Advances in Thin Film Measurement

		At a Glance
	Thin film measurement of layers <200 Å and complex transparent (dielectrics) and opaque (metals) stacks are pushing the technological limits of metrology systems. Confronted with the limitations of series resistance measurements, the development of photoacoustic measurement techniques represents a new avenue for characterizing thin metal layers. Presented in this article is an overview of old and new technologies and how they address current demands.

Scaling devices to accommodate shrinking linewidths is driving a trend toward thinner layers <200 Å and more stacks to engineer electrical and mechanical properties and speed the fabrication process. The layering of thin films, both metals and dielectrics, provides the flexibility to balance and optimize numerous parameters such as adhesion, conductivity, diffusivity and stresses to prevent wafer bending. Precise control over deposition processes, patterning, etching and chemical-mechanical polishing (CMP) is critical in meeting performance and yield objectives (Table 1). Thickness measurements, in particular, play a significant role.

Transparent films

Current optical metrology for transparent films provides noncontact thickness measurements on films ranging  from 1 nm to hundreds of nanometers. Optical techniques irradiate the film surface with a polarized or unpolarized light source of single or multiple wavelengths. Each technique measures the periodic variations in the intensity of reflected light either with changes in polarization and/or changes in wavelength to characterize films.

Ellipsometry, a well-established technique, uses a linearly polarized incoming beam that, upon reflection from the sample, becomes elliptically polarized. By analyzing the polarization change in the ellipsometric parameters over a wide spectral range, information about the physical characteristics of the layer can be obtained. Phase change information makes ellipsometry sensitive to the physical properties of extremely thin films on the order of tens of angstroms, such as gate oxide. The thickness of the film and the optical constants such as refractive index (n) and extinction coefficient (k) can be determined from a comparison of calculated and measured data.

During the past three years, the use of production ellipsometry for analyzing complex film stacks has expanded significantly. Tool manufacturers such as KLA-Tencor (San Jose, Calif.), Nanometrics (Sunnyvale, Calif.), Rudolph (Flanders, N.J.), SOPRA (Acton, Mass.) and Therma-Wave (Fremont, Calif.) provide fully automated thin film  characterization tools that can routinely measure oxide films <40 Å thick with precision better than 0.1% and sub-100 Å ONO stacks with precision in the 1% range.

In principle, thin metal layers <500 Å are considered semitransparent and can be characterized with spectroscopic ellipsometry (SE), noted Jean-Claude Fouere, vice president and general manager of SOPRA. For example, in copper interconnects, thin seed layers of copper used in the electroplating process are typically <500 Å and can be characterized with the SE technique. However, metal layers are generally >1000 Å thick and considered opaque. For this application, new techniques have been developed.

Picosecond ultrasonic laser sonar technology uses ultrafast lasers to characterize metal films and stacks. (Source: Rudolph Technologies)

Other issues

Currently, gate dielectrics are advancing to the 2-4 nm range, and according to Dr. Jeff Bindell, manager for material characterization at Cirent Semiconductor (Orlando, Fla.), discrepancies are already occurring between various thin film metrologies. Even ellipsometric tools based on different configurations do not consistently agree, making tool matching a challenge. At a few nanometers of oxide, surface roughness can vary thicknesses from one point to another. These differences can arise from what is essentially atomic-level layer thicknesses and interfacial layers that may be as much as 30% or 40% of the projected gate oxide thickness. Though the discrepancies are small, when they exist from one tool to the next, the following question must be asked: Which is the right answer?

While optical techniques are non-destructive and fast, transmission electron microscopy (TEM) may resolve the issues of atomic-level films, Bindell contended. Though labor-intensive and destructive, TEM is the only technique that describes what is happening at the atomic level. For film thicknesses of a few nanometers, TEM can provide an understanding of the structure. A model can then be designed for subsequent optical characterization.

Consistent processing will relieve some of the measurement pressure. Even when an actual number cannot be determined, feedback from device performance provides the required control. Once the process is established, tool matching is essential. No matter how tight the process, however, it is still necessary to know the actual layer thickness and understand physically what the measurement really means, noted Bindell, and standards and calibration are a necessary part of this process. At present, there are no nationally accepted standards for these ultrathin materials. The National Institute of Standards and Technologies (NIST), industrial analytical laboratory workers and metrologists at SEMATECH as well as various tool manufacturers are currently trying to address this issue.

Metal films

Below a quarter micron, filling high-aspect ratio structures and making very thin conformal via linings become a bigger issue, impacting electrical and contact resistance. By 1999, metal effective resistivity is expected to drop from 3.3 µmW -cm to 2.2 µmW-cm, which means high conductivity materials such as copper and thinner layers.

