Silicon interfaces

Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

When dealing with polymer blends or blockcopolymers, surface enrichment or microstructures may be observed as already discussed in Sect. 3.1. Quite similar effects may be expected for buried interfaces e.g. between polymer and substrate where one component may be preferentially enriched. In a system of PS, PVP and diblock copolymer PS-6-PVP it has been shown by FRS that the copolymer enrichment is strongly concentration dependent [158]. In a mixed film of PS(D) and end-functionalized PS on a silicon wafer the end-functionalized chains will be attached to the silicon interface and can be detected by NR [159],... 

An important step toward the understanding and theoretical description of microwave conductivity was made between 1989 and 1993, during the doctoral work of G. Schlichthorl, who used silicon wafers in contact with solutions containing different concentrations of ammonium fluoride.9 The analytical formula obtained for potential-dependent, photoin-duced microwave conductivity (PMC) could explain the experimental results. The still puzzling and controversial observation of dammed-up charge carriers in semiconductor surfaces motivated the collaboration with a researcher (L. Elstner) on silicon devices. A sophisticated computation program was used to calculate microwave conductivity from basic transport equations for a Schottky barrier. The experimental curves could be matched and it was confirmed for silicon interfaces that the analytically derived formulas for potential-dependent microwave conductivity were identical with the numerically derived nonsimplified functions within 10%.10... 

Figure 27. Minority charge carrier profiles near the semiconductor/electrolyte junction. calculated for a silicon interface at two different electrode potentials. Uf- -0.25 V and Uf= 5.0 V10 ((//= forward bias = t/ - Ufl>).

Figure 34. PMC lifetime map of n-type silicon/polymer (poly(epichlorhydrine-co-ethylenoxide-co-allyl-glycylether plus iodide) junction at -10 V potential (mostly dropping across the polymer layer), after Li+ insertion has changed the silicon interface. The statistical evaluation shows the drastic drop in the PMC lifetime. For color version please see color plates opposite p. 453.

The resultant tailored interface is often vastly superior for biomedical applications over the native silicone interface. Furthermore, surface modification maintains the low materials cost and favorable bulk properties of the original silicone elastomer. The modification methods can be divided into physical and chemical techniques. 

If eq. (l) were applicable to other materials, approximate values of the maximum growth rates could be obtained by scaling with (T / ). Accordingly, we estimate maximum rates of 400 m/s for nickel and 430 m/s for silicon. Interface velocities of 50 m/s have been measured for Ni... 

Electron injection has been observed during the chemical dissolution of an oxide film in HF [Mai, Ozl, Bi5]. The injected electrons are easily detected if the anodized electrode is n-type and kept in the dark. Independently of oxide thickness and whether the oxide is thermally grown or formed by anodization, injected electrons are only observed during the dissolution of the last few monolayers adjacent to the silicon interface. The electron injection current transient depends on dissolution rate respectively HF concentration, however, the exchanged charge per area is always in the order of 0.6 mC cm-2. This is shown in Fig. 4.14 for an n-type silicon electrode illuminated with chopped light. The transient injection current is clearly visible in the dark phases. 

The origin of the electron injection peak at the end of the dissolution of an oxide film is not understood in detail. Silicon interface atoms with three Si-O bonds and a single Si-Si bond are proposed to be responsible for the effect [Mai]. On the other hand, during the dissolution process silicon interface atoms with one Si-O bond and three Si-Si bonds lead to a configuration identical to the one for which electron injection is observed during divalent dissolution (Fig. 4.3, step 2). In any case, the injected charge exceeds by a factor of 3 to 5 the charge expected... 

Illumination is a relevant parameter in the electrochemistry of silicon because photogenerated carriers may initiate or contribute to the charge exchange at the electrolyte-silicon interface. If an electrode is illuminated, photogenerated electron-hole pairs are generated corresponding to the number of absorbed photons. This number depends on spectral distribution, total illumination intensity and losses due to optical reflection and transmission. The number of electron-hole... 

Evidence for a layered structure is provided by X-ray reflection. For the oxide film present at point A in Fig. 5.4, a thickness of 10.8 nm and a density of 2.1 gcnT3 has been determined. While the film present at point B shows an angular reflectance that could best be fitted assuming a double layer structure with a thin (2.9 nm), dense (2.1 g cm-3) oxide at the silicon interface and a thick (10.2 nm), less dense (1.75 g cm-3) oxide on top. The density value found for the top oxide is in good agreement with density values obtained for thick (100 nm) anodic oxides using gravimetric methods. 

Figure 27. End point detection of a native oxide/silicon interface using chemiluminescence from Sip3 formed by the reaction of the etch product Sip2 with P and P2. Etching was performed with P atoms downstream...

