Wafer with native oxide

Figure 5.11 AFM images of PS-i)-P4VP-[Re(DIAN)(C0)3]+C10j spin-coated on different substrate surfaces (film thickness = 40 nm) (a) silicon wafer modified with (3-aminopropyl)trimethoxysilane (b) silicon wafer modified with A-trimethoxysilylpropyl-AyV,lV-trimethylammonium chloride (c) silicon wafer modified with 3-(p-methoxyphenyl)propyltrichlorosilane and (d) silicon wafer with native oxide layer removed. All the films were prepared from toluene solution. Scan size (a)- c) 1X1 pm (d) 500 X 500 nm. (From Cheng and Chan.71 Reprinted with permission. Copyright 2005 American Chemical Society.)...

AFM method is available to observe a nanometer scale roughness on the surface of the silicon wafer with native oxide. As a matter of course, topographical images of AFM could not imply the thickness information of the over layer film, and AFM methods might be unavailable for the area over than 10 cm due to the instrumentation limits. 

Figure 3. Mean frictional force versus temperature for a silicon(100) wafer with native oxide.

On hydrophilic surfaces under ambient conditions, the formation of a water meniscus by capillary condensation (see Chapter 5) may strongly influence the friction behavior. A logarithmic decrease in friction with increasing velocity was observed when a sapphire ball was slided on a silicon wafer with native oxide layer [1025]. Such dependence was also found for AFM tips sliding on hydrophilic CrN surfaces by Riedo et al. [1026] and explained the kinetics of capillary condensation. 

Figure 3 displays mean friction force as a function of temperature for a Si(lOO) wafer covered with native oxide. These data were collected as a control and exhibit no substantial temperature dependence within our experimental range. All reported variations in friction, on polymers, as a function of temperature are thus attributed to the properties of the material, or its interaction with the SFM tip. 

For some materials, the most notable being silicon, heating alone sufiBces to clean the surface. Commercial Si wafers are produced with a thin layer of silicon dioxide covering the surface. This native oxide is inert to reaction with the atmosphere, and therefore keeps the underlying Si material clean. The native oxide layer is desorbed, i.e. removed into the gas phase, by heating the wafer in UHV to a temperature above approximately 1100 °C. This procedure directly fonus a clean, well ordered Si surface. 

Vapor-phase decomposition and collection (Figs 4.16 to 4.18) is a standardized method of silicon wafer surface analysis [4.11]. The native oxide on wafer surfaces readily reacts with isothermally distilled HF vapor and forms small droplets on the hydrophobic wafer surface at room temperature [4.66]. These small droplets can be collected with a scanning droplet. The scanned, accumulated droplets finally contain all dissolved contamination in the scanning droplet. It must be dried on a concentrated spot (diameter approximately 150 pm) and measured against the blank droplet residue of the scanning solution [4.67-4.69]. VPD-TXRF has been carefully evaluated against standardized surface analytical methods. The user is advised to use reliable reference materials [4.70-4.72]. 

In a subsequent publication [68] the influence of the substrate on the self-assembly process was elucidated. The comparison of a polyimide substrate with a native oxide (Si( )A) covered wafer revealed that differences in interac-... 

It should be noted that approximately 1% of the APS used in the last experiment appears to be APS monomers before dilution. Could this low monomer concentration be responsible for adhesion It should be noted that the residual monomers present in this highly-oligomerized solution correspond to a 0.001 vol % solution if one were to remove the oligomerized APS. As a result of this observation, the dependence of the adhesion of thin films to native-oxide silicon wafers as a function of the concentration of the APS under conditions of T H stress was investigated. Adhesion studies were performed using APS solutions with concentrations that varied from the industry standard of 0.1 vol % down to 0.00001 vol %. The test wafers were prepared and exposed to T(200) and T(500) conditions as discussed above. Adhesion was measured by 90° peel test, as discussed above. The results of this study are presented in Fig. 13. The Lx-axis is APS concentration which decreases from left to right. The y-axis is the adhesion in the units of g mm"The three curves are the results at T(0), T(200) andT(500). 

Oxide layers of various thickness (260, 182, 98, 62, 43, or 20 A) were prepared on the Si wafers. At the beginning we prepared an oxide layer thicker than 200 A by thermal oxidation, and then evaluated the thickness (260 A). Following that, the 260-A thick layer was etched down to each thickness using a 1.6%-HF aqueous solution. Each thickness was evaluated with an ellipsometer. Using Auger electron spectroscopy, we determined the thickness of the native oxide layer on the Si wafer to be 11 A. 