Click to see larger image.
Calculated Layer Thickness Layer Thickness (Å) TiN 507.53 Ti 49.63 AlCu 2309.00 TiN 106.89 Ti 393.59	1. This non-destructive technique can characterize each layer of a TiN/Ti/AlCu/TiN/Ti film stack. (Source: Rudoph Technologies)

To control electromigration, promote adhesion, improve mechanical durability and act as barriers to diffusion into adjacent insulating layers requires the deposition of thin layers placed on both surfaces of the conducting layers. These cladding layers are typically composed of Ti and TiN, with thicknesses on the order of 200 Å. Aluminum layers can range in thickness from 2500 Å to 5000 Å. Absolute thickness is critical in controlling gate dielectric capacitance and contact resistance; uniformity is essential for maintaining planarity and unvarying conductivity a-cross the wafer for balanced delays between circuit elements.

The traditional method for measuring metal film thickness is sheet resistance (R_s). The four-point probe technique passes a current through the layer and measures electrical resistance. Knowing the layer's bulk resistivity (R_B ), a film thickness (t = R_B/R _s) is inferred. However, for very thin layers, the thickness of the film itself begins to influence the bulk electrical conductivity of the material. Boundary scattering occurs when electrons that would normally bounce off atoms as they travel through the thin film instead go through to the cladding layers. At this point, the simple relationship between film thickness to series resistance and bulk resistivity breaks down. The usefulness of this equation depends on the conductivity of the metal. For higher conductivity metals such as Al, 300 Å or 400 Å is the limit of the assumption but may be 100 Å or 200 Å for Ti. For copper, the equation fails sooner, perhaps 1000 Å. In such situations, series resistance results for thickness measurements may be misleading.

Eddy current probes, an inductively coupled resistance measurement, is a noncontact approach to obtaining sheet resistance information. An AC coil is positioned close to the wafer and induces eddy currents in the film. One possible drawback is the restrictive range of film thicknesses this technology can measure, a lower limit of many 100s of angstroms as well as an upper limit.

X-rays 

X-ray beams can be focused to a spot size ~200 µm in diameter, making them suitable for metal film measurements on product wafers. X-ray fluorescence (XRF), for example, uses X-rays to excite the electrons of atoms in the film. These excited electrons then drop down into the ground state, emitting X-ray photons whose energy is indicative of the identity of the atom itself. Layer thickness is inferred from the comparison of the initial to the emitted X-ray flux. A challenge may occur when measuring films with mixed compositions; there may be some ambiguity about the location of Ti in the Ti or TiN layer.

Click to see larger image. 2. Data from an ECD copper layer, collected in ~1 sec, yield a thickness of 5185 Å with ±2 Å repeatability. (Source: AIS)

X-ray reflectivity is similar to optical reflectivity. The X-ray reflectivity from a thin film will vary as a function of the angle of incidence or wavelength. The result is the formation of Bragg-like peaks corresponding to constructive interference from the top and bottom surfaces of the film. The angular positions of these reflectivity maxima, after correction for refraction of the X-rays in the film, can be used to calculate the thickness and density of the thin layer. A stack of two layers can sometimes be modeled and measured by this technique, but it primarily applies to single-layer structures.

Photoacoustic technology 

A new technology, photoacoustics, is proving to be a "tremendous breakthrough in metal thin film measurement," said SEMATECH fellow Alain Diebold. Since opaque layers are impervious to light, to probe metal films, companies like Rudolph Technologies and Active Impulse Systems (AIS, Natick, Mass.) use light-induced sound pulses.

Click to see larger image. 3. Contour map of a PVD copper/oxide/silicon layer thickness extends to within microns of the films edge. (Source: AIS)

Rudolph's MetaPULSE (lead photo) employs light to generate picosecond sound pulses to launch an acoustic pulse vertically into the film stack. When reflected back, a slight change in the reflectivity occurs at the surface. A probe pulse, split off from the excitation pulse, is delayed, and the reflected light from the probing pulse is measured. The time it takes for the excitation pulse to come back (echoes) to the surface is used to deduce the film thickness (Fig. 1). This noncontact technique has a spot size <8 µm, which can probe structures on patterned wafers and can measure layers <20 Å, noted Dr. Robert Stoner of Brown University (Providence, R.I.). Test pads located within the scribe lines on product wafers generally measure 60 µm or less on each side. A manufacturing benefit is the capability of this system to measure metal stacks of up to six or more films, making this technique appropriate for metal cluster tools.

AIS's InSite 300 uses crossed-laser pulses to launch acoustic waves that form a transient "grating" pattern at the film surface. Time-dependent diffraction of probe light by this pattern reveals acoustic oscillations (Fig. 2) that yield the acoustic frequency and speed of sound, which is a sensitive function of film thickness. Professor Keith Nelson, department of chemistry at MIT (Cambridge, Mass.), explained that the complete time-dependent signal is recorded after each laser shot, and 1 sec of data collection yields angstrom repeatability. InSite 300 units are currently used for contour mapping, edge profiling and determining the usable diameter of a metal layer. The small spot size is ideal for use on test pad or scribe line structures on product wafers.

