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The Litmus Test

HF appears to be one of the best chemicals for selectively wet etching favoured high-k material HfO2 but, according to recent research, it must be acidified and heated to work correctly. Kurt Christenson of FSI International explains.

HF appears to be one of the best chemicals for selectively wet etching favoured high-k material HfO2 but, according to recent research, it must be acidified and heated to work correctly. Kurt Christenson of FSI International explains.


The deposition of blanket films with a subsequent etch after gate patterning remains the leading candidate for integrating high-k gate dielectrics at the 45nm node. Early high-k material candidates such as BST etched easily in a range of acidic chemistries, but device and materials studies have now converged on HfO2, its silicates and nitrides [1]. In metal-gate devices, a metal will be present in place of, or in combination with, the polysilicon over the high-k gate dielectric. Which metals will be used is currently under investigation.

The high-k wet etch process will use the metal/polysilicon gate electrode as a mask, but must also be selective to silicon, silicon nitride and deposited silicon oxides. Typical high-k:SiO2 etch selectivity specifications range from 1:1 to 5:1. The traditional plasma and wet etch chemistries used in the formation of SiO2/polysilicon gates have thus far been unable to provide the necessary selectivity and etch rate.

Wet etching
High-k etchants are typically fluoride based. In solution, the HF forms a number of chemical species including H+, F-, HF2-, (HF)2 and HF. The relative concentration of these species varies with the fluoride concentration, the pH, the nature of the solvent and the temperature. High fluoride concentrations favour the fluoride-rich species HF2-, (HF)2. Low pH favours the neutral species HF and (HF)2.

While the etch mechanism and dominant etch species for high-k materials are unknown, it is believed that HF2- is the dominant species in SiO2 etching. Therefore, the etch rate of SiO2 can be suppressed by the use of dilute, acidic solutions.

Figure 1 shows the variation in etch rate as a function of pH for a number of films. As expected, the etch rate of the TEOS - Tetraethylorthosilicate Si(OC2H5)4 - decreases with decreasing pH [2]. The hafnium silicates etch rate, however, increases with decreasing pH. This indicates that the etch mechanisms or dominant etch species differ between the two films.

Figure 2 shows the variation in etch rate of various films with HF concentration. While the etch rate of all films decreases with decreasing HF concentration, the TEOS etch rate drops more quickly than that of the high-k films. This results in increased selectivity with decreasing concentration as shown in Figure 3.

To be used in production, a process must have both sufficient selectivity and a practical high-k etch rate. Acidifying the solution to a pH of 1 serves both needs by suppressing the etching of SiO2 and generally enhancing the etch rate of the high-k film. Other work has shown that the activation energy for the high-k etches is higher than that of SiO2 etches [3]. Therefore, elevated temperatures increase both the high-k etch rate and the high-k-to-SiO2 selectivity.

The etch rate of a film depends strongly on its stochiometry, growth method and thermal history. It appears the key parameter is the crystallinity of the film. Pure, annealed, dense, crystalline films of unary oxides are very difficult to etch with wet chemistries. But any disorder in the film greatly increases the etch rate for a variety of chemistries (Figure 4). The film can be disordered for two reasons.

First, the film can be structurally disordered, having a stoichiometric ratio of metal and oxygen atoms, but with the atoms arranged randomly and not as a periodic crystal. Second, there can be additional atoms present that either prevent or slow the crystallisation of the film, or result in a crystal that is stressed at the atomic level and is vulnerable to chemical attack.

All films with disorder tested to date have been successfully etched with the hot, acidified, dilute HF chemistries listed in Figure 2. Fortunately, recent electrical results indicate that amorphous films may have electrical advantages as well. The grain boundaries form regions of highly enhanced diffusion, providing a path for dopants or metals from the gate to reach the channel. The crystals also act to reduce the electron mobility [4]. Amorphous HfSiO2 and HfSiON are the current leading candidates for production worthy high-k materials [5].

Conclusions
Very dilute, hot, acidified HF has been shown to be a selective etch for HfO2 compounds over silicon, silicon nitride and deposited oxides. The use of this chemistry allows the patterning of the high-k gate with a cost-effective, damage-free wet etch process.
































References
1. H Iawi et al, “Advanced Gate Dielectric Materials for Sub-100nm CMOS,” 2002 IEDM Conf, IEDM Digest, p. 625-628, 2002.

2. DM Knotter, “Etching Mechanisms of Vitreous Silicon Dioxide in HF-Based Solutions,” J Amer Chem Soc, 122(18), 4345-4351, 2000.

3. K Christenson et al, “Selective Wet Etching of High-k Dielectrics,” 2002 UCPSS Conf.

4. T Yamaguchi et al, “Additional Scattering Mechanism for Mobility Degradation in Hf-silicate Gate MISFETs,” 2002 IEDM Conf, IEDM Digest, p621-624, 2002.

5. M Koyama et al, “Effects of Nitrogen in HfSiON Gate Dielectric on Its Excellent Electrical and Thermal Characteristics,” 2002 IEDM Conf, IEDM Digest, p849-852, 2002.

Acknowledgements
We would like to thank International Sematech for providing samples and Steve Nelson for assistance with experimental work and data analysis.



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