摘要:

Although optical lithography has been extended to far smaller dimensions than was predicted 15 years ago, there are definite physical barriers to extending it to the minimum dimensions of 70 nm that are projected to be required 15 years from now. Both focused, point electron beams and ion beams have been used to write dimensions in resist well below 20 nm, albeit at speeds far too slow for production lithography. Projection systems, which employ a mask and, in effect, produce a large array of beams, can provide both small minimum dimensions and high throughput. Ions are particularly well suited for this because they suffer little or no scattering in the resist, the linewidth is not a strong function of dose (good process latitude), and the resist sensitivity is relatively independent to resist thickness or ion energy. IMS in Vienna, Austria has built two generations of ion projection lithography systems which have demonstrated many of the features needed for high throughput lithography. In these systems a stencil mask is irradiated with a uniform beam of light ions,H+,H2+,orHe+,and the transmitted pattern is demagnified (by10×to3×) and focused on a resist covered wafer at energies in the 70–150 keV range. So far, minimum dimensions down to 70 nm line-space pairs have been demonstrated, drift has been eliminated with a “pattern lock” servo system, and field distortion of less than 0.15 μm over8×8mm has been measured in agreement with calculations. Based on these achievements a new generation ion lithography machine has been designed which uses3×demagnification and will expose a20×20mm field at 0.12 μm minimum dimensions with less than 10 nm of distortion introduced by the ion optics. Global and stochastic space charge effects have been modeled and, in some cases, measured with the existing machines. Stochastic space charge effects will not cause unacceptable blur in the new design if the total ion current is kept below 3 μA. At this total current the time to expose one chip is still of order 0.5 s so that the calculated throughput is about 70 wafers (200 mm) per hour. Ion sources with low energy spread(∼2eV) have been developed and will provide uniform illumination of the mask. Stencil mask fabrication on membranes of 2.5-μm-thick silicon has been developed. Distortion of the pattern cut in the membrane due to stress relief has been modeled, and with proper mask design can be kept below 20 nm (6.7 nm on the wafer). According to calculations and measurements mask distortion due to ion beam heating can be reduced to a negligible level if a radiation cooling cylinder is used. As a result of the building and evaluation of the existing machines and the design of the next generation, significant progress has been made in ion projection lithography which we will review in this article. The next step in the development of ion projection lithography is being conducted by theMEDEAprogram in Europe, which will develop a full field processing tool.

ISSN:1071-1023    

DOI:10.1116/1.590052

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

We propose an improvedIn0.5Ga0.5P/GaAsheterostructure-emitter bipolar transistor (HEBT) grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The recombination atp-ninterface can be significantly reduced at low collector current using 100 Å undoped GaAs spacers. Meanwhile, the emitter edge thinning technique is used to reduce the surface recombination current. The passivated device reveals a current gain as high as 360, along with an offset voltage as low as 80 mV. Moreover, the measured results indicate that the current gains vary slowly with temperature. In order to demonstrate that the emitter edge-thinning technique can effectively reduce the surface recombination current, the emitter size effect on current gain is also investigated.

ISSN:1071-1023    

DOI:10.1116/1.590053

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

A process for fabrication of heterojunction bipolar transistors (HBTs) with selectively buried subcollectors by molecular beam epitaxy (MBE) regrowth is described. This process can be used to reduce parasitic base-collector capacitance of HBTs and improve the speed of these devices.In situetching by iodine prior to the regrowth was used for the first time to improve the quality of the substrate epilayer interface in a semiconductor device grown by MBE. The secondary ion mass spectroscopy depth profiles of regrown HBT structures suggest that thein situsurface cleaning by molecular iodine was not sufficient to remove all contamination from the substrate epilayer interface and that the microwave performance of HBTs fabricated by this process may have been affected by that contamination. The dc performance of devices which were fabricated by our process was not affected, however, by the contamination and was comparable to the dc performance of conventional HBTs. Our results suggest that the described process for fabrication of HBTs with selectively buried subcollectors by MBE regrowth would be feasible in conjunction with a more effective surface treatment.

ISSN:1071-1023    

DOI:10.1116/1.590231

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

In small signal GaAs metal semiconductor field-effect transistor (MESFET) fabrication technology where a gate recess is required, the optimum value of residual channel thickness is one of the most crucial parameters for high transconductance operation of the device. The effects of active channel thickness on device characteristics have been investigated. In a different strategy to that of previously reported work, in this study we have fixed gate lengthLgat 200 nm and varied the channel thickness,afrom 100 to 40 nm. This means that the shape of the depletion layer, which is a function of gate length, was largely fixed for all the devices. The variation in the device characteristics depended on the change in the active channel thickness, which modifies the field distribution inside the channel. It was found that, for optimum doping concentration, the value of transconductance increases in a parabolic fashion by decreasing the aspect ratio(Lg/a)of the device, and a highest value of transconductance is observed atLg/a≈2.This aspect ratio is 2–3 times lower than the conventional reported values.

