Plasma Technology for Advanced Devices

Etching of SiCO: Effect of Pores

2008-02-18T21:35:00.000-08:00

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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Etching of porous SiOCH

2007-09-03T22:03:00.000-07:00

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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Materials and Gas Systems in Plasma Etching

2007-02-03T21:26:00.000-08:00

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

2007-02-03T21:24:00.000-08:00

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

2007-02-03T21:20:00.000-08:00

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)

2007-02-03T21:17:00.000-08:00

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

2007-02-03T21:14:00.000-08:00

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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Electron Reactions in Plasmas

2007-02-03T21:09:00.000-08:00

Electron Impact Reactions (slide 1) convert relatively inert molecules into very reactive radicals. The role of reactive radicals is very important in plasma etching: most of the plasma surface chemistry is achieved thanks to radicals. The role of radicals depends also on the plasma density: in low density RIE discharges, they play a fundamental role whereas ions may be strongly involved in the chemistry in high density plasmas.

Electron Impact Ionizations (slide 2) transform atoms and molecules into ions. Dissociation and ionization may often simultaneously occur: this process is called Dissociative Ionization.

Some molecules tend to capture low energy electrons and form negative ions. This process is called Electron Attachment (slide 3). It produces negative ion fragments as well as neutrals and leads to lower plasma densities. The energy acquired by the molecule in the capture process can cause the molecule to dissociate. This process is called Dissociative Electron Attachment.

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Surface Processes in Plasma Etching

2007-02-03T21:03:00.000-08:00

Physical and chemical surface processes are central to plasma processing. Reactive species are created in the discharge and transported to the surface where they can react and desorp. The reaction and desorption is frequently assisted by the energy of impacting ions. Slide 1 describes these processes for the etching of silicon in fluorine based plasmas.

Besides the reaction at the surface, the discharge equilibrium itself affected by surface processes such as positive ion neutralization and secondary electron emission at surfaces. In processing discharges (ion energy between 10-1000 eV), all positive ions are immediately neutralized at the surface (slide 2).

Heavy particles (ions and neutrals) exhibit the same behavior when they impact surfaces:
- at low thermal energies: physi-and chemisorption and desorption can occur.
- in the tens of electronvolts energy range: molecules can fragment into atoms.
- in the hundreds of electronvolts range: atoms can be sputtered from the surface.
- in the thousands of electronvolts range: implantation is important.

Adsorption and desorption are very important for plasma processing since one or the other of these reactions is the rate limiting step for a surface process (slide 3).

Adsorption A + S -> A:S is the reaction of a molecule with a surface, desorption is the reverse reaction.
Physisorption: Weak attractive van der Waals force between a molecule and a surface. Physisorption is exothermic with Ephys ~ 0.01-0.25 eV. Physisorped molecules are so weakly bound to the surface that they can diffuse rapidly along the surface.
Chemisorption is the formation of a chemical bond with the atom or molecule and the surface. The reaction is strongly exothermic, Echem ~ 0.4-4 eV.

Fragmentation: Ionic and neutral molecules with enough impact energy can fragment into atoms that are reflected or adsorbed when they hit a surface (slide 4). The threshold in energy is of the order of the molecular bond (at energies four or five times above the threshold, 50% of the molecules fragment). High energy molecular ions (energy higher than 50 eV) often fragment when they hit surfaces. Large molecules show frequently delayed fragmentation. The kinetic energy of the impact is transferred into internal energy and redistributed among the bonds as vibrational and rotational energy. If this internal energy is higher than the bonding energy, the ion fragments along this bond. Large molecules with only one type of atoms like C60 are especially resilient to collision induced fragmentation.

Sputtering: At energies above 20-30 V, heavy particles can sputter atoms from a surface. The sputtering yield increases rapidly with energy up to a few hundred volts generating collision cascades in the solid bombarded. Above these energies, the yield is independent of the projectile energy and the target atom density.

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Ions and Radicals in Plasmas

2007-02-03T20:57:00.000-08:00

The energy that is transferred to a plasma is ultimately stored in high energy plasma particles such as fast electrons, photons as well as atoms in high energy states: ions and radicals (slide 1).

