Etching selectivity

A powerful feature of wet etching is the abiUty to achieve excellent etch selectivities of one material over another. This can be extremely useful in the fabrication of epitaxial devices with different material layers. Because selective etching allows the removal of specific layers, the final accuracy of the etch can approach that of the epitaxial layers. Etch selectivities of >100 1 have been achieved for citric acid H202 etching of GaAs—AlGaAs and InGaAs—InP stmctures (133). 

The etch rates were measured by a surface profiler and field emission scatming electron microscopy (FESEM), and the etch profile were observed by FESEM. In this study, a C /Ar gas chemistry was chosen to obtain high etch selectivity of Si film to niobium oxide mask since CI2 gas was known to be a good etch gas for Si films. The etch rate, etch selectivity and etch profile of niobium oxide nanopillars and Si films were explored by varying the CI2 concentration, coil RF power and dc bias voltage to substrate. 

The formation of Si nanodot arrays on a substrate was performed by ICPRIE of Si films using self-assembled niobium oxide pillars as an etching mask. The etch rates of niobium oxide pillars and Si films, and the etch selectivity of Si films to niobium oxide were investigated by varying etch parameters in a Ch/Ar gas. The main etch parameters used in this study were the concentration of CI2 gas, coil rf power, and dc-bias to substrate. 

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)... 

Fig. 1. Etch rates of niobium oxide pillar and Si film, and etch selectivity of Si to niobium oxide piUar for the variation of (a) CI2 concentration, (b) coil rf power, and (c) dc-bias voltage to susceptor...

Fig. 3. Etch rate and etch selectivity of Co2MnSi and TiN as a function of O2 concentration...

The incorporation of PDMSX into conventional novolac resins has produced potential bilevel resist materials. Adequate silicon contents necessary for O2 RIE resistance can be achieved without sacrificing aqueous TMAH solubility. Positive resist formulations using an o-cresol novolac-PDMSX (510 g/mole) copolymer with a diazonaphthoquinone dissolution inhibitor have demonstrated a resolution of coded 0.5 pm L/S patterns at a dose of 156 mJ/cm2 upon deep-UV irradiation. A 1 18 O2 etching selectivity versus hard-baked photoresist allows dry pattern transfer into the bilevel structure. 

We have found that hydrophilic oiganic polymers treated with TiCLi have much higher etching selectivities than organosilicon polymers in an O2 plasma. This paper examines some of the parameters that influence the reaction of TiCLi with a variety of polymers. We find that TiCLi, readily functionalizes hydrophilic as well as moderately hydrophobic polymers, but fails to functionalize very hydrophobic films. Rutherford backscattering analysis reveals that TiCl4 is hydrolyzed at hydrophilic polymer surfaces that have sorbed water. Lack of surface water on hydrophobic polymers explains the absence of a TiC>2 layer on these polymer surfaces. 

Several years ago the concept of near-surface imaging was introduced(l,2). The critical step that imparts the etch selectivity by introduction of refractive elements is termed gas-phase functionalization and involves the selective reaction... 

Etch selectivity is crucial to the gas-phase functionalized resist schemes. Since the thickness of the etch resistant Ti02 layer that forms on the polymer film should depend on the amount of water sorbed on the polymer surface, we studied the influence of various processing parameters on the surface water content as measured by the amounts of Ti deposited. 

Of the 8 polymers in Tables III and IV, only the PS and HB-HPR 206 films seemed to behave very differently. HB-HPR 206 films showed zero film thickness loss upon etching, while PS films exhibited the lowest etching selectivity in spite of having more Ti than any other film. For example, HB-HPR 206 films with... 

TiCU readily functionalizes hydrophilic polymers such as poly(vinyl alcohol), m-ciesol novolac and methacrylic acid copolymers as well as moderately hydrophobic polymers such as poly(methyl methacrylate), poly(vinyl acetate), poly(benzyl methacrylate) and fully acetylated m-cresol novolac. HCI4 did not react with poly(styrene) to form etch resistant films indicating that very hydrophobic films follow a different reaction pathway. RBS analysis revealed that Ti is present only on the surface of hydrophilic and moderately hydrophobic polymer films, whereas it was found diffused through the entire thickness of the poly(styrene) films. The reaction pathways of hydrophilic and hydrophobic polymers with HCI4 are different because TiCl is hydrolysed by the surface water at the hydrophilic polymer surfaces to form an etch resistant T1O2 layer. Lack of such surface water in hydrophobic polymers explains the absence of a surface TiC>2 layer and the poor etching selectivities. 

Ion energy and flux represent another important aspect in achieving and controlling selectivity. Because ion bombardment generally imparts different degrees of damage to different materials, etch selectivity can often be varied and even reversed from that obtained via chemical means. In... 

To enhance etching selectivity, in order to reveal local inhomogeneities of the material or to produce on its surface a certain relief. The discussion to follow deals mainly with this latter problem. 

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