Volume 17, Issue 3, May 1999
- regular articles
- brief reports and comments
- rapid communications
- papers from the 17th north american conference on molecular beam epitaxy
- electronic devices
- quantum wells
- in situ monitoring and characterization
- techniques to influence epitaxy and devices
- growth of novel materials and structures
Index of content:
- REGULAR ARTICLES
17(1999); http://dx.doi.org/10.1116/1.590668View Description Hide Description
We have developed a method for producing alignment marks on a wafer using exposed photoresist. Alignment to the exposed pattern is possible without having to develop out the pattern, provided that there is enough contrast between the exposed and unexposed areas. The exposed pattern was easily found for alignment using the He–Ne laser alignment system in our stepper. Although the contrast between the exposed and unexposed resist was rather faint to the human eye, the contrast was greatly improved using a commercially available contrast enhancement layer, so this method can be used in contact lithography where multiple mask levels must be exposed prior to development. The ability to align to the exposed pattern prior to development is especially useful in stepper-based lithography. For example, wafers may be aligned using the stepper for multiple mask exposures or for multilevel resist schemes prior to pattern development. In addition, very thick resist can be exposed using multiple exposures and changing the depth of focus.
17(1999); http://dx.doi.org/10.1116/1.590669View Description Hide Description
A consistent frequency-space analysis of the effects of optical lithography resolution enhancement techniques including optical proximity correction, off-axis illumination, phase-shift masks, and imaging interferometric lithography is presented. The improvements for each of these techniques are directly related to the enhanced frequency-space coverage afforded. Optical proximity correction extends the frequency coverage out to where NA is the optical system numerical aperture and the exposure wavelength enabling ’s of in the context of the usual Rayleigh resolution equation There are many possible configurations for off-axis illumination. For a quadrupole oriented at 45° to the (x, y) pattern axes, the maximum spatial frequency is extended to or Adding pupil plane filters to ensure a uniform modulation transfer function and orienting the quadrupole along the pattern axes allows extension to frequencies of 2NA/λ or Phase shifts at the mask plane emphasize the high frequency image components by increasing the importance of the quadratic imaging terms and allow frequencies to and ’s out to Imaging interferometric lithography further extends the frequency coverage out to either or depending on the details with corresponding s of
17(1999); http://dx.doi.org/10.1116/1.590670View Description Hide Description
Imaging interferometric lithography (IIL) has been proposed recently to extend optical lithography (OL) to the linear systems resolution limit of optics. In this work, a comparison of IIL and OL with off-axis illumination (OAI) is presented with both numerical simulations and experimental verification. In OAI, the optical modulation transfer function decreases at higher spatial frequencies as a result of the inclusion of the same low frequencies in more than one of the incoherently related offset exposures. Pupil plane filters avoid this multiple counting of the low frequency components and improve the pattern fidelity. Different filters provide different tiling of spatial frequency space and result in significant differences in the final patterns. Results of different tiling schemes for both OAI and IIL approaches are discussed. Overall, IIL provides the most robust imaging results.
Amorphous carbon films for use as both variable-transmission apertures and attenuated phase shift masks for deep ultraviolet lithography17(1999); http://dx.doi.org/10.1116/1.590671View Description Hide Description
We describe the development of amorphouscarbon-C) filmsgrown by magnetron sputtering for use in optical elements for sub-0.25-μm deep ultraviolet (DUV) lithography. We have measured the transmittance of -C filmsdeposited onto quartz substrates as a function of film thickness, and find that the films are ideally suited for use in variable-transmission apertures that can be used to improve DUV process latitude: we can achieve essentially any transmittance desired in the range by controlling the film thickness in the range nm with subnanometer precision. We also find that the transmittance remains stable after prolonged exposure to high intensity DUV radiation. We describe a masked deposition technique to produce variable-transmission apertures using -C films of various thicknesses, and also discuss the use of these films in attenuated phase shift masks, given that we can simultaneously achieve transmittance and a phase shift of 180 at either nm or 193 nm.