Positive and negative ions co-exist in plasmas. Positive ions are very important since they are accelerated through the sheath. Negative ions play an important but secondary role since they don’t reach the wafer under normal conditions. They can influence plasma properties but rarely participate in the surface reactions of the etch process. When electronegative gases are used in plasma etching, the density of negative ions in the plasma can be higher than the electron density. In processing plasmas, the ion to neutral fraction ranges from 10-2 for high density plasmas to the 10-4 to 10-6 range for conventional RIE plasmas.

Radicals are more abundant than ions in molecular gas glow discharges because they are generated at a higher rate than ions (lower threshold energy and ionization is often dissociative) and they survive longer in the discharge than ions.

Slide 2 shows typical relative concentrations of plasma species in low density plasmas (Magnetically Enhanced Reactive Ion Etching (MERIE) and Single Frequency Capacitively Coupled Plasmas (SF-CCP)) and in high density plasmas (Inductively Coupled Plasmas (ICP), Ultra High Frequency Capacitively Coupled Plasmas (UHF-CCP), Double Frequency Capacitively Coupled Plasmas (DF-CCP) and Electron Cyclotron Resonance (ECR) Plasmas).

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Plasma Etch Mechanisms

2007-02-03T20:49:00.000-08:00

Plasma etching is a complex process involving several elementary processes or mechanisms. Some of these processes are illustrated on slide 1:
- chemical etching
- ion induced or enhanced etching
- physical etching / ion bombardment
- trenching
- sidewall passivation
- mask erosion

Anisotropic plasma etching has two major components (slide 2), chemical etching (neutrals and radicals of the plasma) and physical bombardment (ion assisted etching reactions). VLSI plasma etch processes are characterized by a varying significance of the chemical and physical components. Aluminum etching is a very chemical etch while SiO2 etching has a very strong physical component. Silicon etching has a strong chemical and physical components. To understand plasma etching mechanisms and improve processes, it is essential to describe these two components and the synergy existing between these two components.

The synergy between the chemical and physical components in plasma etching was first shown in an experiment by Coburn and Winters. In this classical experiment, the silicon etch rate increased by one order of magnitude when XeF2 was present at the surface and the argon ion beam was turned on (slide 3).

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Spontaneous Reactions in Plasma Etching

2007-02-03T20:38:00.000-08:00

Chemical or so-called “spontaneous” etching is the result of the interaction of reactive free radicals with the surface. Free radicals are electrically neutral species that have incomplete outer shells such as CF3 and F.

The mechanism of chemical etching consists of three elementary steps:
- adsorption of reactive species on the surface and dissociation if it is a molecule
- etch products formation (chemical reaction)
- etch products desorption

In a spontaneous etch process, these steps proceed without the need for activation by ion bombardment. Examples for spontaneous etch reactions are given in slide 1.

A classical example of a chemical reaction which is relevant for plasma etching is the reaction of XeF2 with silicon. Coburn and Winters studied this system and could provide experimental evidence that Ar bombardment enhances the etch rate in this system. This was the proof for the concept of ion enhanced chemical etching which is the foundation of plasma etching. The etch mechanism of XeF2 is actually quite complex and very dependent on the surface temperature as shown on slide 2.

Another way to generate free fluorine without inhibitors is to mix CF4 and O2 in the appropriate ratio (slide 3). The etch rate in the experiment by Mogab, Adams and Flamm peaked for an O2 percentage of 10 to 20 % in the gas feed.

Slide 4 describes etch rate effects for etching silicon in a SF6 plasma. The silicon etch rate is driven by radical concentration in the gas phase. A higher source power leads to deeper SF6 dissociation and more free fluorine. A higher SF6 flow reduces the concentration of reaction products in the gas phase. For very high source power / SF6 flow combinations, the etch rate saturates, indicating that surface processes are becoming rate limiting.

