S. B. Christman

Infrared spectroscopy of Si(111) surfaces after HF treatment: Hydrogen termination and surface morphology

Polarized internal reflection spectroscopy has been used to characterize HF‐treated Si(111) surfaces. The silicon‐hydrogen stretching vibrations indicate that the surface is well ordered, but is microscopically rough, with coupled monohydride, dihydride, and trihydride termination.

On the mechanism of the hydrogen-induced exfoliation of silicon

We have investigated the fundamental mechanism underlying the hydrogen-induced exfoliation of silicon, using a combination of spectroscopic and microscopic techniques. We have studied the evolution of the internal defect structure as a function of implanted hydrogen concentration and annealing temperature and found that the mechanism consists of a number of essential components in which hydrogen plays a key role. Specifically, we show that the chemical action of hydrogen leads to the formation of (100) and (111) internal surfaces above 400 °C via agglomeration of the initial defect structure. In addition, molecular hydrogen is evolved between 200 and 400 °C and subsequently traps in the microvoids bounded by the internal surfaces, resulting in the build-up of internal pressure. This, in turn, leads to the observed “blistering” of unconstrained silicon samples, or complete layer transfer for silicon wafers joined to a supporting (handle) wafer which acts as a mechanical “stiffener.”

Mechanism of silicon exfoliation induced by hydrogen/helium co-implantation

Infrared spectroscopy and secondary ion mass spectrometry are used to elucidate the mechanism by which co-implantation of He with H facilitates the shearing of crystalline Si. By studying different implant conditions, we can separate the relative contributions of damage, internal pressure generation, and chemical passivation to the enhanced exfoliation process. We find that the He acts physically as a source of internal pressure but also in an indirect chemical sense, leading to the reconversion of molecular H2 to bound Si–H in “VH2-like” defects. We postulate that it is the formation of these hydrogenated defects at the advancing front of the expanding microcavities that enhances the exfoliation process.

CO diffusion on Pt(111) with time‐resolved infrared‐pulsed molecular beam methods: Critical tests and analysis

The microscopic diffusion of CO on stepped Pt(111) crystal surfaces has been investigated with pulsed molecular beam–time‐resolved surface infrared methods. Following a rapid exposure to CO, we record the time evolution of the CO surface vibrational spectra as the CO diffuse from the initial random distribution to the thermodynamically favored step sites. The data are simulated with a model that describes the kinetics of diffusion across the terraces, as well as the filling of step sites. We critically evaluate this model and the general experimental approach by extending our previous measurements of CO diffusion on Pt(28(111)–(110)) to a surface with higher step density, Pt(l2(111)–(110)), with varying coverages. The model describes both sets of data with the same parameters, confirming the original determination of the diffusion barrier (ΔET) and prefactor (AT) for microscopic surface hopping of CO/Pt(111). This further provides a quantitative means to estimate systematic errors. We report ΔET=4.0±0.7...

Physics and chemistry of silicon wafer bonding investigated by infrared absorption spectroscopy

Silicon wafer bonding is achieved by joining two particle‐free silicon wafers and annealing to elevated temperatures (∼1100 °C). We have used multiple internal transmission infrared absorption spectroscopy to probe the interface between the wafers upon initial joining and also during subsequent annealing steps. For atomically flat hydrophobic wafers (H passivated), we observe a pronounced shift in the Si–H stretching frequency due to the physical interaction (van der Waals attraction) that occurs when the surfaces come into intimate contact. The hydrogen eventually disappears at high temperatures (1000 °C) and Si–Si bonds are formed between the two surfaces. For hydrophilic wafers (oxide passivated), we initially observe three to five monolayers of water at the interface (providing the initial attraction through H bonding), as well as the presence of hydroxyl groups that terminate the oxide at low temperature. Upon moderate heating (<400 °C), the water trapped at the interface dissociates and leads to the...

Infrared spectroscopy as a probe of fundamental processes in microelectronics : silicon wafer cleaning and bonding