More photoacoustics

In situ metrology provides immediate feedback on the process and will play an increasingly important role in device manufacturing. Since photoacoustic systems can operate through glass and into a vacuum, they may be placed anywhere that optical access to the sample can be obtained. The modular head used in the InSite 300 is currently being adapted for in situ film measurements, stated John Hanselman, president of AIS. The head could be mounted onto a cluster tool and could operate as far as 6 in. from the sample. The versatile photoacoustic technique can also be used in a sensor-type application to measure film thickness in metal CMP.

In addition to thickness measurements, photoacoustic technology can simultaneously measure density and a number of the optical properties of metal films. According to Stoner, one area still in development is measuring films of mixed composition and doing this in parallel with film thickness.

New materials

Improved conductivity and control of electromigration are the primary forces behind the transition from aluminum to copper. Since copper diffuses through Ti/TiN barrier layers, new materials such as TaN and WN will be required. However, these layers do not deposit with easily predicted stoichiometry and density calling for techniques such as XRF, which measures atomic composition directly and can provide data on density as well as thickness.

4. Integrated into a cluster tool, thickness monitoring systems can provide the accuracy of a stand-alone, but with high throughput. (Source: Nova)

Characterization of copper is in progress. In conjunction with SEMATECH, AIS is characterizing thickness uniformity, edge profile and usable diameter of seed and copper films using the InSite 300. A contour map of a physical vapor deposited (PVD) copper layer shown in Figure 3 reveals significantly different characteristics from mappings of copper layers produced through CVD or electrochemical deposition (ECD).

Intended to increase packing densities, low-k dielectrics are needed to reduce the capacitance between the lines vertically, as well as in the horizontal plane. Unlike processing oxide layers for interlayer dielectrics (ILD) where a wealth of experience exists, low-k materials present many unknowns. There are three classes of low-k dielectrics being investigated: porous, polymeric and CVD-type materials. According to Stoner, the real challenge in measuring low-k materials is that they may be foggy materials like Teflon or materials that are optically ani-sotropic, having optical properties dependent on the angle of incidence.

To sort through the plethora of unknowns, work is currently under way to measure low-k materials. In particular, KLA-Tencor has successfully characterized paralyene, a birefringent, using SE, noted Gary Bultman, vice president and general manager of its film measurement division. Oxynitride, for example, has the same dispersion for both the horizontal and vertical planes. Parylene has different dispersions. No changes to the SE system were required, however -- only additional algorithms.

CMP

The variations inherent in CMP that are due to the unpredictability of the process, tool set and consumables reduce the process margin, making precise monitoring critical. CMP is typically used for ILD, shallow-trench isolation and tungsten polishing, and copper will soon follow. An ideal CMP metrology tool would be an endpoint detection system that provides good spatial resolution and direct measure of film thickness and can be applied to both metal and oxide layers. The challenge lies in resolving a signal through a dynamic mechanical/chemical environment.

In situ systems currently available have optical sensors embedded in the polishing head and measurements taken through the back of the wafer. Others use a window in the pad and a single-wavelength light source. The most challenging application is ILD, consisting of multiple dielectric and metal layers.

A recent development from Lam Research (Fremont, Calif.) is a multiwavelength, endpoint detection in situ metrology tool that measures thickness directly, rather than by way of an inferred signal or delta removal rate, stated Rahul Jairath, director of CMP technology at Lam. A single measurement is needed to acquire thickness information, and simultaneous measurements at various points on the wafer provide uniformity information across the wafer. Measurement occurs during the polishing process and is transparent to the process itself, Jairath added. The real-time monitor system is expected to reduce the mean time to test by 50%. Comparisons were made with and without Lam's in situ monitor, polishing dielectric layers on 100 wafers. Results indicate a factor of six improvement in accuracy for thickness targets of 500 Å and an ~50% improvement in repeatability.

Stand-alone systems still provide the best resolution; however, they pose limitations on throughput. Systems such as Nova Measuring Instruments' (Rehevoth, Israel) integrated thickness monitoring system (ITM, Fig. 4) can provide the accuracy of a stand-alone but with high throughput by implementing the cluster tool concept. A central handling mechanism transports the wafer between the measurement tool, scrubber and output cassette. Thickness measurements are performed on wet wafers. In contrast to a stand-alone system, the ITM has a small optical head that moves instead of the stage, providing ~5X faster movement and 20% overall equipment efficiencies, noted Giora Dishon, managing director of Nova.