Slide 5 illustrates the importance of the chemical aspect of plasma etching on etch rate selectivities. The silicon to nitride selectivity for a SF6 plasma is shown as a function of Vdc. The lower Vdc, the more chemical the etch. The results show that nitride requires ion bombardment to be etched in a fluorine based plasma while silicon etches spontaneously. Therefore very high selectivities can be obtained by reducing the ion component of the etch. Decoupled ICP or ECR plasma sources and downstream reactors are suited for this application.

The most significant application of chemical etch processes in advanced logic device manufacturing is the so called resist trim process (slide 6). In-situ XPS studies show the presence of a perturbed layer on the sidewall of the resist. It’s thickness depends on the bias power, i.e. ion energy. The thickness of the layer also depends on the microloading conditions, i.e. is different for dense and isolated features. The trim rate slows down for thicker perturbed layers. Bias power, among others, can be used to adjust the dense/iso effect of the trim process.

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The Role of Ion Bombardment in Plasma Etching

2007-02-03T20:33:00.000-08:00

When a surface is exposed to ion bombardment, atoms and molecules can be ejected. This is called Sputtering (slide 1).
Physical sputtering is a non selective phenomenon (materials of different nature can be sputtered at similar rates). It is also a directional phenomenon which helps in obtaining anisotropic etching profiles.
Ion induced damage and mixing: Ion bombardment may favor neutral dissociation at the surface and increase the number of adsorption sites by generating some surface roughness and generating dangling bonds.
Ion enhanced chemical reaction: Ion bombardment may favor the formation of etch products.
Chemical sputtering: Ion bombardment may favor the desorption of etch by products.
Ions as a source of reactants: Ions get neutralized when they reach the surface, they become an additional source of reactive species.

The synergy between ion bombardment and chemical etching was first shown by Coburn and Winters in the classical experiment shown on slide 2.

Besides enhancing the chemical etch, ions also play a major role in removing non-volatile by-products or etch products that require an activation energy to desorp from the surface. The removal of by-products and their re-deposition onto the feature sidewall is the fundamental reason why plasma etching can obtain anisotropic profiles (slide 3)

Factors that influence the anisotropy are (slide 4):
Ion energy flux (ion density and ion energy): Primarily responsible for the etch anisotropy in plasma etching. In general an increase in ion energy flux leads to a better anisotropy.
Neutral to ion flux ratio: The lower the neutral to ion flux ration the better the anisotropy.
Natural species reactivity: Probability for spontaneous reaction of neutral species to react with the surface.
Deposition rate for non-volatile etch products forming a passivation layers at the feature sidewalls.
Substrate temperature: Influences neutral species reactivity and reaction product deposition.

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Etching Profiles

2007-02-03T20:30:00.000-08:00

In most cases, the desired etch profile is square shaped (slide 1). To obtain this perfect profile, etch and passivation have to be carefully balanced and if need readjusted as the aspect ratio of the structure increase. Excessive sidewall passivation leads to tapered profiles which are desired for shallow trench isolation etch. The lack of sidewall passivation and excessive spontaneous chemical etching leads to undercutting. Ion deflection in very narrow spaces can lead to bowing. At the interfaces of layers such as the interface between the gate oxide and the poly-Si layer, notching has to be avoided.

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Trenching

2007-02-03T20:25:00.000-08:00

Profile trenching is caused by ion bombardment (slide 1). Ions arriving at grazing angles on the feature sidewalls get reflected and accumulate leading to a localized higher etch rate. This effect is well known in sputter etching and in plasma processing. At least two possible explanations for trenching can be found in the literature:
- Ion scattering from sloped sidewall surfaces (S. Van Nguyen, D. Dobuzinski, S.R. Stiffler, G. Chrisman; J. Electrochem. Soc. 138 (1991) 1112 / T.J. Dalton, J.C. Arnold, H.H. Sawin, D. Corliss; J. Electrochem. Soc. 140 (1993) 2395 / J.C. Arnold, H.H. Sawin; J. Appl. Phys. 70 (1991) 5314 and others)
- Ion deflection due to differential charging of microstructures (G.S. Hwang, K.P. Giapis; Appl. Phys. Lett. 71 (1997) 458 / M. Schaepkens, G.S. Oehrlein; Appl. Phys. Lett. 72 (1998) 1294 and others).