Abstract In this paper, we review our recent infrared studies of the fundamental physical and chemical processes occurring at the interface of bonded silicon wafers, as a function of surface preparation and annealing temperature. We present a brief overview of the practical aspects of silicon-wafer bonding and the techniques used to evaluate the interface integrity, which highlight the need for fundamental studies of the microscopic interface phenomena. Importantly, we show that the interface between two silicon wafers approximates an ideal spectroscopic environment, in that there is a 28-fold enhancement in the sensitivity to the normal component of the interface absorption over any other surface optical geometry. Furthermore, the interface region is almost infinitely stable at room temperature, but can exhibit partial pressures ranging from near vacuum to several atmospheres upon annealing. We present results for two distinct types of wafer bonding: hydrophobic (hydrogen-terminated) and hydrophilic (oxide-terminated), since the origin of the initial attraction between the opposing surfaces is quite different in the two cases. Specifically, we show that ideally hydrogen-terminated Si(111) surfaces come within a few A, under the influence of a Van der Waals attraction, as evidenced by a pronounced perturbation of the isolated SiH stretch mode. In contrast, the initial attraction between hydrophilic surfaces is via hydrogen bonding, which is mediated by the presence of 2–4 monolayers of water that are trapped at the interface upon room-temperature joining. We demonstrate that vibrational spectroscopy provides unprecedented mechanistic insight into the thermal evolution of the molecular interface, which necessarily has a profound influence on the bonding process. Throughout the paper, emphasis is given to the need for a wide variety of additional (fundamental) studies of the surface phenomena occurring in these novel, technologically important systems.

A VIBRATIONAL STUDY OF ETHANOL ADSORPTION ON SI(100)

The adsorption of ethanol-d0, -d3, and -d6 on Si(100) has been studied in the mid- to far-infrared region using surface infrared absorption spectroscopy. The acquisition of infrared spectra in this frequency range (<1450 cm−1) is made possible by using specially prepared Si(100) wafers which have a buried metallic CoSi2 layer that acts as an internal mirror. We find that ethanol dissociatively adsorbs across the Si(100) dimers near room temperature to form surface bound hydrogen and ethoxy groups. Furthermore, the ethoxy groups are oriented such that the C3v axis of the methyl group is nearly perpendicular to the surface, unlike the case for ethoxy groups bound to metal surfaces. This adsorption geometry is deduced on the basis of the surface dipole selection rule, which applies to these Si(100) samples with a buried CoSi2 layer. Ab initio cluster calculations using gradient-corrected density functional methods confirm the proposed adsorption geometry for ethoxy on Si(100) and accurately reproduce the obs...

Vibrational study of silicon oxidation : H2O on Si(100)

Abstract The vibrational spectrum of water dissociatively adsorbed on Si(100) surfaces is obtained with surface infrared absorption spectroscopy. Low frequency spectra ( −1 are acquired using a buried CoSi 2 layer as an internal mirror to perform external reflection spectroscopy. On clean Si(100), water dissociates into H and OH surface species as evidenced by EELS results [1] in the literature which show a SiH stretching vibration (2082 cm −1 ), and SiOH vibrations (OH stretch at 3660 cm −1 and the SiOH bend and SiO stretch of the hydroxyl group centered around 820 cm −1 ). In this paper, infrared (IR) measurements are presented which confirm and resolve the issue of a puzzling isotopic shift for the SiO mode of the surface hydroxyl group, namely, that the SiO stretch of the OH surface species formed upon H 2 O exposure occurs at 825 cm −1 , while the SiO stretch of the OD surface species formed upon D 2 O exposure shifts to 840 cm −1 , contrary to what is expected for simple reduced mass arguments. The higher resolution of IR measurements versus typical EELS measurements makes it possible to identify a new mode at 898 cm −1 , which is an important piece of evidence in understanding the anomalous frequency shift. By comparing the results of measurements for adsorption of H 16 2 O, H 18 2 O and D 2 O with the results from recently performed first-principles calculations, it can be shown that a strong vibrational interaction between the SiO stretching and SiOH bending functional group vibrations of the hydroxyl group accounts for the observed isotopic shifts.

Oxidation of H-covered flat and vicinal Si(111)-1×1 surfaces

The initial stages of O2 oxidation of H-passivated flat and vicinal Si(111) surfaces are investigated by monitoring the Si–H stretch vibrations with infrared absorption spectroscopy. We find that the incorporation of oxygen into silicon is activated (1.66±0.10 eV on flat surfaces), involving a multistep process. Oxygen molecules are incorporated into Si–Si bonds without removing surface hydrogen and this process is facilitated at steps.

Silicon wafer bonding studied by infrared absorption spectroscopy

The interface of hydrophilic and hydrophobic Si wafers joined at room temperature is studied using multiple internal transmission infrared absorption spectroscopy as a function of 30‐min annealing steps in N2 atmosphere up to 1100 °C. For hydrophilic wafers, water molecules are observed up to 300 °C, silanol groups up to 900 °C, and hydrogen up to 1000 °C. The absence of water spectral features above 300 °C implies that, if present, water molecules are mostly oriented in a plane parallel to the interface. For the hydrophobic wafers, the temperature dependence of the Si–H stretch spectra shows that the majority of H does not participate in the bonding, pointing to the role of impurities or defects.