17(1999); http://dx.doi.org/10.1116/1.590672View Description Hide Description
Exposure of resist with electron or ion beams is a common nanolithographytechnology which is used to fabricateelectronic devices and microstructures. The resolution mainly depends on the beam size and the resolution of the resist. We have developed two new high resolution organic resists, which are calixarene derivatives. 50 keV electron beams and 260 keV ion beams were used to expose the resist, and 10 nm resolution was achieved with the Gaussian electron beam. The electron beam sensitivities of the two resists were 7 and 0.7 mC/ By using them, we produced 10-nm-order resolution patterns, which we exposed with a Gaussian electron beam. We also achieved 10-nm-level resolution by using a low molecular weight (Mw=1100) polystyrene resist, almost the same Mw as that of the calixarene. The resolution of the polystyrene resist improved as lower molecular weights were used. Therefore, the resist resolution depends on the molecular weight or molecular size.
Formation and growth of on (001)Si inside 0.2–2 μm oxide openings prepared by electron-beam lithography17(1999); http://dx.doi.org/10.1116/1.590673View Description Hide Description
The formation and growth of inside 0.2–2 μm linear oxide openings and contact holes prepared by electron-beam lithography have been investigated. A thin, uniform epitaxial was grown inside 0.5 μm or smaller linear openings and 0.7 μm or smaller contact holes by both one- and two-step rapid thermal annealing processes. On the other hand, epitaxial and polycrystalline were found to form on silicon near the edge and central region, respectively, inside 0.6 μm or larger linear openings. The size effect of the oxide openings is correlated to the distribution of local stress induced at the oxide edge. The formation of CoSi at low temperature appeared to be retarded by the local compressive stress near the edge of the linear oxide openings. The relative ease in the epitaxialgrowth of near the oxide edge of the linear openings and of 0.7 μm and smaller contact holes is attributed to the thinness of the CoSi layer.
17(1999); http://dx.doi.org/10.1116/1.590674View Description Hide Description
A focused ion-beam system was used for structuring of silicon by local desorption of hydrogen from the HF-passivated surface. The native oxide which was formed on the depassivated regions was used as an etch mask for KOH. The etch contrast for Ga and Si ions was studied as a function of ion energy and fluence. With a fluence on the order of a feature height of 15 nm was obtained. The method was applied to produce 160 nm wide wires on a heterostructure.
Formation and micromachining of Teflon (fluorocarbon polymer) film by a completely dry process using synchrotron radiation17(1999); http://dx.doi.org/10.1116/1.590675View Description Hide Description
The development of a new fabrication technique of Teflon microparts using synchrotron radiation (SR) irradiation, the SR ablation process, was described. The anisotropicmicromachining and thin film formation of polytetrafluoroethylene, fluorinated ethylene propylene, and perfluoroalkoxy were demonstrated using the SR ablation process. The anisotropicmicromachining of Teflon with hole pattern of 2 μm diam was successfully performed, and the micromachining of Teflon with a high aspect ratio of 50 was achieved. Moreover, Teflon films with flat surface were formed at a high rate by the SR ablation of Teflon at the substrate temperature above
17(1999); http://dx.doi.org/10.1116/1.590676View Description Hide Description
A novel gas source replacing fluorocarbon feed gases has been developed for preventing global warming. The novel gas source was designed to generate fluorocarbon species from polytetrafluoroethylene by laser ablation. The species generated from the gas source were introduced into an electron cyclotron resonance (ECR) plasma employing Ar gases. To characterize the gas source, radical densities with and without plasmas were measured by infrared diode laser absorption spectroscopy. In the ECR plasma employing the novel gas source, radical densities were estimated to be of the order of The gas source has been applied to the selective etching of to Si using the ECR plasma. As a result, the etching characteristics by ECR plasma employing the novel gas source were equivalent to those by a conventional ECR plasma employing gas. Therefore, this novel gas source is applicable to etching processes for preventing global warming.
17(1999); http://dx.doi.org/10.1116/1.590677View Description Hide Description
Reactive ion etching of via holes for grounding of monolithic microwaveintegrated circuits has become the industry standard. It is well known that the via etch rate decreases as a function of decreasing via mask diameter as well as increasing etch depth. A model has been developed which relates the experimental etch rates in plasmas to the ion and neutral fluxes incident on the wafer. This model provides a useful tool for designers and process engineers to predict etch depths and average etch rates as functions of via diameter and total etch time.
