Film monitoring of engineered substrates
03/01/2005
The industry is seeing a gradual migration from standard silicon (Si) substrates to engineered substrates [1, 2]. The types of engineered substrates currently used in production and being studied for future use include silicon-on-insulator (SOI), strained silicon-on-insulator (sSOI), silicon germanium-on-insulator (sGOI), and their variations.
These materials create new requirements and challenges for monitoring the substrates and the films deposited on them. New requirements include measuring thin Si/SiGe single or multilayers on insulator, monitoring strain in the layer, and measuring the gate dielectric on SOI.
Metrology of engineered substrates
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Examples of process flows for both SOI and strained Si are shown in the table. There are multiple metrology steps in each process flow and several parameters are monitored. In the SOI process using wafer-bonding technology, the thickness of the oxide layer is monitored following oxidation, and both superficial Si and buried oxide (BOX) thicknesses are monitored at several steps, leading up to final inspection following CMP. The superficial Si thickness and uniformity are critical.
Similarly, strained Si formation using thick SiGe layers requires monitoring the thickness of the SiGe layers and the Ge concentration, along with the thickness of the strained Si layer. The strain in the Si layer can be inferred from the Ge concentration of the layer beneath, though it is desirable to have a direct measurement of the strain.
The “simplest” variant of SOI consists of unstrained superficial Si on BOX. With the push to fully depleted Si, this superficial Si layer is becoming thinner with each node - usually <200Å for fully depleted SOI and ~500-1500Å for the BOX. Since the process window and metrology scale with the thickness, a typical requirement for stability and matching for the thickness measurement is ~1% of nominal thickness, or 1Å for a 100Å-thick layer of superficial Si.
Reflectometry has been used by some substrate manufacturers for wafer qualification to measure the superficial Si in thick SOI films. With the migration to thinner superficial Si layers, and more stringent requirements to measure the superficial Si and BOX layers simultaneously with tight tolerances, spectroscopic ellipsometry (SE) is required. An example of thin superficial Si measurements using SE is shown in part a) of the figure. Here, the thickness of the superficial Si is measured on the same wafer through successive etch steps. Each etch step removes ~30Å of Si. Excellent correlation is seen between the etch process and the Si measurement even below 100Å of Si, demonstrating the accuracy of the SE measurement.
More complicated variants of the SOI structure currently are seen, largely in development. These include sSOI, where the strain in the Si is “frozen” during the wafer-bonding process prior to removal of the SiGe underlayer. In addition to thickness measurement, an estimate of the strain in such structures is required. The optical bandgap is a function of the strain in the Si; thus, the optical dispersion (variation of refractive index with wavelength) is also a function of the strain. It is therefore possible to establish correlations between the strain in the Si and the measurement of refractive index in the thin Si film. For accurate measurements of index in such thin films, SE is required. Efforts are currently underway to determine these correlations for thin Si films and quantify the capability of measuring strain using SE.
Strained Si layers formed on thick SiGe layers generate the templates from which the strained Si layer is transferred by the wafer bonding process to create the sSOI. From the process flow seen in the table, the thickness of the individual layers and the Ge concentration are parameters that need to be monitored. An example of an optical measurement of the strained silicon layer in such a structure is shown in part b) of the figure. A linescan measurement of the thickness of the strained Si layer from three wafers deposited under similar conditions is shown. The contours are similar for all three wafers, verifying the repeatability of the process.
Monitoring films on engineered substrates
Engineered substrates also introduce new measurement challenges for films deposited on the substrates. For instance, the gate-oxide thickness on Si substrates is monitored using single-wavelength ellipsometry (SWE) using HeNe lasers at 633nm. Thick Si substrates are opaque at this wavelength, and the measurement consists of a single parameter (thickness of gate oxide). The thin superficial Si layers used in SOI substrates are transparent at these wavelengths, however, instead making the gate-oxide measurement on SOI a multiparameter measurement.
In addition to the thickness of the gate oxide, the thickness of the superficial Si and BOX need to be determined because the optical response incorporates this information. Such multiparameter measurements are not possible with standard SWE. In this case, SE is the preferred technique. Part c) of the figure shows an example of a thin-oxide-on-SOI contour map. For accurate estimates of the oxide thickness and uniformity, a laser-based desorption technique was used to remove airborne molecular contamination from the oxide film prior to each measurement. The average thickness and uniformity seen for the gate oxide on Si were verified to be comparable to that seen on standard Si substrates using similar deposition processes.
Conclusion
With the use of thinner superficial Si for SOI and the introduction of thin strained Si layers, SE is becoming the method of choice for monitoring substrates. Thin gate-oxide measurements on SOI substrates pose new measurement challenges. An example of a solution with SE and laser-based desorption was presented.
References
- C. Malville, G. Celler, Yield Management Solutions, Vol. 6, No. 2, p. 6, 2004.
- K. Rim, et al., VLSI Tech. Dig., Vol. 98, 2002.
Arun Srivatsa received his PhD in materials science and engineering from North Carolina State U., and is a staff technologist in the Films and Surface Technology (FaST) Division of KLA-Tencor, 160 Rio Robles, San Jose, CA 95134; ph 408/875-2946, e-mail [email protected].