Al Etch Research Articles

Aluminum Etch and After-Corrosion Characteristics in a m=0 Helicon Wave Plasma Etcher

The etch and corrosion characteristics of stacked aluminum were investigated in a m=0 helicon plasma etcher. Wide process margin was achieved in the aspects of Al etch profile and selectivity of Al to photoresist. The dependence of corrosion resistance on etch parameters was remarkably improved under the conditions of high source power, low pressure, and no N2 addition. The sidewalls of etched Al analyzed by tunneling electron microscopy (TEM) were composed of three different layers: an outermost layer, a middle layer, an innermost layer. When N2 was added, the serious corrosion was caused by the high concentration of residual chlorine captured in the dense sidewall film which mainly consists of Al–O, Al–N, Si–O, and Si–N. It was observed that lowering the source power induced the big increase of both Cl and Cu concentrations.

Modeling magnetically enhanced RIE of aluminum alloy films using neural networks

Aluminum (Al) and its alloy films are widely used for fabricating VLSI interconnections. The discharge behavior of a magnetically enhanced reactive ion etching (MERIE) of Al(Si) has been modeled using neural networks. A 2/sup 6-1/ fractional factorial experiment was employed to characterize etch variations with RF power, pressure, magnetic field and gas mixtures of Cl/sub 2/, BCl/sub 3/, and N/sub 2/. Responses of an Al(Si) film etched in a chlorine-based plasma include etch rate, selectivity to oxide, anisotropy and bias of critical dimension (CD). The generalization accuracy of the models, measured by the root-mean squared error (RMS) on a test set, are 285 /spl Aring//min for etch rate, 5.58 for oxide selectivity, 0.08 for anisotropy, and 3.82 /spl Aring//min for CD bias. Al(Si) etch rate was found to be chlorine-dependent with significantly affected by magnetic field variations. For the other etch responses, RF power was dominant. Gas additives such as BCl/sub 3/ and N/sub 2/ were seen to have conflicting effects on etch outputs. Predicted Al(Si) etch behaviors from neural process models were in qualitative good agreement with reported experimental results.

Effects of O2 Addition on BCl3/Cl2 Plasma Chemistry for Al Etching

Effects of low level (0.5–20%) O2 addition on BCl3/Cl2 plasma chemistry have been investigated using several diagnostic tools: optical emission spectroscopy, microwave interferometry, and mass spectrometry. Experiments were performed using a magnetically enhanced planar 13.56-MHz rf plasma reactor, where aluminum etching was also performed using samples masked with a photoresist pattern of lines and spaces. As O2 was added into a BCl3/Cl2 plasma, the Al etch rate first increased and then dropped above ≈3% O2 added; a transition from reactive-ion-etching (RIE) lag to inverse RIE lag occurred at an O2 percentage of ∼8%. Optical and mass spectrometric measurements indicated that the Cl concentration increases as O2 is added into a BCl3/Cl2 plasma, and that above a critical O2 percentage (∼6% O2) Bx Oy species are formed in the plasma through a reaction between boron chlorides and oxygen and then deposit onto the wafer surface during etching. The Al etching characteristics obtained in BCl3/Cl2/O2 plasmas are interpreted in terms of competitive effects of increased concentrations of Cl and Bx Oy .

Profile of Al etch rate estimated from the analysis of 3-D rarefied flow of Cl 2, BCl 3, and AlCl 3 in a commercial etcher

The etching process of AL wafer in a commercial apparatus is examined by simulating molecular collisions of reactant and product. The surface reaction is modelled by one-step reaction 3 Cl 2 + 2 Al → 2 AlCl 3 with adjustable reaction probability P react. The calculated profile of the etch rate for P react = 0.25 agrees well with the experimental data.

Model for Al Etch-Rate Enhancement at Low Temperatures

A surface reaction model for aluminum etching by chlorine is proposed, which takes into account the temperature dependence of the thermal processes and the ion-assisted processes simultaneously. In the model, chlorine physisorption and chemisorption are treated separately, based on the previous work on chlorine adsorption on aluminum. In particular, cluster formation was introduced into the chlorine physisorption process to allow physisorbed species to exist on the Al surface. The model reproduced the measured Al etch rate, including the Al etch-rate enhancement in the low-temperature regime. This etch-rate enhancement was found to be brought about by the increase in physisorbed chlorine on the Al surface, followed by ion-assisted reaction and desorption.