17(1999); http://dx.doi.org/10.1116/1.590678View Description Hide Description
Inductively coupled plasma(ICP)etchingcharacteristics of GaSb and AlGaAsSb have been investigated in and plasmas. The etch rates and selectivity between GaSb and AlGaAsSb are reported as functions of plasma chemistry, ICP power, rf self-bias, and chamber pressure. It is found that physical sputteringdesorption of the etch products plays a dominant role in ICPetching, while in plasma, the chemical reaction dominates the etching.GaSbetch rates exceeding 2 μm/min are achieved in plasmas with smooth surfaces and anisotropic profiles. In plasmas,etch rates of 5100 and 4200 Å/min are obtained for GaSb and AlGaAsSb, respectively. The surfaces of both GaSb and AlGaAsSb etched in plasmas remain smooth and stoichiometric over the entire range of plasma conditions investigated. This result is attributed to effective removal of etch products by physical sputtering. For a wide range of plasma conditions, the selectivity between GaSb and AlGaAsSb is close to unity, which is desirable for fabricating etched mirrors and gratings for Sb-based midinfrared laser diodes.
Dry oxidation resistance of ultrathin nitride films: Ordered and amorphous silicon nitride on Si(111)17(1999); http://dx.doi.org/10.1116/1.591101View Description Hide Description
The integration of high-dielectrics, such as in metal–insulator–semiconductor structures requires the careful control of interfacial silicon-oxide growth to preserve electrical properties such as capacitance. This has led to the introduction of silicon nitride films as oxidationresistant barriers between the dielectric and silicon. In this study, we examine the oxidationresistance of ultrathin nm) nitride films produced from exposure in ultrahigh vacuum are examined with in situ scanning tunneling microscopy and x-ray photoelectron spectroscopy. We find that ordered nitrides, grown at 950–1000 °C, are resistant to oxidation up to 600 °C for subatmospheric oxygen exposures. Amorphous nitrides, grown at 600 °C, exhibit similar oxidationresistance behavior.
17(1999); http://dx.doi.org/10.1116/1.590679View Description Hide Description
Si-selective epitaxialgrowth (Si-SEG) with gas is performed on clean Si(001)-2×1 open linear windows in an ultrathin mask formed by electron-beam-induced selective thermal decomposition (EB-STD) to fabricate a Si nanowire. We demonstrate that Si-SEG is possible in a 15-nm-wide Si(001)-2×1 open linear window formed by EB-STD. However, the width of the Si wire increases with Si growth, because the oxide mask decomposes thermally at the oxide/Si(001)2×1 boundary. An ultrathin oxide layer grown at higher temperature is effective to suppress the widening of the Si wire, even if the oxide thickness is not changed.
Adsorption of atomic hydrogen on the Si(001) 4×3-In surface studied by coaxial impact collision ion scattering spectroscopy and scanning tunneling microscopy17(1999); http://dx.doi.org/10.1116/1.590680View Description Hide Description
Using coaxial impact collision ion scatteringspectroscopy,scanning tunneling microscopy(STM), and low-energy electron diffraction techniques, we have investigated the interaction of atomic hydrogen with the Si(001) 4×3-In surface phase. During this interaction, Si–In bonds are broken and replaced by Si–H bonds. As a result, the 4×3 reconstruction is destroyed and In atoms form small clusters on the hydrogen-terminated Si(001) surface. The indium clusters are found to be not monocrystalline but, rather, polycrystalline. This is in contrast with other metal adsorbate/Si systems such as H/Ag/Si(111), H/Pb/Si(111), and H/In/Si(111) where metal atoms are known to form epitaxial monocrystalline islands. The size of the In clusters increases with increasing substrate temperature during hydrogen exposure of the 4×3-In phase. Using STM, we have confirmed that, as a result of atomic hydrogen interaction, the Si substrate becomes bare and forms the Si(001) 4×1-H phase, indicating that the underlying Si layer in the Si(001) 4×3-In surface phase is reconstructed with 4×1 periodicity. A structural model of the Si(001) 4×1-H surface has been proposed.
Microstructure characterization of oxidized nanocrystalline Si:H film by transmission electron microscopy17(1999); http://dx.doi.org/10.1116/1.590681View Description Hide Description
Oxidizednanocrystalline (nc) Si:H film has been produced by plasma enhanced chemical vapor deposition with a high hydrogen diluted silane and followed by thermal oxidization in a conventional furnace at a relatively low temperature of 800 °C. The resulting films were studied using cross-sectional transmission electron microscopy, high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. It is found that nc-Si dots embedded in an matrix can be formed by the above method and some of them exhibit the columnar growth morphology. Both the nc-Si:H deposition and the later thermal treatment are revealed to contribute to the film microstructures. It is further suggested that by choosing optimum oxidation time, the size of Si nanoparticles can be well controlled.