Slide 2 illustrates research results on the role of the plasma chemistry on trenching. Severe trenching is observed with pure chlorine chemistry in a bias power range of less than 100 W, while HBr and HCl show little or no trenching. This could be explained by the angular distribution of the impacting ions or the presence of fast protons neutralises the negative charging of the mask in the case of HBr and HCl.

Bogart et al. showed that the microtrench shape and depth seems to be in a first approximation independent on the nature of the mask (oxide vs. resist). Hence charging does not seem to be the primary cause of microtrench formation. They concluded that the angular distribution of ions impacting and subsequently scattering from the etching feature are likely to be the primary cause of non-vertical sidewall and microtrench formation (slide 3).

Schaepens and Oehrlein found microtrenching to be influenced by the direction of a weak magnetic field. This field changes only the angular distribution of the electrons. The ions are too immobile. This result strongly supports the differential charging mechanism (slide 4).

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Sidewall Passivation

2007-02-03T20:20:00.000-08:00

At room temperature etch anisotropy is always obtained thanks to the formation of a sidewall passivation layer: The sidewall passivation layer can be formed by different mechanisms (slide 1):
- Mask etch products sputtered into the plasma gas phase by energetic ion bombardment and get re-deposited on the feature sidewalls.
- Condensation of some molecules or atoms originating from the dissociation of the feed gas stock.
- Etch by products dissociation in the gas phase leading to the formation of partially volatile or non volatile etch by products which get re-deposited on the feature sidewalls.
- Direct line of sight deposition of non volatile etch by -products.

Slide 2 illustrates the various passivation mechanisms.

Different etch processes are dominated by different passivation mechanisms (slide 3). This has extremely important consequences for the response of process results like CD, profile angle and profile microloading to process parameters like pressure, source and bias power. Aluminum etch is dominated by re-deposition of resist etch products on the Al sidewalls generating a carbon-based passivation layer. In dielectric etch, condensation of CFx species from the fluorocarbon gas on the oxide sidewalls and re-deposition of resist etch products both contribute to the CFy passivation layer formation. Re-deposition of silicon etch products from the plasma gas phase and direct line of sight deposition of silicon etch products drive the passivation layer formation during silicon (gate) etch.

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Loading Effects and Aspect Ratio Dependent Etching

2007-02-03T20:10:00.000-08:00

Loading effects in plasma etching can be classified into macroscopic loading, microscopic loading and etch rate dependent etching, ARDE (slide 1).

Macroscopic loading causes an Etch rate decreases when the total area of the material to be etched decreases. The effect is caused by the consumption of reactive species during the etch process.

In addition, identical features are etched with different rates depending on their position with respect to open area features (dense areas, semi dense areas, open areas). This effect is called microloading.

ARDE is an effect where features with a high aspect ration (depth/width) have a higher etch rate then those with a small aspect ratio. Under certain conditions this effect can be reversed and is then called reverse ARDE.

Macroscopic loading (slide 2) is most common for isotropic etches with spontaneous etch mechanism, for instance removal of poly-Si with SF6 based plasmas. Certain silicon gate overetch processes also show significant macroloading. Macroloading is related to a change in plasma composition when wafers with different open areas are being etched, i.e. a change in the loading of the plasma with reactants and reaction products. Simply increasing the concentration of reactive species by increasing the source power is not practical because of the simultaneous production of reaction products. Possible solutions are:
- Very high pump speeds (overall very low concentration of reaction products)
- Dilution with inert gases (He, Ar)
- Change of the process properties such that the limiting step is not the reactant supply for instance by adding passivating gases.
- Find a way to favor reactive species consumption or recombination on the chamber walls
- Plasma pulsing to allow reaction products to be removed.

Microloading is caused by a localized depletion of the reactive species or accumulation of reaction by-products as a result of the local pattern density on the wafer (slide 3). Processes with strong macroloading typically also show strong microloading unless the mean free path of the species is much larger than the wafer diameter (which is not the case for normal operating pressures between 1 and 100 mTorr).