Al Etching Characteristics Employing Helicon Wave Plasma

Characteristics of helicon wave plasma employing Cl2 gas and its application to Al etching have been studied. Production of Cl ions dominates that of Cl atoms at 1200W RF power at 2 mTorr. Liberated oxygen from a quartz tube sputtered with Cl ions oxidizes Al and the probe surface. This is overcome by adding 10% BCl3 to Cl2. Electron density, temperature and ion current in Cl2 plasma at 2 mTorr are 5×1010 cm-3, 5.5 eV and 15 mA/cm2, respectively. Formation of negative chlorine is dominant at higher pressure. Although the Al etch rate is 5000 Å/min, selectivity to resist and SiO2 is poor due to high substrate bias voltage. Initiation time depends strongly on bias voltage, while net etching time is not changed. This implies that Al etching follows a chemical reaction even at 2 mTorr. Considerably uniform etch rate was achieved in zero magnetic field in a reactor. Slight variation which may result in the helicon wave plasma was observed.

Dry Etching of Al Alloy Films Using HBr Mixed Gases

Al/1%Si films were dry‐etched at 20°C using an or plasma. Conventional reactive ion etch equipment was used. Etch rates of the Al films and photoresist, self DC bias, and selectivities for photoresist and thermal oxide were studied as functions of RF power, gas pressure, gas flow rate ratio, and total gas flow rates. A 0.9 μm/min etch rate, a 6.8 photoresist selectivity, and a 18.9 thermal oxide selectivity were obtained for the former plasma. A 2.3 μm/min etch rate, a 7.7 photoresist selectivity, and a 45.5 thermal oxide selectivity were obtained for the latter plasma. Etched Al films showed highly anisotropic profiles with very slight etch residues for both plasma. Consequently, very high selectivities as well as high Al etch rates and anisotropy were obtained using containing gases.

Aluminum–4% Cu interconnect etching in a low‐pressure magnetically enhanced reactor

A novel approach has been used to obtain residue‐free dry etching of Al–4% Cu alloy interconnects with chlorine‐based plasmas. The addition of external aluminum in a magnetically enhanced plasma etch reactor was used to form AlCuClx, a volatile etch by‐product. Response surface methodology was used to explore the parameter space for a hydrocarbon‐free BCl3/Cl2‐based chemistry. Total gas flows between 60 and 80 sccm, rf power levels between 800 and 1000 W, and BCl3/Cl2 ratios of 0.1–0.25 were found to lead to optimal etches. The response variables studied were Al etch rate, photoresist etch rate, SiO2 selectivity, and post‐etch residue. Al etch rates of 4500 Å/min, photoresist selectivities of 1.5:1, and SiO2 selectivities better than 5:1 were obtained. Optical emission spectroscopy was used to monitor the removal of the Cu‐rich residue by observing emission intensities of the 325‐nm Cu line.

Detailed measurements and simplified modeling of wafer charging in different barrel reactor configurations

The detailed profiles of the wafer charging in different barrel reactor configurations have been obtained by using the electrically erasable–programmable read-only memory devices. Charging profile in a parallel electrode system depends strongly on the wafer orientation with respect to the rf electric field, while minor changes are observed by the use of floating Al etch tunnel and by the reduction of the wafer-to-wafer separation. On the other hand, no wafer charging is detected in a co-axial electrode system. A simplified equivalent circuit model, which represents the potential in 2-dimensional rf plasma–wafer system, has been proposed. The charging profile derived from the simplified model coincides with the experimental results. This model gives an analytical explanation of the gate charging.

Fabrication of an all-refractory circuit using lift-off with image-reversal photoresist

A four-stage shift register was fabricated using Nb/Al-Al/sub 2/O/sub 3//Nb Josephson junctions, Mo resistors, Nb transmission lines, and SiO/sub 2/ insulating layers. The circuit has 36 junctions (5 mu m, 1000 A/cm/sup 2/) and 61 resistors (1.2 Omega /square), with a minimum feature size of 2 mu m. An eight-mask process was used in the fabrication. All material layers were deposited by sputtering. Patterning for all but one of the masking levels was done by lift-off using image-reversal lithography in most cases. Lift-off avoided many of the problems common to reactive ion etching (RIE), including the need for etch stops, nonuniformity in etching, and the formation of organic residue (polymer) on the wafer. RIE was used only in the patterning of the Nb counterelectrode, where a natural Al etch stop from the barrier layer is present. The simplicity associated with the lift-off of all other layers, including the trilayer, is thought to be a major factor in the successful fabrication of the circuit which operated properly up to 4.0 Gbit/s.

