Characterization of the Thin Oxide-Nitride-Oxide (ONO) Structure Using Spectroscopic Ellipsometry
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ABSTRACT Spectroscopic ellipsometry has been used to characterize thin oxide/nitride/oxide (ONO) layer structures. Films were deposited on top of the Si/oxide/polysilicon filmstack using a low-pressure chemical vapor deposition (LPCVD) technique. Different approaches to modeling this structure are compared to select an adequate representation. Analysis of the data for the three-layer-with-abrupt-interfaces ONO model using an effective-medium or Cauchy approximation shows strong correlation of the parameters and ambiguity of the results. A more robust and physically plausible model of the ONO structure as an inhomogeneous oxynitride film with Gaussian distribution of the silicon nitride component is suggested. This interpretation of the ONO structure is supported by the results of Auger electron spectroscopy (AES) and transmission electron microscopy (XTEM). Introduction. An Oxide-Nitride-Oxide (ONO) structure on top of a Si substrate/Si0 2 /PolySi (OP) filmstack is widely used in semiconductor manufacturing of memory devices : for stacked cell capacitors in dynamic random access memory (DRAM), as interPolySi insulators in electrically erasable and programmable read only memory (EEPROM) and antifuses in field programmable gate arrays (FPGA). In general, the ONO structure is used as a dielectric insulator, in lieu of Si0 2 , in cases where the film thickness must be < 200- 300 A . Marginal optical contrast between ONO layers, further complicated by their small thickness, makes precise determination of the structure parameters very difficult and beyond the capabilities of nondestructive optical methods. Several methods using single-wavelength ellipsometry of etch-beveled surface [1], in-situ ellipsometry monitoring of ONO reactive ion-etching [2] and IR ellipsometry[3], were developed. These methods are either destructive or cannot be implemented for OP-ONO structures. Sputter profiling using electron spectroscopy - secondary ion mass spectrometry (SIMS) or Auger electron spectroscopy (AES) - has a depth resolution of -,•25 A, however sputtering rate depends on material stoichiometry which makes it difficult to quantify the results. The accuracy of transmission electron microscopy (TEM) is also limited by roughness of the interfaces and low optical contrast of the ONO layers. Spectroscopic ellipsometry is being increasingly used for characterization of challenging thin film structures and its application to the OP-ONO structure has been reported[4]. Polysilicon was represented using the effective-medium-approximation (EMA) and the measurements were made only on the complete OP-ONO sample. The discrepancy of the EMA representation can therefore result in additional errors in determination of the ONO parameters. 1029 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
Analysis of the wafers, taken at the sequential stages of the OP-ONO manufacturing process, and representation of the polysilicon optical constants using a harmonic oscillator (HO) model allowed a more
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