ARDE is caused by the effect of the wafer topography on the etch rate of a certain feature. Possible mechanisms have been described on a review paper by Gottscho at el. (JVST 10 (1992) 2133). These mechanisms include Knudsen transport of neutrals, ion shadowing, neutral shadowing, differential charging of insulating microstructures, charging of a polymer sidewall and the interaction of etch and deposition (slide 4).

ARDE has been shown to be dependent on the composition of the oxide for a SAC (selfaligned contact) etch process (slide 5).

SAC etch is a very polymerizing process and the question arises as to how ARDE depends on the interaction between deposition and etch (slide 6). From the experimental data it is clear, that polymerizing chemistries show a different ARDE than pure etching chemistries. In certain cases, heavily polymerizing processes show reverse ARDE (SAC and Al interconnect etch, for instance). To understand the effect of the polymer formation on ARDE, one has to consider the relative importance of etch and deposition and their transport to the etch front within the high aspect ratio microstructure.

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Knudsen Transport of Neutral Species

2007-02-03T20:03:00.000-08:00

RIE lag or Aspect Ratio Dependent Etching (ARDE) is particularly important for high aspect ratio etching. One of the elementary mechanisms that can contribute to ARDE is the transport mechanism of the neutrals. Plasma etching or reactive ion etching relies on the presence of reactive neutrals and ions at the etch front. For very high aspect ratio features, the neutral flux gets attenuated. Coburn and Winters showed that this reduction of the neutral flux towards the bottom of the feature can be described by the Knudsen transport model used to describe vacuum systems. In this model, neutrals are lost either on the feature sidewall as described by the transmission probability or get reflected from the etching surface as described by the reaction probability. The remaining species contribute to the etching process (slide 1).

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Charging Effects

2007-02-03T19:53:00.000-08:00

Charging effects during plasma etching of high aspect ratio structures can cause gate oxide degradation during gate etching and profile deformation issues such as notching or bowing. Charging effects become important for aspect ratios higher than 2:1. The origin of this phenomenon is due to the difference in directionality between ions and electrons when they cross the plasma sheath and interact with three dimensional structures (slide 1).

At low and medium frequencies (< 10 MHz), the ions enter the plasma sheath at different phases of the RF cycle resulting in a bi-modal ion energy distribution. For higher frequencies, the energy distribution exhibits only one peak since the period of the RF signal is much smaller than the time it takes for the ion to travel through the sheath resulting in the ion experiencing just an average field. Because of the acceleration in the sheath, the angular distribution is very directional for ions.

Electrons can respond to the instantaneous electric field, they enter the sheath with a Maxwellian energy distribution in speed but very isotropic directionality. The difference in directionality between ions and electrons leads to charging effects which may strongly impact plasma processes (slide 2).

The following discussion is adapted from Hwang and Giapis, JVST B15, 70, (1997). For featureless surfaces, ion and electron fluxes onto large open areas are equal. Any vertical surface on the wafer (feature sidewall) screens a part of the electron flux: the net flux of electrons arriving on the surface decreases. In contrast, the ion flux is not impacted. If the surface at the bottom is an insulator, it charges positively, possibly leading to a partial deviation of the ion flux. If the mask is an insulator, it will charge negatively. For two adjacent sidewalls are present (space between two lines or trenches), the shadowing effect becomes even more pronounced. The insulator between the lines will be charged even more (slide 3).

Simulations by Hwang and Giapis show that it takes about 1000 RF cycles for the ion and electron fluxes to reach a steady state. This leads to a strongly asymmetric potential distribution. A very strong peak potential develops inside the structure close to the last polysilicon line. The dramatic re-distribution of potential occurs leads to the conditions necessary for an ion trajectory deflection and ion acceleration towards the silicon sidewalls (slide 4).

In the steady state etching regime, 15 eV ions can get repelled away from the outer sidewall. The large positive potential at the trench bottom can slow down energetic ions so that they can be deflected and accelerated towards the lower part of the poly silicon sidewall. Increasing the ion energy to 30 eV reduces these effects (slide 5).

