Ilsin An

Rotating-compensator multichannel ellipsometry: Applications for real time Stokes vector spectroscopy of thin film growth

A multichannel spectroscopic ellipsometer based on the rotating-compensator principle was developed and applied to measure the time evolution of spectra (1.5–4.0 eV) in the normalized Stokes vector of the light beam reflected from the surface of a growing film. With this instrument, a time resolution of 32 ms for full spectra is possible. Several advantages of the rotating-compensator multichannel ellipsometer design over the simpler rotating-polarizer design are demonstrated here. These include the ability to: (i) determine the sign of the p-s wave phase-shift difference Δ, (ii) obtain accurate Δ values for low ellipticity polarization states, and (iii) deduce spectra in the degree of polarization of the light beam reflected from the sample. We have demonstrated the use of the latter spectra to characterize instrument errors such as stray light inside the spectrograph attached to the multichannel detector. The degree of polarization of the reflected beam has also been applied to characterize the time evo...

Advances in multichannel spectroscopic ellipsometry

Abstract Since the development and perfection of automatic ellipsometers, an effort that began nearly three decades ago, spectroscopic ellipsometry (SE) has increased in popularity as a non-destructive tool for characterizing the optical properties and layered structure of bulk solids and thin films. With the more recent development of multichannel ellipsometers for high-speed spectral scanning, in situ real-time SE has emerged as a very powerful tool for characterizing the time evolution of the optical properties and layered structure of materials during preparation and processing. High-speed spectral scanning has also opened up the possibility of real-time monitoring and control for complex material systems that cannot be fully characterized using well-established real-time measurements at one (or a few) wavelength(s). In this review, we will first describe the most recent developments in multichannel, multiparameter SE, in which spectroscopic information is obtained in addition to the ellipsometric parameters (ψ, Δ). Then we will discuss recent applications of multichannel SE to amorphous semiconductor film growth in order to obtain material properties in complex multilayer and graded-layer device structures. In general in this review, the focus will be on recent examples of the characterization of complex optical systems of technological interest. Practical situations will be described in which the dielectric functions of films are deposition-dependent and evolve with thickness due to particle and grain size effects and intentional compositional gradients, and in which film growth occurs on rough surfaces, leading to optical models for film growth that require interface and surface roughness layers.

Waveform analysis with optical multichannel detectors: Applications for rapid‐scan spectroscopic ellipsometry

A unique rapid‐scanning ellipsometer employing a rotating polarizer optical configuration and a multichannel detector for a 1.5–4.5 eV spectral range, has been developed recently for real time studies of film growth and surfaces. This is a new application of the photodiode array‐based optical multichannel detector that entails waveform analysis of the incident irradiance at each photon energy. For accurate ellipsometric spectra {ψ(hν),Δ(hν)}, the raw data in the form of photon counts, integrated over four or more equal sectors of polarizer rotation, must be corrected for systematic errors originating from the detection system. Simple procedures are described to characterize and correct for the most significant errors including detection system nonlinearity, image persistence, and scattered stray light in the spectrograph/detector enclosure.

In situ determination of dielectric functions and optical gap of ultrathin amorphous silicon by real time spectroscopic ellipsometry

We have developed techniques to determine the near‐infrared to near‐ultraviolet dielectric function and optical gap of ultrathin amorphous silicon [a‐Si:(H)] using real‐time spectroscopic ellipsometry during preparation and processing. The techniques have been applied to ∼50 A a‐Si:H films prepared by plasma‐enhanced chemical vapor deposition, and to ∼250 A pure a‐Si chemically modified by atomic H exposure. For the latter, the time evolution of the bonded H content can be estimated along with the evolution of the gap.

Preparation of ultrathin microcrystalline silicon layers by atomic hydrogen etching of amorphous silicon and end‐point detection by real time spectroellipsometry

The etching of hydrogenated amorphous silicon (a‐Si:H) in thermally generated atomic hydrogen has been investigated in detail, utilizing real time spectroellipsometry for characterization and end‐point detection. When properly controlled, etching can yield ultrathin microcrystalline Si (μc‐Si:H) films of relatively high density on virtually any substrate material. These films are unique in that their microstructure is established by the crystallization of the near‐surface a‐Si:H, rather than by the nucleation of crystallites on the substrate, as occurs for plasma‐enhanced chemical vapor‐deposited μc‐Si:H films.

