Monday, February 18, 2008

Etching of SiCO: Effect of Pores

The following experiments investigate the role of pores in etching of porous SiCO low k materials. Slide 1 shows the SiCO etch rate as a function of etch chemistry and porosity. For etch chemistries with low polymerization such as mixtures of CF4 and Ar, the etch rate increases with higher porosity. The first data point in the graph represents 50 sccm CF4 and 400 sccm Ar and the second datapoint 50 sccm CF4 and 200 sccm Ar. The etch rate for the less diluted CF4 is higher for all three materials and the effect is strongest for the material with the highest porosity. Considering the accuracy of the etch rate measurements, it is reasonable to assume that the etch rate increase is caused by the lower density of the material and the removal rate of the actual SiCO material is constant for all three materials.

The third data point represents a mixture of 30 sccm CF4, 200 sccm Ar, and 10 sccm CH2F2. This chemistry readily forms fluorocarbon based polymers on the wafer surface and reactor walls. The etch rates for all three materials drop significantly. The etch rates for the materials with 30 and 40% pores are almost equal and the etch rate for the material with 50% pores is only 30% higher than that of the material with 50% pores. When the CH2F2 flow is increased to 20 sccm, the etch rate trend as a function of porosity reverses. The etch rate for the material with 50% pores is the lowest and close to the removal rate for Ar sputtering.

One possible explanation for this effect is that is the etch chemistry if sufficiently polymerizing and the etch process close to etch stop, the pores at the surface of the material offer additional adsorption places for the polymer precursors and that they can be filled with polymers.

Slide 2
shows results from XPS measurements for SiCO with 50% pores for Ar/CF4 and Ar/CF4/CH2F2. For both chemistries, a CFx layer is formed. The case of the CH2F2 added chemistry, the combined C and F signal comprise 96% of the total signal indicating the the CFx surface layer is quite thick.

This is confirmed by SEM cross sections of etched samples in slide 3. When the etching proceeds without etch stop in the case of CF4/Ar, significant surface roughness is induced on the top surface of the porous material. In the case of the highly polymerizing CH2F2 process, a thick fluorocarbon layer is formed on top of SiOC closing the pores and potentially leading to etch stop.

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Monday, September 3, 2007

Etching of porous SiOCH

In this study, the effect of the mask material on the etch behavior of porous SiOCH was studied with cross section SEM, decoration methods and XPS. Slide 1 shows the chemistry effect for an oxide hardmask. The results show that the addition of more polymerizing gases like CH2F2 helps to generate more vertical profiles because a thicker and/or more stable sidewall passivation layer is formed. Free fluorine etches Si and carbon and therefore without polymer protection, a more isotropic etch is observed. The selectivity to the oxide hardmask is poor and in the order of 4 to 1 SiOCH to oxide. The more polymerizing chemistry shows lower mask selectivity due to the slower SiOCH etch rate.

Slide 2 summarized the results of the analysis of the composition of the mask surface and the sidewall and etch front with XPS. The mask is covered in a layer of C and F polymers for the CH2F2 added process. Carbon is almost missing at the mask etched with the Cf4/Ar only chemistry. The etch front is rich in the polymer forming species C and F for the CH2F2 added process and contains mostly Si and O when this gas is missing. The XPS analysis reveals that the reason for the more vertical profiles obtained with the CH2F2 added process is a thick F and C containing passivation layer. In sharp contrast, the composition of the sidewall for the sample etched with pure CF4/Ar resembles closely the composition of the etch front. This is a clear indication of isotropic etching.

Slide 3 shows an experiment which was designed to measure the thickness of the perturbed layer on the sidewall of the low k material. The samples were stripped and coated with nitride. The sample was then dipped in diluted HF which removes any damaged SiOCH faster then material with the original structure. The surprising finding of this experiment is that the sample which was etched with the addition of CH2F2 shows deeper material damage then the sample etched with pure CF4/Ar. This means that while the polymer is effective to stop isotropic etching, it allows fluorocarbon species to diffuse through the pores and to alternate the composition. If this modified material stays in the device, it can potentially increase the effective k value of the final structure and ultimately slow down the device
The etch chemistry and process can be adjusted to obtain vertical etch profiles while minimizing the diffusion of fluorocarbon into the low k material. The main conclusion from this experiment is that cross section analysis alone is insufficient to judge the quality of a low k etch.

Slide 4 shows results for SiOCH etches with the same etch recipe but different mask materials. When TiN is exposed on top of the structure, very strong profile distortions can be observed. A very thick layer is deposited on the SiOCH sidewall. Since this effect is absent when the TiN is covered with resist, TiN etch by-products must play a role in the formation of this layer.

Slide 5
shows the temperature effect of etching SiOCH with TiN hardmask. As the temperature is increased, the profiles become more vertical. The thickness and shape of the TiN hardmask is not changed significantly. This means that temperature influences the re-deposition process but not the TiN etch, i.e. the amount of Ti containing by-products remains roughly the same but the amount of these species ending up on the sidewall of the structure is increased at lower temperatures.

Slide 6 provides more evidence for this conclusion. A thick TiFx layer is detected on the sidewall and the bottom (etch front) of the sample etched at lower temperatures. This supports the main learning from the experiments with TiN hardmask: The substrate temperature is a key factor in controlling the amount of TiFx based etch by-products and hence in generating well controlled etch by-products.

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Saturday, February 3, 2007

Materials and Gas Systems in Plasma Etching

Slide 1: General Overview of materials and gas systems relevant for VLSI production

Slide 2: Plasma Etch Chemistries for Materials Systems with Giant (GMR) and Colossal (CMR) Magneto Resistance: NiFe

Slide 3: Plasma Etch Chemistries for Materials Systems with Giant (GMR) and Colossal (CMR) Magneto Resistance: NiMnSb

Slide 4: Plasma Etch Chemistries for Materials Systems with Giant (GMR) and Colossal (CMR) Magneto Resistance: CMR materials

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Non-equilibrium Plasmas

Plasmas used in plasma processing are non-equilibrium plasmas. Non-equilibrium plasmas are characterized by charged species with a much higher kinetic energy than neutral species (slide 1).

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Reactive and Condensable Species

Neutral species that arrive at the wafer surface can stick to the surface and react. Depending on the sticking coefficients and reaction probabilities, reactive and condensable species can be distinguished among the species in the feed gas and the reaction products (slide 1). The balance between reaction and condensation influences the etch profile.

Reactive species: React chemically with surfaces. Reactions are not very temperature sensitive because of low activation energies for the reactions. Surface coverage is typically saturated at a few monolayers.

Condensable species: Form liquid or solid films on surfaces. Surface coverage dependents strongly on the substrate temperature.

Slide 2 shows reactive and condensable species for the example of tungsten Etching with Cl2/O2.

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Electron Energy Distribution Function (EEDF)

The electron temperature of the plasma is generally lower than the threshold energies for dissociation of the feed gas molecules. Dissociation and ionization are induced by the high energy tail of the EEDF (slide 1). The EEDF for inductive and capacitive plasmas are different.

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Electron - Molecule Collisions

Electron Molecule Collisions are the main channel for the creation of species that are used in plasma etching: ions and radicals. Three fundamental reactions can occur when an ion strikes a molecule: electron attachment, ionization and dissociation (slide 1).

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