System Abandons Proxy Measurements

Directly or indirectly, several metrology problems originate from the fact that traditional proxy measurements (which measure thin films on scribe lines outside the die to correlate results to actual electrical phenomena within) are increasingly inadequate for production film control, because die have different pattern densities unrepresented by scribe lines. Other problems include airborne molecular contamination (AMC) and the need to measure film stress as a 2-D map across the wafer.

Additional challenges are new materials, which require different measurement parameters. Multilayer ultrathin films of high-k or ONO dielectric stacks must be measured simultaneously, because they often interact.

Visible light (as well as parts of UV) provides insufficient information to distinguish films of slightly different compositions. These materials' bandgap lies in the deep UV (DUV) region. By extending this spectrum, composition could be measured optically. However, this requires a vacuum ambient; otherwise, moisture and oxygen absorb energetic photons <190 nm, because these bonds' molecular resonance frequency is approximately the same.

KLA-Tencor (San Jose) has introduced its SpectraFx 200, a production films control platform. Its optional module, the 150 SE system for 150 nm spectroscopic ellipsometry (SE), maintains the wafer at atmosphere while its optics and light path are in a controlled atmosphere that eliminates ppm levels of oxygen and moisture. This results in high photon counts in the <190 nm region, enabling vacuum UV SE applicable to material composition monitoring. This is important to DRAM manufacturers using ultrathin Al₂O₃ high-k films and HfO₂ laminates. The module measures the thickness and composition of the HfSiO₄, and the interfacial layer's thickness between it and the silicon, which controls the high-k gate oxide's properties.

Another application is to monitor ARC layers for 193 nm lithography. Designed to prevent reflections, an ARC layer typically has <3% reflectivity, making it virtually black to instruments and requiring a high photon count. Under atmospheric conditions, oxygen and water molecule attenuation deplete the photon count, while the film absorbs most of the photons producing a negative signal-to-noise condition. The module's ambient provides a three-fold increase in actinic photons at 193 nm, and of an order of magnitude at 150 nm, improving throughput and measurement precision.

Because patterned die films have different thickness or composition caused by topography and loading effects, proxy measurements are not useful — in-die measurements are needed. Traditionally, films are measured on product wafers on a copper pad in the backend, underneath the dielectric films, to reflect light. Because copper CMP dishing effects on the pad can result in excessive topography, a copper grating instead of a pad is preferred underneath dielectric films. However, a grating can produce unwanted signal "noise" caused by light reflection and diffraction. Since it is difficult for traditional algorithms to separate that noise, measurements made on top of the grating may be undependable. The platform has algorithms that simulate the grating underneath, using a remote coupled wavelength approximation approach that improves measurement accuracy on top of a grating. This dielectric pattern metrology provides the capability to measure on top of underlying patterns that can also be used for measurements inside the die in critical yield-limiting areas.

With ultrathin layers or stacks, a 2 or 3 Å difference in thickness created by AMC condensation on wafer surfaces can become significant since, typically, oxides today are down to 20 Å, requiring control to within 10%, or ±2 Å. The platform has a laser desorption system that desorbs the AMC deposited on films. Then, a single-wave ellipsometer or SE measures the correct thickness. Enabled for all SE measurements, the system works on multilayer as well as ONO stacks, monitoring ultrathin film stacks used in 65 nm production.

AMC distorts measurements at 90 and 65 nm nodes and alters device capacitance and leakage performance. AMC layers are typically 0.1-3 Å. (Source: KLA-Tencor)

Stressed low-k films are becoming common in the BEOL. These can crack and delaminate, yet are still checked with monitor wafers lacking discontinuous films, patterns, or complicated film stacks. The present measurement limit is a single-line axis scan and global fit to arrive at a fit number for film stress. Where before an average number was sufficient, now a 2-D map of all wafer die locations is required. The system can be programmed with any spatial pattern over the wafer — monitor or patterned — providing a contour-maplike 2-D representation of film stress. It can cross-correlate a failing die to a certain measured location, showing hot spots that might be otherwise missed by 1-D scans.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt