HfO2 Etching Research Articles

Plasma reactor dry cleaning strategy after TiN, TaN and HfO2 etching processes

The authors have investigated the etch chamber recovery after TiN, TaN, and HfO2 metal gate etching processes. The deposits formed on the reactor walls after etching these materials in various chemistries have been analyzed by x-ray photoelectron spectroscopy. They found that after a complete polysilicon/metal/high-k gate stack patterning process, the reactor walls are typically covered by a composite layer such as SiOClx–MO–HfBClx (where M is the metal). The SiOClx layer is deposited on the reactor walls during silicon etching (HBr∕Cl2∕O2 plasma); it is subsequently covered by a halogen-rich metal oxide layer during the metal etch step and, finally, by a BClx polymer containing traces of Hf deposited during HfO2 etching (in a BCl3 plasma). They have then investigated the best plasma chemistry to clean the reactor walls after these processes. In particular, they have analyzed the efficiency of several plasma cleaning chemistries to remove each metal from the reactor walls. This allowed them to conclude that after a metal gate etching process, a two step cleaning strategy is required in most cases to remove the deposit from the reactor walls. A Cl2 plasma is first used to remove the HfBClx layer and the Ta coatings (in the case of TaN etching), and it is followed by a SF6∕O2 plasma that is able to remove Ti and SiOClx deposits from the reactor walls. This two step cleaning strategy will always restore clean chamber conditions before processing the next wafer, thus ensuring a good wafer-to-wafer reproducibility.

Etching mechanisms of HfO2, SiO2, and poly-Si substrates in BCl3 plasmas

B Cl 3 based plasmas exhibit promising plasma chemistries to etch high-k materials and, in particular, HfO2, with a high selectivity over SiO2 and Si substrates. The authors report on the mechanisms involved in the etching of HfO2, SiO2, and poly-Si substrates in BCl3 plasmas. X-ray photoelectron spectroscopy analyses help in understanding the mechanism driving the high etch selectivity between HfO2 and silicon-containing substrates. The ion energy plays an important role in the etching mechanisms since it controls a transition between a BCl-like deposition on the substrate and its etching by ionic bombardment. The ion energy threshold above which etching occurs is different from one substrate to another, being lower for HfO2 than for Si substrates. Indeed, BClx deposition forms more easily on poly-Si or SiO2 rather than on HfO2 surfaces, because boron reacts with Si atoms to form Si–B bonds initiating the growth of BClx polymer on Si-containing surfaces, while on HfO2 surfaces, boron is directly involved in the etching and reacts with oxygen to form volatile BOCl etch products.

Effects of additive C4F8 during inductively coupled BCl3∕C4F8∕Ar plasma etching of TaN and HfO2 for gate stack patterning

In this work, the authors investigated the etching characteristics of TaN and HfO2 layers for gate stack patterning in BCl3∕Ar and BCl3∕C4F8∕Ar inductively coupled plasmas and the effects of C4F8 addition on the etch selectivity of the TaN to the HfO2 layer. Addition of C4F8 gas to the BCl3∕Ar chemistry improved the TaN∕HfO2 etch selectivity because adding the C4F8 gas enhances the formation of the CFxCly passivation layer on HfO2 surface and decreased the HfO2 etch rate more rapidly than the TaN etch rate in a disproportionate way. Reduction in the etch time for HfO2 layer also increases the TaN∕HfO2 etch selectivity because the etch time gets closer to the initiation time for HfO2 etching.

Development of a Selective Tantalum Carbide Etchant

A superior nonflammable selective TaC etchant has been developed. This etchant possesses a TaC etch rate of 13 Aå/minute as well as TaC to HfO2 etch selectivity of 140:1. The TaC to SiO2 etch selectivity is high enough to allow the use of this etchant with a SiO2 hard mask.

Influence of ion mixing on the energy dependence of the ion-assisted chemical etch rate in reactive plasmas

Recently, Stafford et al. [Appl. Phys. Lett. 87, 071502 (2005)] have shown that in contrast to the etch yield on a saturated surface, the ion-assisted chemical etch rate cannot universally be modeled by a simple square-root energy dependence. This results from the surface coverage by reactive neutral species being also a function of the ion energy. In this work, we further point out that depending on the plasma-material combination, the etch rate can exhibit two regimes that are characterized by different dependences on the ion energy. While these results are inconsistent with currently available models, we show that they can be interpreted by taking into account ion mixing effects on the desorption rate of volatile reaction products involved in the model of Stafford et al. Application of this rate model to the etching of Si, SiO2, HfO2, and ZrO2 in chlorine and fluorine plasma chemistries provides an excellent description of the simultaneous dependence of the etch rate on ion energy and on ion and reactive neutral fluxes.

Selective etching of high-k HfO2 films over Si in hydrogen-added fluorocarbon (CF4∕Ar∕H2 and C4F8∕Ar∕H2) plasmas

Inductively coupled hydrogen-added fluorocarbon (CF4∕Ar∕H2 and C4F8∕Ar∕H2) plasmas were used to etch HfO2, which is a promising high-dielectric-constant material for the gate of complementary metal-oxide-semiconductor devices. The etch rates of HfO2 and Si were drastically changed depending on the additive-H2 flow rate in C4F8∕Ar∕H2 plasmas. The highly selective etching of HfO2 over Si was done in the condition with an additive-H2 flow rate, where the Si surface was covered with the fluorocarbon polymer. The results of x-ray photoelectron spectroscopy indicated that the carbon content of the selectively etched HfO2 surface was extremely low compared with the preetched surface contaminated by adventitious hydrocarbon in atmosphere. In the gas phase of the C4F8∕Ar∕H2 plasmas, Hf hydrocarbide molecules such as metal-organic compounds and Hf hydrofluoride were detected by a quadrupole mass analyzer. These findings indicate that the fluorine species, carbon, and hydrogen can work to etch HfO2 and that the carbon species also plays an important role in selective etching of HfO2 over Si.

Plasma etching of HfO2 at elevated temperatures in chlorine-based chemistry

Plasma etching of HfO2 at an elevated temperature is investigated in chlorine-based plasmas. Thermodynamic studies are performed in order to determine the most appropriate plasma chemistry. The theoretical calculations show that chlorocarbon gas chemistries (such as CCl4 or Cl2–CO) can result in the chemical etching of HfO2 in the 425–625K temperature range by forming volatile effluents such as HfCl4 and CO2. The etching of HfO2 is first studied on blanket wafers in a high density Cl2–CO plasma under low ion energy bombardment conditions (no bias power). Etch rates are presented and discussed with respect to the plasma parameters. The evolution of the etch rate as function of temperature follows an Arrhenius law indicating that the etching comes from chemical reactions. The etch rate of HfO2 is about 110Å∕min at a temperature of 525K with a selectivity towards SiO2 of 15. x-ray photoelectron spectroscopy analyses (XPS) reveal that neither carbon nor chlorine is detected on the HfO2 surface, whereas a chlorine-rich carbon layer is formed on top of the SiO2 surface leading to the selectivity between HfO2 and SiO2. A drift of the HfO2 etch process is observed according to the chamber walls conditioning due to chlorine-rich carbon coatings formed on the chamber walls in a Cl2–CO plasma. To get a very reproducible HfO2 etch process, the best conditioning strategy consists in cleaning the chamber walls with an O2 plasma between each wafer. The etching of HfO2 is also performed on patterned wafers using a conventional polysilicon gate. The first result show a slight HfO2 foot at the bottom of the gate and the presence of hafnium oxide-based residues in the active areas.

