Reflectance-difference spectroscopy (RDS) is a recently developed normal-incidence optical probe that uses symmetry to enhance the typically low sensitivity of reflectance measurements to surface phenomena. In RDS, the difference between reflectances parallel and perpendicular to the two principal optic axes in the plane of the surface are determined experimentally by modulation techniques. Contributions from the bulk and randomly oriented surface species largely cancel in subtraction, leaving those from the lower-symmetry surface. Sensitivities of 0.01 monolayer to surface species have been demonstrated with averaging times of 100 ms. Being an optical probe, RDS is well suited either to the reactive, relatively high pressure sample environments in organometallic chemical vapor deposition (OMCVD) reactors or to the ultrahigh-vacuum environment of molecular beam epitaxy (MBE) chambers. This allows comparisons of various growth chemistries to be made. Our MBE results for the (001) AlGaAs system show that reflectance-difference (RD) signals respond to either surface chemistry or surface structure depending on photon energy, and can distinguish Al- from Ga-terminated surfaces. Our OMCVD results for the (001) GaAs-trimethylgallium-arsine system follow submonolayer coverage of reacted species and provide the first microscopic information about crystal growth by OMCVD. The time, temperature, and pressure dependences of this coverage show OMCVD growth to be kinetically limited by a combination of reversible excluded-volume chemisorption (at 26 kcal/mole) and subsequent irreversible decomposition (at 39 kcal/mole) of trimethylgallium at surface lattice sites. Further work, especially in combination with a probe such as spectroellipsometry that can detect optically isotropic species, is expected to lead to new understanding of crystal growth and better control of growth processes.
This paper reviews some recent developments in the use of contactless modulation spectroscopy techniques for the in-situ characterization of the growth and processing of semiconductors. Photoreflectance (PR) measurements at 600°C on GaAs and Ga0.82A1 0.08As have demonstrated the potential of this method for in-situ monitoring during growth. Investigations PR, electron beam electroreflectance and differential reflectometry have shown that post growth (processing) information can be obtained about very thin Ga1AlxAs/GaAs epitaxial films, Ga Al As alloy composition, deep trap states, surface electric fields and carrier concentrations, lattice-mismatched strain, ion-implantation and annealing, and sputtering. In addition, characterization of semiconductor heterostructures can be performed.
Low energy cathodoluminescence spectroscopy (CLS) is a powerful new technique for characterizing the electronic structure of semiconductor surfaces and interfaces. CLS provides information on localized states, deep level surface and interface defects, and compound formation at semiconductor interfaces. This electron microscopy technique provides direct identification of reaction-induced metal/semiconductor interface states, which evolve in energy and density with multilayer metal coverages of the particular metal, and extrinsic surface states due to lattice disruption. All of these interface states can play a role in Schottky barrier formation. Unlike surface science techniques sensitive to only the outer few monolayers, low energy CLS reveals electronic structure of "buried" interfaces.
This paper is a review of the characterization of III-V heterostructures by Raman spectroscopy and a discussion of the potential of Raman spectroscopy as an in situ technique for the monitoring of epitaxial growth. Raman spectroscopy can be used at the high temperatures of epitaxial growth, and can yield information on the thickness, doping, composition, and quality of the growing epitaxial film. Raman measurements of 111-V materials at temperatures typical of those used in epitaxial growth are presented.
The applications of optical emission spectroscopy and optical interferometry to the monitoring and control of plasma processes are considered. Results obtained by each method during reactive ion etching of a tri-level resist system and silicon dioxide are presented for several sets of etching parameters and a variety of mask patterns. Successful process control is illustrated by sub-micrometre size etched features in the tri-level system with aspect ratios up to 10:1. The value of each diagnostic tool is illustrated by reference to details and subtleties of the etch processes. Comparisons made between the informations obtained by each diagnostic method allow the complementary and often supplementary roles of optical emission spectroscopy and interferometry to be identified.
We present a summary of our efforts to develop a general purpose probe for plasma and other gas phase processing techniques based on laser absorption spectroscopy. This method offers several advantages for gas phase point monitoring, including line-of-sight optical access, small solid angle detection, absolute species concentration calibration and simple interpretation of spectra. Drawbacks to overcome include limited sensitivity due to laser technical noise and limited spectral coverage of high resolution tunable dye and diode lasers.
