K.M. Horn

Ion beam enhanced epitaxial growth of Ge (001)

An enhancement in surface smoothness during molecular beam epitaxial growth of Ge on Ge (001) by 200 eV Xe ion bombardment has been measured with reflection high‐energy electron diffraction. We show that the steady‐state surface is smoother during simultaneous ion bombardment and growth at 400 °C than during ion bombardment or growth alone. The smoothest surface is obtained when the rate of deposition is approximately equal to the rate of production of ion‐induced surface defects. It is suggested that the smoothening is the consequence of the ion‐induced vacancy‐like defects annihilating with surface atoms and/or destabilizing small clusters.

Single event upset imaging with a nuclear muprobe

Abstract An entirely new ion beam analysis technique is described: single event upset (SEU) imaging. SEU-imaging utilizes the scanning of a mu-focused MeV ion beam across an integrated circuit. This beam generates both electrons and logic state changes which are monitored by a computer. The data is collected in a way that permits the generation of visual images which depict both the physical appearance of the scanned region (through the ion-induced electron signals) and the areas of the IC which are susceptible to upset (through detection of chip malfunctions). Comparison of these images with the chip design facilitates matching the individual transistor components with the upset-sensitive region. While our initial results with 1 μm resolution ion beams have demonstrated the viability of this new technique in directly identifying the sources of upset in mun-scale integrated circuits, the trend toward submun feature size will necessitate higher-resolution muprobes and improved appearance-imaging systems in future applications of this new technique.

Surface roughening of Ge(001) during 200 eV Xe ion bombardment and Ge molecular beam epitaxy

The kinetics of surface roughening of Ge(001) during 200 eV Xe ion bombardment and during Ge molecular beam epitaxy (MBE) are studied by real-time reflection high-energy electron diffraction. In both cases, initially smooth surfaces reach a steady state roughness which depends on temperature and incident ion or adatom flux. The data are analyzed in terms of a phenomenological model in which beam-induced roughening competes with beam-induced smoothening, and in which the defect creation rate and surface diffusivity are fitting parameters. For comparable fluxes, the temperature dependences for the net roughening induced by ions and adatoms are strikingly similar, implying a similarity in the surface diffusivities of vacancies and adatoms. For the case of ion-induced roughening, approximately one surface defect (in units of displaced surface atoms) is created per ion which is consistent with calculations assuming that a large fraction of atomic displacements recombine without producing surface defects at these ion energies.

Low-energy ion beams, molecular beam epitaxy, and surface morphology

Abstract The use of low-energy ions in conjunction with molecular beam epitaxy is discussed. Energetic ions can create surface defects; they can also create unwanted bulk defects. In the first part of this paper, we present calculations showing how the partitioning of these two processes depends on incident ion mass and energy. Because epitaxy is a surface process, mediated by particular kinds of surface defects (e.g., adatoms, clusters, steps) it will be strongly influenced by other surface defects (e.g., surface vacancies) created by ion bombardment. Therefore, we are interested in the interaction of these various surface defects. The evolution of these defects manifests itself in changes in surface morphology, which we monitor by reflection high-energy electron diffraction (RHEED). In the second part of this paper, we present two examples of the first such dynamic measurements of surface morphology during ion bombardment and epitaxy: one in which chemically reactive H + ions smoothen static Ge surfaces, and one in which chemically inert Ar + ions roughen static Ge surfaces, but smoothen dynamic (growing) Ge surfaces.

Ion Beam Induced Charge Collection (IBICC) microscopy of ICs: Relation to Single Event Upsets (SEU)

Abstract Single event upset (SEU) imaging is a new diagnostic technique recently developed using Sandias nuclear microprobe. This technique directly images, with micron resolution, those regions within an integrated circuit which are susceptible to ion-induced malfunctions. Such malfunctions are an increasing threat to space-based systems which make use of current-generation 1C designs. A complementary technique to SEU imaging involves measurement of the charge collection volumes within integrated circuits; charge collection is the underlying physical process responsible for single event phenomena. This technique, which we term ion beam induced charge collection (IBICC), has been used here and elsewhere to generate micron resolution maps of the charge collection response of integrated circuits. In this paper, we demonstrate the utility of combining the SEU imaging and IBICC techniques in order to gain a better understanding of single event upset phenomena. High resolution IBICC images are used to extract more detailed information from charge collection spectra than that obtained from conventional broad-area ion exposures, such as from radioactive sources. Lastly, we suggest the application of IBICC as a replacement for electron beam induced conduction/current (EBIC) measurements. As reductions in circuit feature size continue in the submicron regime, IBICC could certainly prove to be a technologically valuable replacement for EBIC and an important business opportunity for all nuclear microprobe facilities.