17(1999); http://dx.doi.org/10.1116/1.590682View Description Hide Description
A new method for silicon-on-insulator (SOI) is presented that has very few stacking faultdefects and produces multiple layers of single crystalsilicon surrounded by thermal The technique requires selective epitaxialgrowth,epitaxial lateral overgrowth, and chemical mechanical planarization to form SOI islands stacked in multiple layers. Islands of silicon as small as 150×150×40 nm thick were fabricated. Larger SOI islands in two SOI layers, with grown vertical interconnections between layers, were 5×500×0.1 μm. Only one stacking fault was observed in 85 000 μm2 of the first layer and none in the second layer. P-channel metal–oxide–semiconductor field effect transistors with gate lengths of less than ∼100 nm were fabricated in the thin SOI islands. They had normal current–voltage plot characteristics with less than 0.2 pA/μm of leakage current, illustrating the quality of the material in both SOI layers and at the silicon to thermal-oxide interfaces. The devices had measured subthreshold slopes of 76 mV/decade and good saturated current drives.
Role of film conformality in charging damage during plasma-assisted interlevel dielectric deposition17(1999); http://dx.doi.org/10.1116/1.590683View Description Hide Description
While observations of charging damage during plasma-assisted deposition have been erratic thus far, concern abounds that it may worsen as aspect ratios increase and high-density plasmas are used more frequently. Simulations of pattern-dependent charging during interlevel dielectricdeposition reveal that the initial conformality of the dielectric film plays a crucial role in metal line charge up and the subsequent degradation to the buried gate oxide, to which the metal line is connected. For moderate aspect ratios, significant charging damage occurs for nonconformal step coverage.
17(1999); http://dx.doi.org/10.1116/1.590684View Description Hide Description
A diode structure consisting of a polar epilayer on a nonpolar substrate grown by metalorganic vapor phase epitaxy often faces problems of antiphase domain formation in the polar semiconductor and cross diffusion across the heterointerface. Ge outdiffusion into GaAs epilayers was studied by low temperature photoluminescence spectroscopy after etching the film from the surface. The absence of p-n junction formation inside the Ge substrate from interdiffusion of Ga and As has been studied by current-voltage characteristics using mesa diodes. These observations were confirmed by electrochemical capacitance voltage polaron profiler and secondary ion mass spectroscopy techniques. To understand the material quality and current conduction mechanism across the GaAs/Ge heterojunction,I-Vcharacteristics of Si-doped n-GaAs/n-Ge isotype heterojunctions using Au Schottky diodes have been studied for different doping densities. A plethora of growth conditions appear in the literature concerning the attempt to grow antiphase domain (APD)-free GaAs on Ge. In the present case, even though the growth temperature regime is close to reported values, the main difference in minimizing APD formation may arise from the growth rates (∼3 μm/h) and the V/III ratio (∼88).
17(1999); http://dx.doi.org/10.1116/1.590685View Description Hide Description
High throughput epitaxial wafer production is demonstrated by using a newly designed multiwafer gas sourcemolecular beam epitaxy apparatus. The actual application data show excellent results of uniformity, cost performance, and material performance through practical mass production operation. Electron mobility as high as 124 000 cm2/V s is obtained at 77 K for a 7-μm-thick GaAs layer with a carrier concentration of A typical surface defect density of 25 cm−2 is achieved for continuously grown 1.7-μm-thick metal–semiconductor field effect transistor (MESFET) structures. The uniformity of sheet resistance in n-GaAs and AlAs mole fractions in AlGaAs is less than 2.0% (1.5% and 0.27%, respectively) over a 27 cm diameter area. A quantitative throughput number for a typical growth of a MESFET structure is four 4 in. or seven 3 in. diameter wafers per 2.5 h in a continuous process flow.
Improvement of properties of dynamic random access memories capacitors by plasma doping process after the formation of hemispherical-grained silicon17(1999); http://dx.doi.org/10.1116/1.590686View Description Hide Description
A novel technology plasma doping process” is used to improve the reduction of ratio due to the depletion layer generation on the surface of storage electrode. The storage electrode with hemispherical-grained silicon (HSG–Si) made from amorphous silicon(a-Si) with low concentration of phosphorus has no bald defect but low ratio. In plasma doping process, phosphorus diffuses in Si by interacting with vacancies. Using the storage electrode with HSG–Si doped in plasma, the capacitor has 96% of ratio and no degradation of capacitorproperties caused by excess phosphorus incorporation into dielectrics.