ISSN:1071-1023    

DOI:10.1116/1.590054

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

We show that epitaxial layers suitable for fabrication of AlGaAs/GaAs heterojunction bipolar transistors with a carbon base doping level of up to1020cm−3can be grown by solid-source molecular beam epitaxy usingCBr4as a doping precursor. We have observed that the gain of devices fabricated from these layers is improved using an abrupt conduction-band discontinuity at the emitter–base heterointerface. The observed relationship between current gain and the base widthWbof these devices deviates from the1/Wb2dependence predicted by diffusive electron transport for base widths that are shorter than 60 nm. The relationship is better approximated by a1/Wbdependence, which is in better agreement with the theories of ballistic or quasiballistic electron transport. Current gain of the devices drops rapidly as the base thickness exceeds 60 nm.

ISSN:1071-1023    

DOI:10.1116/1.590224

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

We have studied the epitaxial growth of GaSe, a layered van der Waals material, on GaAs, a zinc-blende-structure semiconductor. This heterostructure exhibits a 6% lattice mismatch, and is a prototypical example of van der Waals epitaxy, where the weak van der Waals interaction allows the misfit to be accommodated without the formation of electronically active defects. GaSe was supplied to the growing surface from a single GaSe Knudsen cell. Reflection high energy electron diffraction and x-ray photoemission spectroscopy studies of the nucleation of GaSe indicate Se reacts with the GaAs surface to remove the surface dangling bonds prior to GaSe formation. This is followed by the oriented growth of stoichiometric GaSe layers, that are rotationally aligned with the underlying GaAs substrate. The termination of the GaAs dangling bonds most likely occurs by Se substitution for As in the surface layer of GaAs(111)Band by direct bonding of Se to surface Ga on GaAs(111)Asurfaces. In addition, photoemission measurements indicate that the subsurface Se uptake into the GaAs(111)Alattice is higher than that in the (111)Blattice.

ISSN:1071-1023    

DOI:10.1116/1.590055

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

SiOx(x>1.5)overlayers have beenin situgrown on ZnS (111) and CdTe (111) substrates. Synchrotron radiation photoemission spectroscopy has been used to measure the electronic structures and band lineups of these two heterojunctions. The valence band offsets ofSiOx/ZnS(111)andSiOx/CdTe(111)derived from the measurements are 2.8±0.2 and 4.7±0.2 eV, respectively. Harrison’s “tight binding” theory is extended into the theoretical estimation of the band lineups ofSiO2related heterojunctions. The agreement between the experimental and theoretical results is good. Our result also explains the positive role ofSiO2layers in ZnS-based thin film electroluminescence devices.

ISSN:1071-1023    

DOI:10.1116/1.590056

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

Valence band x-ray photoelectron spectra (XPS) from oxide films grown on GaAs surfaces by thermal oxidation and electron cyclotron resonance (ECR) plasma oxidation are compared. The present work along with previous studies clearly show that the ECR grown oxides are nearly stoichiometric and close toGaAsO4while thermally grown oxides are closer toGa2O3having significant amounts ofAs+3oxidation states. Metal oxide semiconductor (MOS) structures were prepared by coating both the thermal and ECR plasma grown GaAs oxides with ECR plasma enhanced chemically vapor deposited (PECVD)SiO2and then with evaporated Al as top contact. Interface charges in terms of fixed oxide charge and surface electronic states were obtained from conventional capacitance–voltage measurements. Despite the significant difference in the GaAs oxides structure, most notably theGaAsO4like ECR plasma oxides, the interfaces prepared by both thermal and plasma oxidation were found to be equally unpassivated in terms of high levels of interface electronic states and Fermi level pinning.

ISSN:1071-1023    

DOI:10.1116/1.590057

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

We have shown that small concentrations of metal atoms on UV/ozonized GaAs wafer surfaces can be quantitatively determined by time-of-flight secondary ion mass spectrometry (TOF-SIMS). As standard reference materials, we produced defined submonolayer concentrations of Ca, Mg, Al, Cu, Zn, Si, Ni, Cr, Co, and Fe on the wafer surface by sputter deposition from different single- and multielement targets. The concentrations of the elements withm>27u involved in these calculations were verified independently by total reflection x-ray fluorescence spectroscopy. For all ten metals, a linear relation exists between the relative metal signalMe+/71Ga+and the surface concentration of the metal. By this relation we established TOF-SIMS sensitivity factors for these metals on UV/ozonized GaAs. The detection limits for almost all elements are in the order of109atoms/cm2.

ISSN:1071-1023    

DOI:10.1116/1.590058

出版商:American Vacuum Society    

年代:1998

数据来源: AIP

摘要:

We report on the use of an atomic force microscope (AFM) to directly measure the quality of InGaAsP/InP laser facets formed by aCH4:H2:Arreactive ion etching process. From the AFM data, we obtain values for rms surface roughness and transverse angular tilt that have previously only been estimated through laser operating characteristics or inferred from scanning electron microscope analysis. The etched facet reflectivity, calculated from device slope efficiency data and far-field beam profiles, is in excellent agreement with AFM measurements. We show methane-based dry etching can achieve a rms facet roughness of 22 nm. This value surpasses the generally accepted roughness parameter ofλ/10and is competitive with the best chlorine-based facet etching to date. We also report a transverse angular facet tilt of 2°, which we believe to be the most vertical,CH4-based facet etch reported to date. The output performance of these devices is nearly identical to cleaved devices.

ISSN:1071-1023    

DOI:10.1116/1.590059

出版商:American Vacuum Society    

年代:1998

数据来源: AIP