Effectiveness of dilute H2 plasmas in removing boron from Si after etching of HfO2 films in BCl3 plasmas

B Cl 3 -containing plasmas are used to etch HfO2, a high dielectric constant (“high-k”) material. We have investigated several plasmas for their effectiveness in cleaning boron from the underlying Si surface after BCl3 plasma etching of HfO2, while removing a minimum amount of Si. X-ray photoelectron spectroscopy with vacuum sample transfer was used for surface analysis. B cleaning was optimum in dilute H2 plasmas (in Ar) compared to pure H2 plasmas. Dilute H2 plasmas slowed B cleaning process to a controllable time. In a 1% H2–Ar plasma, 20s was required to clean ∼90% B from the surface after a 60s overetch of HfO2 in BCl3 plasmas. The Si substrate was etched <3nm during this cleaning period, which was considerably suppressed comparing pure H2 plasma cleaning. Dilute I2 plasmas (in Ar) can also clean B from the Si surfaces in a short time (10s) but more Si substrate was etched compared to H2–Ar plasmas. BCl3–H2 plasmas deposited a thick B-containing layer on the Si surface instead of combining etching and cleaning into one step.

Dry Etching of TaN/HfO2 Gate Stack Structure by Cl2/SF6/Ar Inductively Coupled Plasma

The dry etching characteristics of the TaN/HfO2 gate stack structure using Cl2/Ar, Cl2/SF6/Ar and Cl2/SF6/O2/Ar inductively coupled plasmas (ICPs) were investigated and the etch rates of the TaN and HfO2 layers and TaN/HfO2 etch rate selectivities were compared. The results obtained for the TaN/HfO2 etching by varying the Cl2/Ar gas mixing ratio, the top ICP electrode power, and the dc self-bias voltage (Vdc) in the Cl2/Ar plasmas showed that low etch selectivities were obtained, due to the high HfO2 etch rate. The effects of adding SF6 to the Cl2/Ar plasmas and adding O2 flow to the SF6/Cl2/Ar chemistry were investigated for the purpose of improving the etch selectivity. Etch experiments performed by varying the Cl2/SF6/Ar gas mixing ratio and Vdc value in SF6/Cl2/Ar plasmas, combined with X-ray photoelectron spectroscopy measurements, showed that the etch rates were reduced compared to those in Cl2/Ar chemistry, due to the heavy fluorination of the surface, however the etch selectivity was increased, due to a disproportionate decrease in the TaN and HfO2 etch rates. The addition of O2 flow to the SF6/Cl2/Ar plasma also increased the etch selectivity at an O2 flow rate of 5 sccm, due to the TaN etch rate being increased, while the HfO2 etch rate remained almost constant.

Selective Dry Etching of HfO2 in CF4 and Cl2/HBr-Based Chemistries

We investigated HfO2 etching characteristics in conventional Si gate etching chemistries, namely, CF4 and Cl2/HBr/O2-based chemistries. We obtained an adequate etch rate of 2.0 nm/min for both chemistries and a selectivity of 1.9 over SiO2 for Cl2/HBr/O2-based chemistry. We examined the etch rate dependence on source power, bias power, O2 flow rate, and Cl2 flow rate in the Cl2/HBr/O2 chemistry. It was clarified that a physical component is dominant in HfO2 etching in this chemistry. The possibilities of achieving a higher HfO2/SiO2 selectivity and of controlling the anisotropic/isotropic component in HfO2 patterning were also discussed. Moreover, it was clarified that the surface portion of the damaged layer created by the dry-etching step can be removed by a subsequent wet etching. Based on these results, the sub-100 nm patterning of poly-Si/HfO2 gate stacks was successfully demonstrated.