This paper discusses the design and operation of a multichamber integrated processing system with in situ surface analysis capabilities. The system has been designed specifically for the deposition of silicon based dielectric thin films by the process of remote plasma-enhanced chemical-vapor deposition (Remote PECVD), and for the formation of microelectronic device heterostructures. In order to achieve these objectives the system includes the following: (1) two substrate-introduction load-lock chambers; (2) a semiconductor substrate processing chamber; (3) a dielectric deposition chamber, specifically configured for the remote PECVD process; (4) a surface analysis chamber including Reflection High Energy Electron Diffraction (RHEED) and Auger Electron Spectroscopy (AES); and (5) inter-chamber substrate transfer in a UHV compatible environment. We will discuss the deposition chamber in some detail and describe the way in which it is designed to meet the requirements for the Remote PECVD process reactions. We also describe an auxiliary deposition/analysis system, which provides both deposition process diagnostics, Mass Spectrometry (MS) and Optical Emission Spectroscopy (OES), and thin film deposition by Remote PECVD. These two systems taken together have provided a research capability for: (1) identifying the deposition process reactions; and (2) fabricating elementary microelectronic device structures, such as MOS and/or MIS capacitors.
A multiple chamber molecular beam epitaxy (MBE) system has been used to investigate a novel material system: GaAs/ZnSe heterostructures. Growth of ZnSe on GaAs shows that two dimensional nucleation of ZnSe occurs only on As rich GaAs surfaces while island growth occurs on the Ga rich surfaces. Studies of the inverted interface, GaAs on ZnSe, reveal a special disorder and roughening at the interface. These results are explained as manisfestations of the electronic imbalance which exists at the ZnSe/GaAs interface. Also, improved ZnSe crystalline quality is achieved by the incorporation of thin epitaxial layers of AlAs or InGaAs between the GaAs substrate and the ZnSe layer. Finally, an assessment of the interface quality resulting from the transfer between growth chambers confirms that extremely high quality interfaces can be obtained by this multiple chamber process.
The molecular beam epitaxy (MBE) and the microstructural, optical, and electrical characterization of two technologically important heterojunctions, the CdTe/InSb and the ZnSe/GaAs, are described. The II-VI/III-V heterointerface is formed by the epitaxial growth of each layer in either separate MBE growth chambers, or by each layer growth occurring in a single growth chamber. For the case where separate growth chambers are used, the active interface is preserved by employing passivation techniques, or by transferring the sample between growth chambers in an ultrahigh vacuum transfer module. The aforementioned growth approaches allow for the formation of an 'epitaxial' heterojunction, to be utilized as an active part of a heterojunction device. The CdTe/InSb heterointerface is approximately lattice matched (<0.05% mismatch), and is motivated by possible device applications provided by InSb quantum wells. The low temperature growth of InSb quantum wells is achieved by the use of an antimony cracking oven to provide Sb2 molecules for the growth. No clear indication of mixed interfacial layers of In2Te3 is observed by Raman spectroscopy or transmission electron microscopy. The ZnSe/GaAs heterointerface, having a 0.25% lattice constant mismatch, has potential for use in passivation of GaAs devices. The highly resistive, stoichiometric ZnSe is employed as an insulator in two GaAs device configurations: a field effect transistor structure and a metal-insulator-semiconductor capacitor. Electrical characteristics of the ZnSe/GaAs interface provide evidence of the electrical integrity, with measurements of the interface state density resulting in numbers comparable to those reported for the (A1,Ga)As/GaAs interface.
There has been a growing interest in the application of low pressure plasma environments to III-V etching techniques. This interest has been generated by the need to etch sub-micron features with better linewidth and resolution than that which can be obtained with wet etching. This paper presents the initial results of combining RIE, the popular technique of etching III-V semiconductors, with MIE, a technique that has found considerable success in silicon processing. The combination of these two types of etchers to the fabrication of OEIC devices has yielded nigh performance electronic and opto-electronic structures. We present results that show initial calibration data taken from the MIE and RIE, and where etches are made by each system during the fabrication sequence. Using the MIE, undercuts in GaAs/A1GaAs have been formed beneath the gate to prevent source/drain shorting from ion implantation. The MIE is also used to define tungsten gates and silicon dioxide mesas in the DOES laser structure. The RIE has been used to define the DOES laser mesa with a photoresist mask. It has also been used with a wet chemical or MIE etch to form undercuts needed for the laser structure. Our goals are (1) to provide an undercut into the HFETs and HFETPDs barrier layer to prevent source/drain to gate shorting from ion implantation and similarly in the BICFET and DOES to prevent source to emitter shorting; (2) to provide a vertical undercut into the main mesa of the DOES laser which will aid in processing and the eventual etching of facets.
This paper develops a model of n-doped polysilicon etching in a SF6/C2C1F5 (Freon 115) plasma using a parallel-plate reactor. Plasmas created using a range, or space, of gas flowrates, pressure and applied R.F. power were monitored using optical emission spectroscopy. Polysilicon etchrates and etch anisotropy were then correlated to the relative intensity of many emission peaks using multiple linear regression techniques. The attempt was made to relate etch anisotropy to species in the bulk plasma that are not suspected to participate in the chemical etching of the polysilicon; ions that enhance the vertical etchrate by ion bombardment relative to the lateral etchrate, along with reactive ionic species. The resulting empirical model contains many factors (individual emission peaks) that may be handled by microprocessor controlled optical emission spectrophotometers to monitor the etching process or to model the feature profile with changes in etching parameters.
