Si etching

Figure 1(a) shows the etch rates of niobium oxide pillar and Si film, and the etch selectivity of Si to niobium oxide as a function of CI2 concentration. The etch condition was fixed at coil rf power of 500 W, dc-bias to substrate to 300 V and gas pressure of 5 mTorr. As the CI2 concentration increased, the etch rate of niobium oxide pillar gradually decreased while Si etch rate increased. It indicates that the etch mechanism of niobium oxide in Cl2/Ar gas is mainly physical sputtering. As a result, the etch selectivity of Si film to niobium oxide monotonously increased. The effect of coil rf power on the etch rate and etch selectivity was examined as shown in Fig. 1(b). As the coil rf power increased, the etch rates of niobium oxide and Si increased but the etch rate of niobium oxide showed greater increase than that of Si. It is attributed to the increase of ion density with increasing coil rf power. Figure 1 (c)... 

The hydrogen was introduced both as an additive gas (CF, —mixtures) and intramolecularly by using CHFj as the etch gas. This concept has been extended by several groups and impressive SiOj-to-Si etch-rate ratios have been obtained. Ephrath , for example, using CF —mixtures in a reactive-ion etching system has observed ratios of 30 1. An example of this work is shown in Fig. 3.2. Lehmann and Widmer using CHFj gas also in a reactive ion etching mode have obtained ratios of 15 1. We have previously shown that mixtures of CF, and CjF behave similarly to CF —Hj mixtures with respect to the SiOj-to-Si etch-rate ratio. 

The mechanism we believe is responsible for the large SiOj-to-Si etch-rate ratios which have been obtained in fluorine-deficient discharges is based on several experimental observations. First of all, it has been shown that there are several ways in which carbon can be deposited on surfaces exposed to CF, plasmas. One way is to subject the surface to bombardment with CF ions which are the dominant positive ionic species in a CF plasma. The extent to which this can occur is shown by the Auger spectra in Fig. 3.3. Curve (a) is the Auger spectrum of a clean silicon surface and curve (b) is the Auger spectrum of the same surface after bombardment with 500 eV CFj" ions. Note that the silicon peak at 92 eV is no longer visible after the CFj bombardment indicating the presence of at least two or three monolayers of carbon. Another way in which carbon can be deposited on surfaces is by dissociative chemisorption of CFj or other fluorocarbon radicals. 

Heavily doped (>1018/cm3) n-type Si and poly-Si etch faster in Cl- and F-containing plasmas than do their boron-doped or undoped counterparts (103a, 105, 111, 112). Because ion bombardment is apparently not required in these cases, isotropic etch profiles (undercutting) in n + poly-Si etching often occur. Although the exact mechanisms behind these observations are not completely understood, enhanced chemisorption (103b, 111) and space charge effects on reactant diffusion (112) have been proposed. 

Fig. 14. Fluorocarbon film thickness on Si and Si etching rate as a function of the gas mixture in CF4—H2 plasmas (reprinted with permission from Mat. Res. Soc. Symp. Proc., 98 (1987) 229 [64]).

Fig. 16. Si etching rate in ion-assisted gas-surface chemistry mechanisms as compared to pure chemical reaction or ion sputtering (reprinted with permission from J. Appl. Phys., 50 (1979) 3189 [66]).

In conclusion Si etching is essentially controlled by the surface chemistry and diffusion of fluorine through the overlayer, as in the previous example, whereas germanium etching depends mainly on the fluorine flux with a weak effect of the surface chemistry. 

In NaOH the dissolution of Si can follow a chemical and an electrochemical route (Fig. 27, top and bottom paths respectively). The chemical path represents 90% of the dissolution [122]. This route underlines the role of water molecules in the dissolution of Si at high pH in accordance with the weak dependence of the etch rate with respect to the OH concentration. Hydroxyl ions are catalysts of the reaction and do not react directly. This is consistent with the fact that Si etching in boiling water also produces atomically flat Si(lll) surfaces, as has been made evident by IR absorption spectroscopy [124] and UHV-STM observations [117]. In this case the temperature is sufficient to overcome the activation energy since no OH ion is available. The remarkable anisotropy of Si etching in strong bases stems from the fact that the chemical route represents about 90% of the dissolution and begins with a hydrolysis step in Fig. 27 [122]. The hydrolysis of Si-H bonds by water molecules... 

Fig. 28. Molecular models for Si etching in acidic HF (a) Electrochemical route under chemical etching condition, (b) Dissolution initiated by holes, here under illumination at n-Si (after [123 a]).

The effect of potential is shown in Fig. 7.25 the etch stops at the passivation potential and decreases with cathodic polarization for both n-Si and p-Si." The etch rate dependence on potential is rather different from that in KOH, where the p-Si etch rate varies only slightly with cathodic polarization (Fig. 7.15). Also, the etch rates at... 

Figure 8.1. Conventional gate process, a) before poly-Si etching, b) implantations, c) oxide regrowth and spacer formation.

Increase of Aspect Ratio for DT Si Etch up to 60 The maximum achievable DT depth is not limited by an Si Reactive Ion Etch (RIE) stop, but by hard mask erosion [277-279]. A boron doped silicon oxide (BSG) is used as the hard mask material (CVD deposition). The BSG is patterned by conventional photoresist technology. A photoresist to BSG RIE selectivity of above 4 1 is possible. In turn, the BSG hard mask to Si RIE selectivity was found to depend on the DT top dimension (lateral DT perimeter). The corresponding experimental relationship is shown in Figure 1.66 for a conventional Si RIE tool set. For each data point the required BSG... 

Figure 1.65 DT capacitance roadmap. The six curves represent different options 1, Si-etch with an aspect ratio (AR) of 45, straight DT profile 2, AR = 45 + bottle shape DT profile 3, AR = 60 4, AR = 60 + bottle 5, AR = 45 + bottle + HSC (hemispherical silicon grains) 6, AR = 45 + bottle + high e dielectric materials such as TiOj, TajOs and AI2O3.

Figure 1.67 Left DT Si etch with an aspect ratio AR of 45 (single step DT etch) DT depth 6 pm, DT width 0.135 pm. Right two-step DT Si etch with AR = 60 DT depth 8.6 pm, width 0.15 pm.

The reported results can be explained by considering the different processes occurring at the OCP on the Si electrode immersed in a fluoride solution. The Si etching reaction can be outlined as [9] ... 

The last step is surface anisotropic etching to remove the remaining adatoms and islands and leave terraces atomically flat (Fig. 1(c)). This technique of preparation is specific to Si and is made possible by the high quality of the Si/SiOa interface and the extreme selectivity of Si02 to Si etching in HF (Si is almost not etched in acidic HF). With other materials like compound semiconductors procedures are generally different. 

---