Etching of Zirconium Oxide, Hafnium Oxide, and Hafnium Silicates in Dilute Hydrofluoric Acid Solutions

Oxides and silicates of zirconium and hafnium are actively being considered and tested to replace SiO2 as the gate material. Though these materials have the high-dielectric constant (k ∼ 20–25) needed to provide a larger equivalent oxide thickness, they are very refractory and difficult to etch by wet and dry methods. In this paper, work done on wet etching of ZrO2, HfO2, and HfSixOy in dilute hydrofluoric acid (HF) solutions is presented and discussed. Experiments were done on various high-k films deposited by metalorganic chemical vapor deposition. It was found that the as-deposited high-k films can be dissolved with a good selectivity over SiO2 in dilute HF solutions, but heat-treated high-k films are difficult to etch with good selectivity over SiO2 under the same conditions.

Plasma etching of high dielectric constant materials on silicon in halogen chemistries

Plasma etching of ZrO2 and HfO2 was studied in BCl3/Cl2 plasmas, as functions of the ion energy, chamber pressure, microwave power, and gas compositions. MClx is found to be the major etching product in Cl2 plasmas while MBxCly is the major etching product in BCl3 plasmas. The etching selectivity to Si is increased at lower ion energies and higher electron temperatures. Increasing microwave powers and reducing chamber pressures in BCl3/Cl2 plasmas increased the Cl and BCl2+ densities in the gas phase and consequently increased the metal oxide etch rate. A phenomenological model that takes into account the Cl density, BCl2+ density, and metal oxygen bond strength is proposed to describe the etch rate of ZrO2 and HfO2 in pure BCl3 plasmas as functions of the ion energy, microwave power, and chamber pressure. More accurate Cl flux measurement is needed to improve the model predictions.

Ion-enhanced chemical etching of HfO2 for integration in metal–oxide–semiconductor field effect transistors

High-density chlorine plasmas were used to chemically etch HfO2, a promising high dielectric constant material, where the etch rate scaled up linearly with the square root of ion energy at energies above 50 eV. Higher etch rates were obtained at lower pressures and high microwave powers, where the electron temperature and ion densities were high. Optical emission spectroscopy and quadrupole mass spectrometry were used to identify the etching products, which are mainly highly chlorinated hafnium (HfCl3 and HfCl4) and ClO. Surface chlorination was confirmed after etching was confirmed by x-ray photoelectron spectroscopy. The addition of BCl3 in the Cl2 plasmas was found to significantly enhance the HfO2 etch rate and improve the etching selectivity to Si from ∼0.01 in a pure Cl2 plasma to ∼0.9 in a pure BCl3 plasma at an ion energy of 75 eV. The etching selectivity was improved to 4 as the ion energies reduced towards the etching threshold energy in a pure BCl3 plasma. BCl3 plasmas were found effective in patterning HfO2 for fabricating the metal–oxide–semiconductor field effect transistors, enabling a complete removal of HfO2 from the source and drain regions yielding high electron mobility.

Formation of polycrystalline silicon germanium/HfO2 gate stack structure using inductively coupled plasma etching

A gate stack structure consisting of a polycrystalline silicon germanium (poly-SiGe) conductor and a HfO2 dielectric on a silicon substrate was formed by inductively coupled plasma etching using HBr/Cl2/O2. Etch rates of poly-SiGe with 46% Ge were 1.7–2.0 times higher than those of polycrystalline silicon, depending on processing conditions. In the small feature sized devices of 100 nm gates, the notching at the sidewall of poly-SiGe was pronounced up to the depth of 50 nm. The amount of notching increased with increasing inductive power and pressure, and decreasing rf bias power. A HfO2 etch rate of 950 Å/min was obtained at the condition of 550 W inductive power, 360 W rf bias power, and 10 mTorr pressure. Etch rates of HfO2 increased with increasing inductive power and rf bias power. Etching selectivity of poly-SiGe with respect to HfO2 increased significantly with the addition of 3.8% O2 to HBr, and it was possible to control the selectivity in the range of 15–70 by changing the rf bias power. The change in etching selectivity was considered mainly due to the change in HfO2 film property, originating from the incorporation of O into the remaining nonvolatile Hf and the reformation of HfOx during etching.