The very strong peak potential that develops close to the edge of the structure induces a ion trajectory distortion. Close to the edge of polysilicon, ions never reach SiO2 at the bottom. They get deflected by the high potential at the bottom that develops on the SiO2 surface and reach the polysilicon sidewalls. There, the ions can generate some etching if their energy is high enough to punch through the passivation layer (slide 6).

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Temperature Effects in Plasma Etching

2007-02-03T19:50:00.000-08:00

The wafer surface temperature depends primarily on the chuck temperature, the ion density and ion energy and the exothermicity of the etching reaction. Surface temperature influences etching processes (slide 1):
- The Reaction probabilities of incident species depends on substrate temperature.
- The vapor pressure of etch products is temperature dependent.
- The re-deposition of reaction products on feature surfaces depends on temperature.

Tight control of the wafer surface temperature is an engineering challenges caused by sudden changes in the plasma condition during the transition between process steps. In addition, the true substrate temperature is difficult to monitor.

Slide 2 summarizes the findings on th selectivities and profile control for silicon etching in a SF6 plasma by S. Tachi et al. (Appl. Phys. Lett. 52 (1988) 617):
-At very low temperature, the silicon etch rate in SF6 based plasmas is not impacted
- SiO2 and photoresist etch rates decrease strongly for decreasing temperatures
- Spontaneous etching reaction between fluorine atomes and silicon are frozen for temperatures below -90°C (no under cut below an SiO2 hard mask).

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Silicon Etching Mechanisms with HBr and Cl2

2007-02-03T19:46:00.000-08:00

The effects of exposing a single crystalline silicon surface to a halogen plasma can be studied with spectroscopic ellipsometry (SE). The measurements show that the surface layer is partially amorphized after plasma exposure. In addition, the existence of a halogen containing layer can be shown (slide 1).

Similar information can be obtained by XPS. The thickness of the amorphized layer is derived from the FHWM of the Si-Si peak (slide 2).

Slide 3 compares the thicknesses of the brominated top layer as measured by SE and XPS. Similarity between etch rate and halogen coverage suggests that the etch rate is limited by the ability of halogenated products at the surface to form volatile species. Halogen coverage can be limited by: thermal flux of neutrals, ion flux, steric hindrance. At etch rate saturation (20 As-1), the silicon atom removal rate is 1016 cm-2 s-1, thermal flux of HBr at 2 mTorr is higher than 1017 cm-2 s-1.

From the studies, the following conclusions can be drawn (slide 4):
- XPS and SE both indicate that the effective thickness of amorphized and halogenated layers are in the range 5-30 Å and increases with ion energy.
- The amorphization effect is almost identical for Cl2 and HBr plasmas.
- The halogenated layer is thicker for HBr.
- The etch rate saturates for HBr due to halogen coverage saturation.
- Halogen saturation is most likely attributed to steric hindrance by Br atoms.

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Plasma and Electrode Potentials

2007-02-03T19:42:00.000-08:00

An RF plasma system can be represented by an equivalent circuit in the capacitive sheath approximation (slide 1). The cathode sheath and the anode (wall) sheath are represented by parallel ohmic and capacitive resistors as well as diode elements. The bulk plasma is an ohmic resistance. The RF signal generates a dc potential on the electrode which can be calculated knowing the RF voltage and the capacitances of the cathode and anode sheaths. For a symmetric reactor (areas of cathode and anode are approximately equal), the average plasma voltage is half of the sum of the RF and dc voltages.

Slide 2 illustrates the plasma and excitation electrode potentials for dc and capacitively coupled plasmas for different cathode to anode area ratios. Most plasma etch reactors are capacitively coupled and the anode (wall and reactor lid) have a much larger area compared to the cathode. For this type of reactor, the dc voltage is negative (which attracts positively charged ions to enhance the etch process) and the RF signal is positive with respect to ground only for a very short period of time. At this point in time, the plasma and RF voltages are equal. The plasma voltage is zero when the RF voltage reaches it's minimum.

Slide 3 shows the ion and electron currents to the powered electrode and the reactor walls for the capacitively coupled asymmetric reactor. A constant ion current is interrupted by a short burst of electron current when the peak voltage of the RF signal becomes positive with respect to ground. The time integrated ion and electron currents are equal maintaining the overall charge balance of the plasma.