Ion-beam-induced charge-collection imaging of CMOS ICs

Charge collection regions of the Sandia TA670 16-Kbit SRAM have been directly imaged using a technique we call ion-beam-induced charge-collection (IBICC) imaging. During the IBICC measurement, the integrated circuit is connected through its power (VDD) or ground (VSS) pins to a charge sensitive preamp whose output is pulse-height analyzed while the IC is exposed to a scanned 0.1-μm resolution microbeam of heavy ions. The IC, in effect, functions as its own detector of the magnitude of charge collected following a heavy-ion strike. In this work, we examine the effect on IBICC imaging of varying power supply bias over a range of 0 to 15 V. Comparison of the IBICC image with the design layout for this integrated circuit unambiguously identifies source and drain regions of n-channel transistors and drain regions of p-channel transistors in the memory array. We were not able to image p-channel source regions in either the VDD or VSS configuration. This result is clearly explained on the basis of the IC design. Comparison of IBICC images with previously measured single-event-upset (SEU) images of the TA670 provide a more complete understanding of the mechanisms that govern single-event upset in this SRAM. IBICC holds great promise as a diagnostic tool to quantify the underlying charge collection processes that are responsible for single event upset in complex integrated circuits. It can also be applied to device failure analysis in a manner similar to EBIC, with potentially higher resolution.

Effects of ion damage on IBICC and SEU imaging

The effect of displacement and ionizing dose damage on ion-beam-induced-charge-collection (IBICC) and single-event-upset (SEU) imaging are explored. IBICC imaging is not affected by ionizing dose damage, and its dependence on displacement damage is a complex function of the structure of the samples used in this study. Degradation of the IBICC signal is controlled by displacement damage that occurs at different rates in the heavily doped substrate and lightly doped epitaxial silicon layer, leading to a non-linear dependence of inverse degradation versus ion fluence. The effect of ion exposure on the electrical performance of complementary metal-oxide-semiconductor (CMOS) static random access memories (SRAMs) is solely related to ionizing dose effects in the transistor oxides. With SEU imaging, we found that an additional region became sensitive to upset with ion fluence probably as a result of ionizing dose effects on the restoring transistors. Finally, SEU during IBICC imaging resulted in charge collection from both p-drains of a memory cell. Implications of damage on the use of these microbeam techniques are discussed.

Dynamics of growth roughening and smoothening on Ge (001)

We present reflection high‐energy electron diffraction measurements of the evolution of surface morphology during molecular‐beam epitaxy of Ge on Ge(001) and subsequent annealing. We find that there is a critical ‘‘kinetic roughening’’ temperature (375 °C) above which a smooth surface remains smooth during growth, but below which it roughens during growth. Surprisingly, smooth starting surfaces never appear to roughen without bound, but reach steady‐state roughnesses which depend on temperature and deposition rate. The results can be fit empirically with simple phenomenological equations based on a competition between growth roughening and growth smoothening of a ‘‘pseudo‐statistical’’ surface. Furthermore, growth‐roughened surfaces tend to smoothen, after growth, at a rate consistent with a third‐order power‐law ripening mechanism.

Surface defect production on Ge(001) during low-energy ion bombardment

Low‐energy ion bombardment of the Ge(001)‐(2×1) surface produces surface point defects, which are detected and quantified using in situ reflection high‐energy electron diffraction. Surface defect production rates are determined for a range of ion energies and ion masses. At low substrate temperatures (T≊−100 °C), copious production of surface defects is observed, with defect yields as high as 20–30 defects per ion for 500 eV Ar and Xe bombardment. The observed He surface defect yields exceed the surface yield predicted by binary collision simulations, indicating that defects created in the subsurface region migrate to the surface for these conditions. The observed surface defect yield is reduced at elevated substrate temperatures. Based on a simple model this reduction is attributed to surface recombination of point defects created within the same cascade. A constant surface defect yield is reached at temperatures greater than 100 °C which is consistent with the net defect production due to the vacancies ...

Oxygen roughening of Ge(001) surfaces

Abstract The interaction of molecular oxygen with the Ge(001) surface is studied in terms of the immediate disordering effects which occur as oxygen impinges on the germanium surface. Using reflection high energy electron diffraction, the surface morphology of initially clean, smooth Ge(001) surfaces is monitored in situ, in real time, during exposure to molecular oxygen. Changes in the (001) surface dimer reconstruction, surface disordering, and ultimately, multi-level roughening of the surface are observed. The pressure and temperature dependence of the oxygen-induced surface disordering rate is presented. Details of the initial oxygen-germanium surface interaction are discussed in the context of changes to surface morphology and compared to previously proposed roughening and oxidation mechanisms which are based upon both mass-sensitive and chemically-sensitive surface analysis techniques.