A process for improved Al(Cu) reactive ion etching

The standard reactive ion etching (RIE) process for etching Al and its alloys in a single-wafer tool is basically a high-pressure (>100 mTorr), multistep etching process. A significant modification to this process is reported which incorporates the standard high-pressure Al(Cu) etch followed by a low-pressure overetch step. The etch gases used for Al(Cu) etching are a mixture of (BCl3/Cl2/CHCl3/N2). At pressures <100 mTorr, the cathode dc self-bias voltage is increased, enhancing the ion bombardment energy and hence directionality normal to the substrate. Then the component of etching by ion assisted or reactively sputtered etching increases relative to the reactive component, increasing the ability to remove Cu-rich residues. The low-pressure overetch step also exhibits a factor of 2 smaller etch bias than the standard process. This is estimated to be 0.05 μm/image in RIE of 1-μm-thick Al(Cu) films. We have demonstrated that lowering the pressure of the overetch step, hence increasing the ion energies but keeping the same reactive species, offers a more effective clearing process. Combining this clearing process with the overetch does not increase the total time required for the process. Using this process, 1.5-μm-thick metal lines of Ti/A1(2.5% Cu)/Si 0.6 μm wide were obtained with vertical sidewalls.

Surface passivation by adding fluorocarbons and chlorofluorocarbons into an Al dry etch reactor

Adding fluorocarbon or chlorofluorocarbon compounds into an Al etch reactor can alter the induction time, change the Al etch rate, and inhibit the Al etching from the grounded electrode and chamber wall. These phenomena are explained by a surface passivation mechanism. During an etching process, the aluminum can react with chlorine-containing radicals and fluorine-containing radicals simultaneously to form either a volatile aluminum chloride or a stable passivation layer. The important parameters which control the etch and passivation rates, for a given system geometry, are the gas flow rate, the fluorine atomic concentration, and the bias voltage of the bottom electrode.

Cermet as an inorganic resist for ion lithography

Al/O cermet has been found to act as a high‐resolution resist for ion lithography. The change in chemical etching produced by the ions appears to be similar to that produced by excimer laser irradiation. Both a focused Ga‐ion beam and collimated Ar ion or proton beams with stencil masks have been used for exposure. Cermet films 500 Å thick were e‐beam evaporated in an atmosphere of O2 and were exposed to ion doses of 5×1014 to 1.6×1016 cm−2 at energies ranging from 800 eV to 75 keV. The etch rate in a phosphoric acid based, Al etch falls rapidly to about 1/2 of the rate for unexposed material after exposure to 70 keVAr+ ions with a dose of 1×1015 cm−2 and is near zero for doses above 8×1015 cm−2. The etch rate in BCl3 plasma is less sensitive to dose and decreases by 30% for a dose of 8×1015 cm−2. Although comparison of Auger analysis of exposed and unexposed cermet indicates that ion bombardment increases the concentration of Al bound in Al2O3 on the surface compared to Al in the metallic state, the mechanism of the ion induced etch resistance is at this point unclear. A film of SiO2 on Si was patterned using the exposed cermet as a mask to reactive ion etch in CHF3. Linewidths down to 0.1 μm were produced.

Etching Characteristics of n+ Poly-Si and Al Employing a Magnetron Plasma

Phosphorous doped Poly-Si and Al etching characteristics in and outside a magnetron plasma have been investigated. Although the ion accelerating voltage is very low in a magnetron plasma, both materials with resist mask are etched anisotropically owing to the side wall Protection film which arises from resist sputtered by intense ion bombardments. Outside the plasma, though, some undercutting occurs due to poor protection film caused by an insufficient amount of ions, despite the higher ion energy. Thus, anisotropic etching of the whole wafer is achieved by scanning the magnetron plasma. Magnetic field dependencies for Al and n+ Poly-Si etch rates reveal that Al etch rate due to plasma scanning is not enhanced by the magnetic field, in contrast to the case for n+ Poly-Si.

Aluminum plasma etch rate limitations

Etch rates of pure Al films were measured using BCl3–Cl2, SiCl4–Cl2, and He–SiCl4–Cl2 gas mixtures in a parallel plate reactor. Reactant loading was avoided by the use of 1-cm2 etch specimens. Experiments were designed to decouple the affects of Cl2 impingement and ion bombardment upon the Al etch rate. Under conditions of constant ion bombardment, the Al etch rate increased linearly with Cl2 partial pressure. Variations in the slope of Al etch rate versus Cl2 partial pressure were attributed to the presence of B or Si etch residues. The concentrations of these residues were controlled by the energies of ions incident upon the etched surfaces.