Oxide: nitride selectivity

EFFECTS OF CHEMICAL ADDITIVES TO OXIDE NITRIDE SELECTIVITY... 

In order to achieve selective oxide and nitride etching, additives to F-source plasmas are chosen to make a F-deficient chemical environment. These include H2, C2H4 and CH4 which are quite efficient F scavengers. The amount of additive necessary remains more an art than a science because oxide and nitride selectivity requires operation in an environment very close to the demarcation between etching and polymerization shown in Figure 10. In fact in some cases (57,59) polymer deposition on Si occurs... 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

Polishing rates with standard processing parameters are in the 100-200 nm/min region for oxide. Polishing times are therefore normally in the order of several minutes. Oxide-to-nitride selectivity depends significantly on the composition of the slurry, the deposition technique, and the thermal treatment of oxide and nitride and polishing conditions. Minimum and maximum values for commercially available slurries are several to one and several hundred to one, respectively. 

The key requirements for an acceptable STI slurry include adequate removal rate (> 3000 A/min) on oxide, desirable selectivity of oxide over nitride (between 200 1 and 400 1), high planarization efficiency across the die regardless of the patterned density, low scratch count, and low particle residue. The typical operating downpressure for an STI process is 3-5 psi. The typical platen and carrier speeds of a rotary tool are 100 and 60 rpm, respectively. The... 

FIGURE 13.11 Oxide removal rate and selectivity (oxide nitride) as a function of the action term. The action term is defined as the head pressure times the linear velocity. 

Additive, wt% Nitride Polish Rate, nm/min Oxide Polish Rate, nm/min Selectivity (oxide nitride)... 

Oxide-to-nitride selectivity for a 1-4% glycine-ceria slurry ranges from 16 to 70. The proline-ceria slurry is capable of achieving an even higher selectivity of over 500 1. The pH of the proline-ceria slurry has a significant influence on the selectivity. Fig. 13.19 shows the removal rates of oxide and nitride films for different values of the slurry pH. 

Fig.4 shows the calculated and experimental removal rate ratio of the mixed area at various pattern densities and pitch sizes. The calculated data increases with decreasing active pattern density and it is in good agreement with the experimental data. As can be seen in the diagram blanket oxide to nitride selectivity is 4.3 and decreases as active density increases. The selectivity change is greater in the low active density areas than in the high doisity areas. 

In (his work, we investigated (he dependence of the removal rate upon the oxidizer (peroxide) addition into commercially available slurries for a variety of films such as aluminum, titanium, titanium nitride and oxide. We found (hat (he barrier layer materials were extremely sensitive to the peroxide addition while (he removal rate varied only slightly for aluminum and oxide. The selectivity to titanium and titanium nitride drops from as high as 1000 to almost close to I as the mixture ratio (peroxide slurry) increases. We proposed that the barrier layer be used to protect the oxide from being over-exposed and suppress the erosion eventually. This can be easily realized by dividing the process into two steps with each step being run at a specific peroxide mixture ratio. The experimental result unambiguously proved, for the first time, the effectiveness of this approach. 

The key ingredients to a successful STI process are the achievement of well-dispersed abrasive ceramic particles having high oxide-to-nitride selectivity and producing few microscratches on the wafer. Silica slurry had been conventionally used in the STI CMP process, however, ceria (Ce02) slurry with high oxide-to-nitride selectivity has been introduced as the thickness of silicon nitride film is decreased by design rule restrictions. 

Figure 3.5a shows the result of CMP field evaluation. Average PETEOS removal rate of slurry A was 2883A/min and B was 672A/min. The within-wafer non-uniformity (WIWNU) shows that ceria slurry B (0.7%) is better than ceria slurry A (1.9%). Average nitride removal rate of slurry A was 5lA/min and B was 44A/min as shown in Figure 3.5b. Thus, oxide-to-nitride selectivity was 56 for ceria slurry A and 15 for ceria slurry B. CMP field evaluation of ceria slurries having different crystallinity showed...

In order to avoid photoresist deposition over severe topography, a peeling mask (nested mask) can be used two mask patterns are fabricated on a starting surface, using a suitable combination of mask materials, e.g., oxide-resist or oxide-nitride. After the first DRIE step, one of the mask materials is selectively removed (oxygen plasma ashing of resist mask or HF etching of... 

The selectivity and product distribution on various Mo-based catalysts show several common features that differ from group 8-10 metals (95,113). First, they show a high paraffin/olefin ratio. This seems due to a secondary reaction of initially formed olefins with dihydrogen to form paraffins. Second, they form a large amount of hydrocarbons of C2-C5, and the concentration of hydrocarbons heavier than C5 is very low. The last feature is the production of a large amount of CO2 due to the secondary water gas shift reaction of initially produced water and CO. These features are similar to those reported for Mo metal, oxides, nitrides, and sulfides, and reflect the fact that the active phase of molybdenum under working conditions seems to be molybdenum carbide irrespective of the phase present in fresh catalysts (112,114). 

The goal of the POC process is to remove this sacrificial oxide and expose the nitride spacer and cap nitride. In POC, within-die/within-wafer oxide, nitride thickness control, and defect control are keys for successful implementafion of a RMG. The key process performance that must be met in POC is to minimize both polishing scratches and nitride loss by maximizing MRR selectivity between oxide and nitride. The remaining nitride thickness after POC can directly impact on the final gate height. Therefore, it is critical to achieve the maximum removal rate selectivity between oxide and nitride and, therefore, to minimize any nitride erosion after the POC process. The typical removal rate selectivity required in the POC process is more than a 50 1 (oxide nitride) ratio. 

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