The dependence of etch rate of photo-CVD silicon nitride films on NH4F content in buffered HF

Microelectronics Journal, 26 (1995) 563-567

0026-2692(95)00017-8

The dependence of etch rate of photoCVD silicon nitride films on NH4F content in buffered HF V.K. Rathi, Manju Gupta and O.P. Agnihotri SemiconductorEngineering Laboratory, Department of Physics, Indian Institute of Technology, New Delhi 110016, India

The influence was studied of the concentration of ammomum fluoride (NH4F) on the etch rates of silicon nitride films deposited by the mercury-sensitized photochemical vapour deposition method. The composition of the buffered HF was varied between 0 and 40 weight percent (wt%) NH4F with 2 to 12wt°/0 hydrofluoric acid (HF). The etch rates as a function of buffered HF composition were measured for films deposited under various process parameters, viz. reactant gas ratio, substrate temperature and chamber pressure. The resuhs of etch rates as a function of process parameters were correlated to variations in material density and silicon content (Si/N ratio) in the films.

1. Introduction n device manu~lcturing technology, various insulating films have been researched over recent years. Today, the dielectric films o f silicon nitride, silicon oxide and silicon oxynitride occupy a dominant position in IC technology. Silicon nitride, owing to its superior masking and protecting capabilities with respect to

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0026-2692/95/$9.50 © 1995 Elsevier Science Ltd

moisture and to various diffusates such as aluminium, phosphorus and arsenic, represents an efficient alternative to silicon dioxide [1,2]. L o w temperature processes are the most important aspect o f m o d e m solid state technology, enabling minimization of undesirable effects such as wafer warpage, defect generation and dopant diffusion [3]. In chemical vapour deposition (CVD) processes, the temperatures required to cause the gases to decompose and react are as high as 900°C [4, 5]. The extension o f this technique towards lowtemperature applications led to the development o f photochemical vapour deposition (photoCVD). In this process, the energy needed to dissociate the reactant gases is supplied by external energy sources (UV radiation emitted from a low-pressure mercury lamp or an excimer laser), thus enabling the films to be deposited at substrate temperatures as low as

563

V.K. Rathi et al./Etch rate of Si3N4 films

100-400°C [6, 7]. A small amount of mercury vapour introduced into the reaction chamber resonantly absorbs UV radiation and transfers energy to the reactant gases, so that the efficiency with which UV energy is conveyed to the chemical system is significantly enhanced [8]. Silicon nitride films of various compositions are utilized in device applications. The rate of etching of these films, an important step in production, is greatly influenced by the structure and composition of the deposited layers. Different types of films can be etched in one step by varying the composition of the solution. Silicon nitride may be etched in fluoride solutions (a mixture of HF and NH4F), by non-fluoride solutions (H3PO4) and by gaseous HC1 [9-12], the most common method being solutions containing HF. The dissolution rate of nitride films is controlled by the film porosity, density and stoichiometry, by bond strains and by impurities. To date, although the etch properties of photoCVD silicon nitride have been reported [1,6, 13], no data are available on etch characteristics as a function of the etchant composition. In this study, the etch characteristics of silicon nitride films deposited by the Hg-sensitized photo-CVD method are examined as a function of the etching solution composition. The effect on the films of NH4F concentration in the etching solution is determined, exhibiting a wide variation in refractive index and material density.

2. Experimental details Thin films of silicon nitride (Si3N4) were deposited on p-type (100) polished silicon wafers by a mercury-sensitized decomposition reaction of SiH4 (2% in argon) and ammonia gases. Before deposition, the substrates were cleaned using the well-known tLCA procedure. The chosen deposition system (SAMCO UVD-10) uses a low-pressure mercury fight source emit-

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ting 185 and 254 nm lines. The mercury vapours were added by flowing silane gas through a temperature-controlled vaporizer containing mercury, before introducing it into the reaction chamber. The temperature of the mercury reservoir was held at 100°C. Films of varying composition were deposited by changing the SiH4/NH 3 flow rate ratio, with a constant flow of NH3. The refractive index and thickness of the films were measured at 632.8nm with a Gaertner ellipsometer (model Ll17). The stoichiometry of the films (defined as the Si/N ratio) was determined by Auger electron spectroscopy). The film etching solutions were prepared from 48% HF and NH4F. The experiments were performed for 2, 6 and 12 wt% HF with NH4F concentration varying from 0 to 40 wt%. The films were etched successively for 5-30 s in the prepared solutions, rinsed in deionized water and blown dry in nitrogen, and the refractive index and thickness of the films were measured.

3. Results and discussion Figure 1 gives the etch rates of silicon nitride films deposited at different reactant gas ratios, as a function of ammonium fluoride concentration. It may easily be concluded that the etch rate was strongly dependent on Si content in the film. From the variation of the Si/N ratio (or Si content) with the reactant gas ratio as shown in Fig. 2, it was found that a lower Si content gives a higher etch rate. The refractive index of the stoichiometric silicon nitride films (,-,1.95) was obtained at a reactant gas ratio of,,~0.04. Figure 3 shows material density as a function of the gas ratio; this ratio corresponded to a maximum film density. A maximum in etch rate was obtained for all the films at an approximate NH4F concentration of 10 wt% in BHF. The slopes of the etch rate curves were found to be least steep for the films corresponding to stoichiometric composition. A steeper slope indicates greater

Microelectronics Journal Vol. 26, No. 6

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concentrations, however, the activity of the charged HF2- is reduced by coordination with NH4 ÷, suppressing the reaction of Si3N4 with HF2-, and the reaction becomes dominated by the HF species.

influence of NH4F concentration, which seems to be determined by the film density. Figure 4 plots etch rate vs NH4F concentration in solutions containing 2, 6 and 12wt% HF. Again, a maximum is obtained in all curves. The results for etch rate vs. NH4 F concentration in BHF can be exl:,lained by a change in the reaction mechanism in the region of the maxim u m etch rate. In the system containing NH4F and HF many species, such as H ÷, F-, HF, HF2- and possibly (HF)xF-, exist in the solution in equilibrium [14]. The rate constant of the HF2- species has been reported to be at least six times higher than that of HF for different dielectric materials [15-17]; it is therefore suggested that HF2- is the principal factor in the etching of nitride films. In the steepest region of etching curves, i.e. at lower NH4 F concentrations, the rising concentration of HF2- results in an increase in etch rate. At higher NH4F

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565

V.K. Rathi et al./Etch rate of Si3N4 films

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Fig. 6. Etch rate vs NH4F concentration in BHF with 6wt% HF for films deposited at different substrate temperatures. be consistent with those in Fig. 3, exhibiting a

decrease in density for the films deposited at higher pressures. N o change in the Si content o f

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Fig. 5. Variation of etch rate with NH4F concentration in BHF with 6 wt% HF for films deposited at various chamber pressures.

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Etch rate vs. NH4F concentration for the films deposited at substrate temperatures 100-400°C is depicted in Fig. 6. Loss o f hydrogen followed by densification o f the network could be responsible for the large decrease in etch rate

Microelectronics Journal, Vol. 26, No. 6

slower slope on either side o f the m a x i m u m etch rate.

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4. C o n c l u s i o n s

By appropriate optimization o f process parameters, films o f various compositions were deposited and the influence o f NH4F concentration in BHF on the etch rates o f the films was studied. For all films, the etch rate was found to reach its m a x i m u m at around 1 0 w t % NH4F. The effect of NH4F was found to be less pronounced fbr denser films, with a

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