Transition Metal-Silicon Interfaces Chemisorption Sites and Sllicide Growth 114... 

Sodium contamination and drift effects have traditionally been measured using static bias-temperature stress on metal-oxide-silicon (MOS) capacitors (7). This technique depends upon the perfection of the oxidized silicon interface to permit its use as a sensitive detector of charges induced in the silicon surface as a result of the density and distribution of mobile ions in the oxide above it. To measure the sodium ion barrier properties of another insulator by an analogous procedure, oxidized silicon samples would be coated with the film in question, a measured amount of sodium contamination would be placed on the surface, and a top electrode would be affixed to attempt to drift the sodium through the film with an applied dc bias voltage. Resulting inward motion of the sodium would be sensed by shifts in the MOS capacitance-voltage characteristic. 

As an example of the use of this technique, a silicon wafer lightly doped with phosphorus is doped with additional phosphorus by ion implantation (dose of 3.5 x 10ncm"2). A thermal oxide film of 857 A thickness was initially grown on the wafer. The variation of dopant concentration with depth from the oxide-silicon interface is shown in Figure 16. The rise in dopant close... 

Now that the top-down internal state variable theory was established, the bottom-up simulations and experiments were required. At the atomic scale (nanometers), simulations were performed using Modified Embedded Atom Method, (MEAM) Baskes [176], potentials based upon interfacial atomistics of Baskes et al. [177] to determine the conditions when silicon fracture would occur versus silicon-interface debonding [156]. Atomistic simulations showed that a material with a pristine interface would incur interface debonding before silicon fracture. However, if a sufficient number of defects were present within the silicon, it would fracture before the interface would debond. Microstructural analysis of larger scale interrupted strain tests under tension revealed that both silicon fracture and debonding of the silicon-aluminum interface in the eutectic region would occur [290, 291]. 

K.A. Gall et al Atomistic simulations on the tensile debonding of an Aluminum-Silicon interface. J. Mech. Phys. Solids 48, 2183-2212 (2000)... 

This chapter will focus on organic/silicon interfaces formed via solution phase reactions using hydrogen-terminated crystalline silicon surfaces as a starting point. While some of the surface chemistry issues have been reviewed previously [7,8], more recent developments will be emphasized here. We will not discuss the considerable literature of reactions with porous silicon [8], or studies of molecules reacting with clean silicon surfaces under ultrahigh vacuum (UHV) conditions [9-11] which have been reviewed elsewhere. 

While the detection of the Si-H and Si-C modes indicates HREELS can probe the buried molecule/silicon interface, in general this method will be most sensitive to the terminal groups at the vacuum/monolayer interface. This is illustrated in Fig. 9 where spectra for several modified surfaces with different terminal functionalities are shown. In each case this terminal group is tethered to the surface via a Cio alkyl linker yet the spectra are significantly different. This is particularly evident in the spectra for the thienyl terminated surface in which the aromatic C-H stretch is clearly observed. In contrast this mode is quite small in the FTIR spectra, which are dominated by the contributions of the alkyl linker chain [51]. The observation of strong terminal group modes in the HREELS spectra indicates that these functional groups are likely present at the surface of the film and not buried back towards the H-terminated surface. This is consistent with their availability for sequential reactions as discussed in the previous section. 

G. Ottaviani and M. Costato have investigated the process of compound formation at the platinum-silicon interface. Platinum films, 115, 205, 268 and 357 nm thick, were sputtered on Si(l 11) single crystals under hard vacuum. The interaction of the Pt and Si phases during isothermal annealing was followed using Rutherford backscattering spectroscopy of helium ions. 

The particular metals and metallization processes may be very significant. For devices involving Schottky barriers the metal work functions may be important, but the process by which the metallization is created may be the dominant factor. Surface states at the silicon interface could make the particular metal insignificant. In some cases the metallization is governed by compatibility with available lithographic procedures. 

Data were accumulated for a set of samples made of MSSQ with porogen loads from 0 to 90% (Figure 7.6). The samples have different thickness (350 to 780 nm) and the density drops as the porogen load increases. At the silicon interface the density changes and positronium annihilates fast into two photons, causing the near vertical drop in the ratio to the equivalent of no 3-photon events. 

A fast and reliable measure of the mean pore sizes is to extract a mean lifetime from the data. The values are shown in Figure 7.17 from data collected with and without the aperture. For samples with porogen loads in excess of 20% the values are smaller when the aperture is in place, indicating that positronium escapes from the samples from the mean depth of positron implantation. Approximate values are shown on the top axis. The mean lifetimes drop dramatically for porogen loads above 50%, independent of aperture configuration, because fast pick-off at the silicon interface, consistent 3-to-2 photon ratio measurements presented earlier. 

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