Glass is a popular substrate for flat panel display technology. Reactive ion etching (RIE) is a method for thin film device patterning but has not yet been widely applied to this technology. One complication is the effect of RIE on glass. A conventional glass contains a complicated structure that makes its etching characteristic very different from that of SiO2. Corning 7059 glass was reactive ion etched with CF4-02 plasma (1). The optimum condition was low oxygen concentration, low pressure, and high cathode self bias voltage. The etch bottleneck step was the removal of aluminum and barium oxides. In this paper both CF4, and CF3C1 were used separately to etch Corning 7059 glass under various conditions. The effect of Teflon material on the electrode during the etch is also examined.
The myth exists that plasma processing is synonymous with dirty processing. while plasma processing can be a dirty Process, it can also be an exceptionally clean process. Diamond etching by a Plasma process is, in tact, a cleaner process than experienced in any other semiconductor by any other means --- the residue (carbon monoxide) is gaseous and the etch recess/trough is exceptionally clean and undamaged. Artifact diamond films are grown by highly non-thermal equilibrium processes. Most of these processes are plasma processes. Virtually all types of plasmas have been used to synthesize diamond. Among the most-used types are R.F., D.C., and remote plasmas. Plasmas are also used to provide in situ cleaning of the substrate surface prior to growth. Although the diamond surface is one of exceptionally high surface energy (virtually nothing sticks to it) and requires comparatively little cleaning, the economical growth of diamond requires non-diamond substrates which do require cleaning. Artifact diamond grown by various plasma processes are compared with each other and with other growth methods.
Plasma enhanced chemical vapor deposition has been used in the semiconductor industry for device passivation and interlayer dielectric applications. The primary function of the glow discharge has been to decompose the reactants to allow lower deposition temperatures. However, in addition to reducing the deposition temperature, RF plasmas can be used to control film stress, improve step coverage and conformality, and obtain exceptional uniformity and process control. Moreover, process induced defects (hillocks, particulates and pinholes) can be greatly reduced by using high rate localized deposition and in situ reactor cleaning. Examples discussed include the deposition of doped and undoped Si02/ SiN and TEOS oxides.
The preparation of transition metal chalcogenides using classical techniques is often confronted with serious problems. We have studied the preparation of a variety of transition metal sulfides, as thin films and bulk powders, comparing thermal and rf plasma techniques. The rf plasma approach allows us to use a simple sulfurizing agent H2S to prepare materials at low temperature. Without the aid of the rf plasma we must use either exotic sulfurizing agents or higher temperatures, which may have detrimental affects on the materials. Specifically we have studied the preparation of TiS2, MoSx and VSx. In all cases we have been able to prepare films or bulk powders at temperatures below 2500C.
We have studied the chemical and physical properties of silicon oxide films plasma deposited from TEOS (tetraethoxysilane), to gain an understanding of the origins of (1) step coverage and (2) film stability. TEOS was diluted in helium/oxygen mixtures and deposited as a function of discharge frequency (150 kHz and 14 MHz) and 02 flow in a parallel plate reactor. The typical deposition conditions were 1 torr total pressure, 320°C substrate temperature, 1 -9% TEOS, 1 -80% 02, and -0.1 W/cm2 discharge power. Films deposited at high frequency with excess oxygen were generally oxygen-rich, chemically unstable and hygroscopic, while films deposited at low frequency were stable to moisture and slightly deficient in oxygen. However, coverage profiles of high frequency films showed an unusual degree of directionality, which could be used to advantage for the coating of high aspect ratio features. We suggest that a judicious combination of high and low frequency discharges may improve film properties while maintaining directional step coverage. Isotopic labeling experiments were performed using 1802 to gain insight into the origins of the oxygen that is contained in these PECVD films. Complete isotopic scrambling was not observed. Film composition data suggest that there is one tenacious Si-0 bond which remains with the silicon from the original TEOS molecule during the reaction to form Si02.
The effect of deposition conditions on the structure and composition of plasma deposited (PD) silicon nitride films was studied using X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). This study focused on the line position and shape of the Si 2p and N is XPS peaks and the Si KLL Auger peak. PD silicon nitride films were transferred directly from the deposition chamber into a surface analysis system, thereby permitting analysis of as-deposited films without modification due to ion sputtering. Post-deposition Ar+ milling of PD silicon nitride films at ion energies as low as 500 eV, caused preferential removal of NH species and was thus unsuitable for surface cleaning before analysis. All films oxidized upon exposure to ambient; however, those deposited at low temperature with large quantities of NH, oxidized extensively.