Secondly, the wafer must be very "clean." Even a clean substrate will have 20 to 50 A layer of native oxide on it, and/or some carbon, and this will be enough to impede nucleation and give rise to many defects.1S After wafers are cleaned and inserted into the reactor, there is still the oxide layer to be removed as well as possibly some carbon on the surface. The traditional way of dealing with this phenomena is to operate a high-temperature HCI (1200°C) etch before attempting depositions. This etches away the native oxide, and any carbon on the surface diffuses into the bulk at this temperature. 

Another approach to this problem involves heating the wafer at 750 F at very low pressures (<10 10 Torr prior to deposition.28 This has the effect of removing the native oxide by evaporation of SiO. Depositions were achieved in the temperature range of 750° to 850°C in SiH4 + H2. Since the authors were developing a hot-wall system with many wafers stacked close to each other, the deposition was carried out at 2 mTorr. Deposition rates of 20 to 45 A/min were achieved. As expected, dopant transition widths were very narrow, several hundred angstroms. Again, device studies on such a system have not yet been done. 

Physico-chemical characterization. For the physicochemical characterization of the photoresist, flat model surfaces are needed that are accessible for the methods described below. They were prepared by spin-coating the photoresist directly onto a silicon wafer with a native oxide layer. Then, they were exposed to DUV radiation with various doses by open frame exposure. The exposed wafers... 

In an early HREELS study of Cr deposition onto polyimide (2b.81. bonding interactions of the Cr atom affecting the carbonyl stretching vibrations were clearly evident. In a further attempt to gain more details on the chemistry developing at the metal-polymer interface, another preliminary set of spectra was recently collected during the metallization of a polyimide film deposited directly onto a silicon wafer (with its native oxide) (Fig. 7). 

Yagi Y, Imaoka T, Kasama Y, and Ohmi T, Advanced ultrapure water systems with low dissolved oxygen for native oxide free wafer... 

FIGURE 2.21. Time dependences of the number of Si atoms in native oxide and the number of dissolved Si atoms in ultrapure water. Nsi is defined as the number of Si atoms per unit area on wafer surface. (Reprinted with permission from Morita et 1989, American Institute of Physics.)... 

With regards to this it is clear that the wafer pretreatment is of key importance for obtaining good selectivity. Such pretreatments can vary from wet clean steps to in situ dry clean steps. In situ clean steps have the advantage that they can be done in vacuo in an integrated (cluster) tool. Unfortunately not much has been published in the literature about the in situ pretreatments. In one study a NF3 plasma is reported to be able to remove native oxide from silicon [Kajiyana et al.84]. 

In practical terms the coexistence conditions are determined as follows. A layer of some hundreds of nm thick of one of pure components (B say) is spin cast on a silicon wafer with a native oxide or covered with an evaporated metal. A similar layer of the other component (A) is laid on the top of the precast film B using standard [74] - or modified [91] (for hydrophobic polymers) - floating techniques. It is possible to ensure that the surface segregation and wetting effects do not perturb final phase configuration by arranging the surface preferred component to be located near this surface. Also the substrate may be modified (by metal evaporation) to cancel the possible polymer-substrate interactions... 

The microfabrication procedure for the TEM membrane windows is shown schematically in Fig. 4.22. A more detailed outline of the procedure is described in [124,125]. The windows are typically fabricated on 3-in. double side polished, n-type Si(lOO) wafers. In the first step, the wafers are cleaned with the two-step standard cleaning procedure used in the semiconductor industry. This procedure ensures the removal of all the organic particles by oxidation, metals by forming soluble complexes, and the native oxide with HF as shown schematically in step 1 of Fig. 4.22 (these steps are hazardous and... 

Preparation of the thin SOG film required special precautions to eliminate any effects due to the presence of native oxide which may be initially on the Si substrate. The Si wafer was first etched with HF vapors to remove any oxide followed by immediate (15-28 sec. delay) application of the SOG film. After a bake at 100°C for 15 min. the sample was cured for 1 hour at 400 C. Despite these precautions, regrowth of native oxide on the Si substrate may have occurred during the elapsed time between the HF etch and application of the SOG film or after application due to the presence of H2O and O2 in the uncured film. This possibility could not be verified. 

The other films were produced to study the influence of the interface between the metal top contact and the organic semiconductor. Here, silicon wafers with a native oxide were used as substrates. The samples were provided by the group of Pflaum at the University of Stuttgart. To minimise impurities, like in the case of DIP, Pc (purchased from Fluka) is purified twice by gradient sublimation before being used as starting material. The films were prepared in UHV at a base pressure of about 7 x 10 mbar from a graphite effusion cell. The evaporation rate was about 3 A/s it was controlled by a quartz microbalance located next to the sample [8]. 

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