Etching, silicon

In-situ IR spectroscopy has proved that the surface of Si is exclusively covered by Si-H bonds when it contacts fluoride solutions [110] or strong bases [20, 111]. Ex-situ IR spectroscopy [57] and more recently ex-situ scanning probe microscopy [17, 20, 112, 113] have also shown that the topography of Si depends critically on the pH of fluoride solutions. These results have prompted many studies to elucidate the mechanism of etching. 

Hessel et al. [112] imaged Si(lll) after etching in NH4F solutions of increasing pH, and rapid transfer of the samples into a UHV chamber. Images paralleled very well 

At an atomic level (100)-H surfaces are 2x1 reconstructed [115] (see above). (Ill) faces have the ideal (Ixl)-H arrangement after treatment in buffered NH4F [105, 116, 117] or in hot water [118]. The 1 x 1-H/Si(lll) surface has been imaged in dilute H2SO4 [17] and in NaOH [20] (although etching proceeds in the latter case). Etching in 1% HF apparently leaves (111) terraces covered by Si trihydrides 

In addition to the visualization of topographic transformations, sequences of in-situ images yield a measure of the local kinetics of the reaction. The etch rate of Si has been evaluated in [20] by using the expression R = (AS/S) /i/Af, with AS/S the surface area of terraces removed per cm of electrode in one sequence, h the step height (3.14 A) and At the time elapsed. The quantity (AS/S)h in fact represents the volume of material which has been removed per cm of electrode, because the dissolution occurs layer by layer. The experimental determination of AS is sketched in Fig. 22 f, in which the hatched area represents AS. In other sequences AS includes the surface of eventual pits. The bias dependence of the etch rate and the current voltage curve are shown in Fig.26 for n-Si(lll) in a 2M NaOH solution [20]. 

Allongue et al. [122] and Gerischer et al. [123] have recently published models describing, at a molecular level, the etching of silicon in strong bases and in fluoride solutions respectively. They are shown in Figs. 27 and 28 respectively for NaOH and acidic HR 

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A SiC buffer layer was grown on a silicon wafer at 1150-1300°C from one to 45 minutes using C3Hg and H2 as reactant gases. The thickness of the film increased gradually by diffusion of Si into the deposit until a thickness controlled by temperature and silicon etching was reached. 

MOSFETT s, and silicon oxide is deposited. The source/drain positions where electrical contact is to be made to the MOSFETs are defined, using the oxide-removal mask and an etch process. For shallow trench isolation, anisotropic silicon etch, thermal oxidation, oxide fill and chemical mechanical leveling are the processes employed. For shallow source/drains formation, ion implantation techniques are still be used. For raised source/drains (as shown in the above diagram) cobalt silicide is being used instead of Ti/TLN silicides. Cobalt metal is deposited and reacted by a rapid thermal treatment to form the silicide. Capacitors were made in 1997 from various oxides and nitrides. The use of tantalmn pentoxide in 1999 has proven superior. Platinum is used as the plate material. 

Cross sectional area Coefficient matrix Chemical species Coefficient matrix Numerical coefficients Advanced silicon etching Internal surface area Attenuated total reflection Numerical coefficient... 

Silicon etch rates in alkaline solutions commonly increase monotonically with temperature. For KOH, for example, the etch rate r can be calculated according to ... 

Table III Etch Rates and Activation Energies for Silicon Etching in F-Source Plasmas...

Figure 12. Silicon etch rate versus F-atom concentration. Arrows indicate increasing O2 concentration in the feed. (Reproduced with permission from...

Figure 17. Degree of isotropy observed in silicon etching in CI2/CIF3 plasma as a function of CIF3 content of the feed. Conditions are 5 seem total flow, 0.02 Torr pressure, 100 W rf with 30 V cathode bias at a frequency of 13.6 MHz. (Reproduced with permission from Ref. 39J...

The mechanism of silicon etching in alkaline solutions is a process of material dissolution with a simultaneous hydrogen evolution. The main soluble product is a silicic anion Si02(0H)2 that can further be condensed to form polysilicic anions. In fact, due to the acido-basic ionization of OH radicals in a highly alkaline solution, Eq. (19) should be modified as follows ... 

Fig. 3.5. Silicon etch rate as measured with a quartz crystal microbalance as a function of the bias voltage applied to the silicon surface in CF4 and Ar glow discharges. The discharge intensity was not significantly influenced by the application of the negative voltage to the silicon surface...

The possibility of any chemical contribution from the ions can be eliminated in the case of silicon etching with ions simply by noting that there is not enough fluorine per ion to chemically remove even one silicon atom as SiF and in addition this ignores the problem of the carbon which also arrives at the surface. The observation mentioned earlier involving CF ion beam bombardment of silicon in which the silicon surface is soon obscured by a thin carbon layer emphasizes the fact that CFj" " ions alone will not etch Si chemically (by forming SiF ) at a significant rate. 

Fig. 3.9. Reciprocal of the silicon etch rate versus silicon area illustrating the relative consumption of the F atoms by the etching process (data from Ref. )...

An interesting demonstration of profile control via alteration of the specific chemistry is that of silicon etching in C1F3 mixtures (86). Because a pure chemical (isotropic) etchant (F atoms) is combined with an ion-bombardment-controlled (anisotropic) etchant (Cl atoms), a continuous spectrum of profiles with varying anisotropies is generated by changing the gas composition. 

Another reason for the preference of up-numbering is actually due to a manufacturing problem. Tolerances in silicon etching are absolute tolerances which means that with decreasing size of the channels the geometric errors increase. This influence is balanced if a large number of channels are combined. 

The equipment used in the unit operations is complex and microprocessor controlled to allow the execution of process recipes. However, advanced control schemes are rarely invoked. The microprocessor adjusts set points according to some sequence of steps defined by the equipment manufacturer or the process operator. Flows, pressures, and temperatures are regulated independently by off-the-shelf proportional-integral-derivative controllers, even though the control loops interact strongly. For example, fluorine concentration, substrate temperature, reactor pressure, and plasma power all influence silicon etch rates and uniformity, but they are typically controlled independently. 

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