The change of the electrode and wall potentials during the RF duty cycle is illustrated in slide 4 (compare to the lower right figure on slide 2).

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Capacitive and Inductive Coupling

2007-02-03T19:34:00.000-08:00

Most low and medium plasma density reactors utilize capacitive coupling while high density plasmas can be generated by inductively coupled, electron cyclotron resonance (ECR) and some high frequency capacitively coupled reactors (slide 1). The capacitive coupling requires a high capacitance between the electrode and the plasma (large amplitude RF voltages). The inductive coupling requires a high inductance between a coil and the plasma (large RF currents). Capacitive coupling results in a high energy ion bombardment while the ion bombardment energy is much lower in inductively couples discharges. In a capacitive discharge, the periodic electron current flow to the electrode causes a modulation of the plasma potential. In an inductive discharge, the time varying current induces a time-varying magnetic field which induces a time varying electric field that can sustain the plasma.

An ideal capacitively coupled discharge is a vacuum chamber with two flat electrodes one of which connected to a rf power supply (typically the bottom electrode which supports the wafer (cathode) (slide 2). The ion density is weak, between 1E9 and 1E10 ion/cm-3. The discharge works in a pressure range between 10 and 100 mTorr. The self bias voltage (Vdc) can reach several hundreds of volts. Major drawback of this design is that it is impossible to control independently the ion density and ion energy.

Slide 3 illustrates the effect of the frequency of the RF signal on the ion energy distribution. Generally, the ion energy distribution function (IEDF) for very high frequencies is monoenergetic. For lower frequencies, the IEDF splits into two peaks with one low energy and one high energy component. The IEDF is ion mass dependent. The IEDF become distorted at higher pressures for which collisions can take place in the plasma sheath.

In addition to the cathode, one of the chamber surfaces, typically the lid, can be RF powered (slide 4). The frequency of the top electrode is usually higher than the frequency of the bias electrode. The higher the delta of the two frequencies, the better the decoupling. The high frequency contributes to the plasma density (anywhere between 1010 and 1012 ion/cm-3 depending on frequency and power) and the low frequency is used to tune the ion energy. The discharge works in a pressure range between below 10 and several 100 mTorr. The high frequency RF power can also be applied to the bottom electrode.

Slide 5 explains the differences between ohmic and stochastic heating in capacitively coupled reactors.

Generally, the plasma density increases when the excitation frequency is increased (slide 6). The exact correlation between plasma density and excitation frequency is however still subject of theoretical and experimental investigations. Nonlinearities have been reported repeatedly (see H. Goto et al., JVST A 10 (1992) 3048

In Magnetically Enhanced Reactive Ion Etching (MERIE), a magnetic field around the source suppresses electron neutralization on the chamber walls and increase the plasma density. The plasma generated is non-uniform due to the drift imposed by the magnetic field ( v x B where v is the electron velocity and B the local magnetic field). Electrons get accumulated on one side of the wafer leading to a strong plasma density and Vdc non-uniformity across the wafer (slide 7).

The plasma uniformity can be improved by introducing a magnetic field gradient close to the wafer or by using a rotating magnetic field. The self bias voltage (Vdc) across the wafer becomes therefore uniform allowing plasma induce damage to be strongly reduced (slide 8). In MERIE sources, ion density and energy cannot be independently controlled unless a second high frequency electrode is introduced.

The effect of the magnetic field in inductively coupled plasmas ICP) is described in slide 9. In ICP sources, a time varying current circulates in the coil and induces time varying magnetic and electric fields in the plasma which sustain the plasma.

Electron Cyclotron Resonance (ECR) plasmas are based on the coupling of an AC electric field, E, with a frequency which matches the frequency at which the electrons rotate in the constant magnetic field, the so called Larmor frequency (slide 10).

