CMOS production methodologies. HBTs use a thin (under 1,000 angstroms) multilayer stack of epitaxial Si with varying thicknesses and Ge concentrations to form the emitter region of a bipolar transistor.

Layering on the difficulties

Compared with conventional Si, the high-Ge-concentration layers supply high-mobility electrons, while the low-concentration layers provide low contact resistance to enable devices capable of gigahertz switching speeds at increasingly smaller geometries. Accurately measuring the thickness and Ge concentration for each of the epitaxial Si layers is essential for effective process control of HBT devices. But since all of the layers grow in a single pass in an epi reactor, designers cannot take film measurements until after all the layers have formed. Therefore, film metrology must be able to take independent thickness and composition measurements of each layer within the film stack.

Another application for SiGe is strained silicon, wherein the traditional Si active device layer is replaced with a thin (under 200 angstrom ) strained Si layer deposited epitaxially on a several-micron-thick bed of epitaxially grown SiGe. Since the lattice spacing of SiGe is wider than that of Si, the crystal lattice of the top Si layer is literally stretched, or "strained," to match that of the SiGe layer below. That increases the mobility of electrons in the transistor channel, producing faster transistor switching speeds.

In this application, the most significant measurement parameters are the thickness of the strained Si layer and the concentration of Ge in the strained Si, which can diffuse from the underlying SiGe during wafer processing and change the strain of the film. These measurements challenge designers because of the thinness of the strained-Si layer.

Metrology matters

Also, if Ge diffusion does occur, the interface between the strained-Si and SiGe layers can become indistinct, making it difficult to measure individual layer thickness.

Another important application for SiGe is raised source and drain, a fea-ture of advanced CMOS and MOSFET designs typically used in conjunction with strained silicon or silicon-on-insulator (SOI) processes. In this application, a layer of SiGe (500 to 800 angstrom ) is grown epitaxially on the shallow implanted source and drain region of the device. The SiGe is then heavily doped by implantation, forming a source and drain with the bulk of the structure above the active device area (see figure). The resulting ultrashallow junction reduces junction capacitance, while the heavily doped SiGe region provides low contact resistance. Both advantages allow for effective device scaling.

For this application, control of the SiGe thickness and Ge concentration are most important. Because of the potential for local pattern-related effects on SiGe deposition, measurements for this process should be made on product wafers rather than on monitor wafers.

Secondary ion mass spectrometry (Sims) is an offline analytical technique that can provide independent measurements of film thickness and Ge concentration on each layer with a high degree of sensitivity-typically parts per billion-and accuracy within 5 percent for concentration and 2 percent for depth. Developers often consider Sims to be the benchmark technique for thickness and elemental composition measurements.

A typical scan, however, can take an hour or more and results in the destruction of the device, making the technique inappropriate for production monitoring. Further, Sims suffers from measurement artifacts, such as enhanced count rates at interfaces, which can distort the true SiGe concentration profile of a multilayer film stack.

X-ray techniques have been used for many years to characterize semiconductor materials in a laboratory environment. These techniques have excellent intrinsic measurement capability but historically have lacked the high throughput, small spot, ease-of-use and low cost of ownership required for production monitoring. The primary X-ray methods can be broadly divided into three techniques: X-ray diffraction (XRD), X-ray reflectometry (XRR) and X-ray fluorescence (XRF).

Closer look at X-ray options

XRD provides the most comprehensive information on film characteristics, since stress, thickness and Ge concentration can be theoretically extracted from diffraction spectra. Small-spot systems are only now becoming available thanks to recent advances in X-ray hardware and algorithm software. But these systems have a high cost of ownership and have not been fully proved in a production environment.

XRR, meanwhile, can provide information on multilayer thickness, density and interfacial and surface roughness, both independently and simultaneously. But XRR typically suffers from a poor signal-to-noise ratio, since the electron density contrast between the various SiGe layers is low. This results in poor sensitivity to small changes in layer thickness. New small-spot systems are becoming available, but significant challenges remain in making this technique production-worthy.

Finally, XRF has been in production for more than 10 years and can be used to measure Si and Ge composition individually. But the technique is limited to measurements on single layers on unpatterned monitor wafers. That is because X-rays penetrate several microns into the substrate, in turn causing the Ge signal that is collected from all of the layers to be counted as a single, "lumped" Ge signal.

Optical edge

Optical metrology is the most widely accepted method of monitoring thin films in advanced semiconductor production. Measurements are non-destructive, and throughputs are typically much higher than for other metrology techniques-both of which are key to enabling use on the production floor.

Of the various optical measurement techniques available today, spectroscopic ellipsometry (SE) is the most capable for meeting the requirements for monitoring silicon germanium film stacks. Until recently, however, conventional SE systems had difficulty in achieving a high correlation (within 1 percent) to offline analytical techniques such as Sims.

This difficulty was primarily due to errors in the optical spectrum arising from component-level optical aberrations. Such errors make it difficult if not impossible to distinguish between changes in thickness and Ge concentration.

That difficulty, in turn, results in poor measurement sensitivity to real excursions in the fab.

Nonetheless, by carefully designing out these intrinsic aberrations in the optical train and coupling them with improved measurement calibrations and algorithms, thin-film metrology systems based on spectroscopic ellipsometry can now achieve exceptional precision (0.5 to 1.0 percent, 1 sigma for thickness; 0.1 percent, 1 sigma for concentration) and tool-to-tool matching (approximately 1.0 percent for thickness and ~0.5 percent for concentration).

These results are achieved on product wafers with extremely small spot sizes (down to 40 x 40-micron measurement area) and at throughputs of between 40 and seventy 300-mm wafers per hour, depending on the number of layers being measured. A current constraint of SE is that measurement accuracy depends on knowing the optical constants of the films to be measured, which may need to be determined experimentally. Once the baseline optical properties are determined and the measurement recipe is calibrated, however, SE provides accurate, robust and cost-effective measurements on product wafers.

Because of the sheer number and disparate nature of SiGe measurements, no single metrology technology completely covers all requirements. With its advanced film characterization capabilities, for example, X-ray metrology shows significant promise for future SiGe applications, but the technology requires further refinement to sufficiently address its inherent speed, cost-of-ownership and maturity limitations before it can achieve status as a production-worthy line monitor.

Optical precision

Among the methods that are currently available, SE-based optical metrology comes closest in attaining the required precision and matching for critical film thickness and germanium concentration measurements at production-level speeds, provided that the appropriate calibrations have been prepared in advance.

Looking ahead, as the march to ever-smaller design rules drives even tighter film metrology specifications for advanced SiGe applications, the semiconductor industry will be faced with a critical challenge. To keep pace with Moore's Law, the industry will need to establish an optimal film metrology strategy that looks to provide the best possible measurement capability in a highly production-efficient and cost-effective manner, either through further refinements in existing film metrology techniques or through new, "leapfrog" technology developments.

Murali Narasimhan is senior director of marketing for the Films and Surface Technology Division at KLA-Tencor (San Jose, Calif.).

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