REAL TIME SPECTROELLIPSOMETRY STUDY OF THE INTERACTION OF HYDROGEN WITH ZNO DURING ZNO/A-SI1-XCXH INTERFACE FORMATION

Using real time spectroellipsometry (SE), we have studied the interfacial interactions that occur when i‐ and p‐type hydrogenated amorphous silicon‐carbon alloys (a‐Si1−xCx:H) are deposited from hydride‐containing plasmas onto transparent, conducting films of ZnO. The SE spectra collected during the nucleation of a‐Si1−xCx:H onto ZnO reveal a widening of the near‐interface optical gap of ZnO by ∼0.1 eV, an effect attributed to the penetration of atomic H from the plasma. The SE data, along with ex situ secondary ion mass spectrometry, reveal that the H diffuses into ZnO to depths ≳200 A. The defects that result from H incorporation in ZnO (e.g., O vacancies) lead to a shift in the near‐interface Fermi level higher into the ZnO conduction band and to an estimated enhancement in the electron concentration by ∼1020 cm−3.

Alignment and calibration of the MgF 2 biplate compensator for applications in rotating-compensator multichannel ellipsometry

Biplate compensators made from MgF2 are being used increasingly in rotating-element single-channel and multichannel ellipsometers. For the measurement of accurate ellipsometric spectra, the compensator must be carefully (i) aligned internally to ensure that the fast axes of the two plates are perpendicular and (ii) calibrated to determine the phase retardance delta versus photon energy E. We present alignment and calibration procedures for multichannel ellipsometer configurations with special attention directed to the precision, accuracy, and reproducibility in the determination of delta (E). Run-to-run variations in external compensator alignment, i.e., alignment with respect to the incident beam, can lead to irreproducibilities in delta of approximately 0.2 degrees . Errors in the ellipsometric measurement of a sample can be minimized by calibrating with an external compensator alignment that matches as closely as possible that used in the measurement.

Simultaneous real‐time spectroscopic ellipsometry and reflectance for monitoring thin‐film preparation

An expansion of the capabilities of high‐speed, multichannel spectroscopic ellipsometry (SE) is described that involves simultaneous measurement of the reflectance spectrum along with the two spectra in the ellipsometric angles (ψ, Δ). Previously, a novel rotating‐polarizer spectroscopic ellipsometer has been perfected that employs a photodiode array detector for high‐speed acquisition of (ψ, Δ) spectra, designed for real‐time studies of thin‐film growth. For a polarizer angular rotation frequency of ω0, the (ψ, Δ) values at a given photon energy are deduced from the 2ω0 Fourier components of the detector irradiance, normalized by the dc component. A third parameter, the weighted reflectance RA, can be obtained from the dc component and from a calibration based on the known optical properties of the substrate measured prior to film growth. With (ω0/2π)=12.5 Hz, three‐parameter data sets, [ψ(hν), Δ(hν), RA(hν); 1.5≤hν≤4.5 eV], can be acquired with a time resolution as short as 40 ms. Although RA provides c...

Real-time spectroscopic ellipsometry studies of diamond film growth by microwave plasma-enhanced chemical vapour deposition

Abstract We have applied real-time spectroscopic ellipsometry to monitor the growth of highly uniform, nanocrystalline diamond films by microwave plasma-enhanced chemical vapour deposition. In this study, a unique multichannel instrument is employed to collect full ellipsometric spectra from 1.5 to 4.0 eV. Here we focus on two capabilities. First, we will describe a method to calibrate the true temperature of the top 200Aof the Si substrate under diamond growth conditions. Second, we describe the full microstructural evolution of the diamond films. The parameters derived include the time evolution of the void and optically absorbing, non-diamond (sp 2 ) carbon volume fractions in the film. In addition, the nuclei, bulk and surface roughness layer thicknesses during the nucleation, coalescence and bulk growth regimes are determined. These results reveal reproducible and remarkably internally consistent behaviour that provides new insights into the growth mechanisms for nanocrystalline diamond. We find that in the coalescence process, a large volume fraction of sp 2 carbon is trapped in the grain boundaries under all conditions of growth. After coalescence is complete, further generation of sp 2 carbon is impeded under optimum conditions.

Real time spectroscopic ellipsometry characterization of the nucleation of diamond by filament‐assisted chemical vapor deposition

The recently developed technique of real time spectroscopic ellipsometry (SE) has been applied to characterize the nucleation of diamond on c‐Si by W filament‐assisted chemical vapor deposition, leading to improved control over the process. Specifically, techniques are developed which minimize W contamination at the diamond/substrate interface; calibrations are performed which determine the temperature of the top ∼250 A of the substrate under growth conditions; and alterations in gas flow conditions are implemented in response to diamond growth for a reduced induction time. With these procedures in place, real time SE provides the induction time, nucleation density, and mass thickness, and is in quantitative agreement with ex situ scanning electron microscopy.