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MERIE and high density plasmas

2007-02-03T19:28:00.000-08:00

Slide 1 shows a comparison between low / medium and high density plasmas. Magnetically Enhanced Reactive Ion Etching (MERIE) employs a medium density plasma. Alternative ways to generate low and medium density plasmas are capacitively coupled plasma sources with relatively low excitation frequencies and powers. High density plasma are usually generated by inductively coupled plasma (ICP) sources or sources based on the effect of electron cyclotron resonance (ECR). High density sources operate typically at lower pressures and are characterized by lower ion energies (slide 1).

Typical high density sources for plasma processing have a one order of magnitude higher radical and two order of magnitude higher ion density than medium density plasmas. These properties benefit chemical surface reactions and hence high density plasma sources are the sources of choice for silicon and metal etch (slide 2).

Slide 3 illustrates that for MERIE sources generate conditions under which the etch reaction requires the presence of neutral active species at the surface because the ion densities are too low. For high density plasmas, the theoretically explained by ion driven reactions only. This is most likely the reason why high density plasmas show very high resist erosion rates in oxide etching, the ion flux densities are too high (slide 3).

Slide 4 compares the sources with respect to the effects observed on the wafer. High density reactors were introduced primarily because ion density and energy are decoupled. This gives major advantages in gate etching with very thin gate oxides. The drawback of high density systems is the high density of ions, electrons and photons which can lead to enhanced resist erosion and bending, especially 193 nm resist. Dual frequency capacitive systems can operate in the low and medium density regime and are quasi decoupled. They are widely being used in dielectric etching. The degree of decoupling is higher the higher the source and the lower the bias frequency. High density etchers are used when the etch is very chemical (too high a radical density would make the etch hard to control) and when the passivation comes at least partially from the eroding resist, for instance metal etching.

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Influence of Mask Marterials in Silicon Gate Etching

2007-02-03T18:44:00.000-08:00

The choice of the mask material for silicon gate etching depends on the process requirements. These materials can be grouped into carbon based materials (photoresists, bottom antireflective coatings (BARC) and carbon hardmasks) and silicon based dielectric masks (oxides, nitride, dielectric antireflective coatings (DARC)).

Slide 1 shows results of ellipsometry studies of the influence of the mask materials on the gate oxide etch rate. Under identical process conditions (HBr/Cl2/O2 standard chemistries), the gate oxide consumption is increased by a factor of 4 when going from a hardmask to a resist mask. This corroborates the common notion that resist masks tend to impact gate oxide selectivity negatively.

One possible explanation for the lower gate oxide selectivity is that carbon is liberated from the resist mask during the etch process and deposited on the gate oxide. Oxide tends to etch faster in the presence of carbon due to the formation of volatile carbon oxides. In-situ XPS studies of the gate oxide surface show that while Carbon is present on the gate oxide with the resist mask it is absent on the gate oxide with the SiO2 hardmask (slides 2 and 3).

The loss of gate oxide loss and the carbon concentration on the gate oxide surface both increase with the local resist coverage. When etching resist masked poly silicon gates, the poly-Si/SiO2 selectivity across the wafer is strongly affected by the local resist coverage (slide 4).

Besides concerns about the gate oxide selectivity, other reasons to use dielectric hardmasks in advanced gate etching include the dramatically reduced resist thickness / budget for advanced gate etching as well as mask charging (slide 5).

In-situ reflectometry measurements with a commercial predictive endpoint system provide additional evidence that the gate oxide erodes faster in the presence of photoresists on the wafer. In addition, the experiment reveals that the presence of silicon also lowers the gate oxide selectivity. This effect is smaller than for resist but measurable. A very uniform etch rate across the wafer is therefore mandatory to avoid local gate oxide pitting or punch through (slide 6).

Advanced poly-Si gate stack for high performance devices are frequently doped. Fluorine addition is frequently used to reduce the doping effect in advanced gate etching. CF4 addition is much more efficient than non-carbon containing gases like NF3. With respect to dielectric hardmasks, this has a double negative impact on mask selectivity: Both, fluorine and carbon increase the oxide or nitride etch rate and lower therefore the mask selectivity (slide 7).

The need for fluorocarbon addition drives the resurgence of resist schemes and the emergence of carbon and other alternative hardmasks (slide 8).

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