Vladimir Bliznetsov

Improving aluminum nitride plasma etch process for MEMS applications

We present a new plasma etch process optimized for etching piezoelectric aluminum nitride (AlN) film deposited on thin molybdenum (Mo) metal electrode. Such film stack finds application in the integration of AlN-based RF microelectromechanical systems devices. The process is based on Cl2/BCl3/Ar gas chemistry with added buffer gas in inductively coupled plasma reactive ion etching system. The new gas mixture overcomes a generic problem of etched surface roughness without significant drop in AlN etch rate. Using design of experiment, the process window is optimized for improving selectivity to Mo and reducing microtrenching while maintaining smooth etched surface. Finally, an etching rate of 280 nm min−1 with reliable etch stop on Mo electrode and smooth bottom surface is reported. The integration suitability of the developed etch process is tested by etching 2.0 to 5.0 µm size square shaped via holes in 1.0 µm thick (0 0 2) oriented piezoelectric AlN on 0.2 µm thick Mo electrode while integrating contour mode resonators.

High-throughput anisotropic plasma etching of polyimide for MEMS

This note describes a new high-throughput process of polyimide etching for the fabrication of MEMS devices with an organic sacrificial layer approach. Using dual frequency superimposed capacitively coupled plasma we achieved a vertical profile of polyimide with an etching rate as high as 3.5 µm min−1. After the fabrication of vertical structures in a polyimide material, additional steps were performed to fabricate structural elements of MEMS by deposition of a SiO2 layer and performing release etching of polyimide.

Deep SiO2 etching with Al and AlN masks for MEMS devices

Silicon oxide-based materials such as quartz and silica are widely used in microelectromechanical systems (MEMS). One way to enhance the capability of their deep plasma etching is to increase selectivity by the use of hard masks. Although this approach was studied previously, information on the use of hard masks for the etching of silicon-oxide based materials on 200 mm substrates is scarce. We present the results of etching process development for amorphous silicon oxide using Al and AlN masks with a view of the application of the results for the etching of silica and quartz. Three gas chemistries (C4F8/O2, CF4 and SF6) and their mixtures were compared in an industrial reactive ion etch (RIE) chamber with two plasma sources. It was established that pure SF6 is the best etchant and AlN is a better mask than Al for providing higher selectivity and a sidewall angle close to vertical. A range of etching parameters for micromasking-free etching was established and etched structures of up to a 4 : 1 aspect ratio were created in 21 μm-thick oxide using the process with an etch rate of 0.32–0.36 μm min−1 and a selectivity to AlN mask of (38–49) : 1.

Effect of Etching Process Deviations and Photoresist Stripping on Contact Yield of Copper Dual Damascene Metallization

Dual damascene technology imposes strict requirements on etch and strip modules. One of them is to avoid exposure of copper on via bottoms during etching and subsequent plasma photoresist stripping (PRS) as it may lead to oxidation and contamination of copper and thus decrease contact yield. Therefore, etching should he reliably tuned for retaining some thickness of the protective cap dielectric in vias of targeted size. However, deviations of etching parameters in mass production may deteriorate selectivity and uniformity, resulting in partial or full exposure of copper in vias. In this case the effect of PRS on copper surfaces must be taken into consideration. This paper presents the study of copper surface oxidation and contamination and their effect on contact resistance and yield for three different PRS recipes: O 2 plasma at 40°C in a magnetically enhanced reactive ion etching (MERIE) reactor and O 2 and O 2 + I % CF 4 downstream plasma at 250°C in an inductively coupled plasma (ICP) reactor. It was found that O 2 PRS in the MERIE reactor and O 2 PRS in the ICP reactor oxidized the surface of copper to different depths hut both processes did not reduce the yield of Kelvin vias and via chains. At the same time, O 2 + 1% CF 4 plasma in an ICP reactor resulted in total failure of via chains in which copper was exposed before PRS (via size larger than 0.32 μm). Further implications of these results on dual-damascene process flow, mechanism of failure, and advantages of low-temperature PRS are discussed.

Control of corrosion on aluminum MEMS structures after post etch clean

During the fabrication of aluminum MEMS (Microelectromechanical Systems) structures for display applications, surface corrosion or pitting appeared intermittently on the aluminum structures after aluminum (Al) plasma etch and post etch cleaning. Such surface corrosion severely limited the light reflecting capabilities of the planar aluminum micromirrors. The formulated organic solvent cleaners or post etch residue removers used after aluminum plasma etch such as NE14 from Air Products (AP) and ST250 from Advanced Technologies and Materials Incorporated (ATMI) were suspected to be the main culprits of the Al corrosion but without conclusive evidences. In order to better understand the causes of the post etch aluminum corrosion, extensive work has been carried out to look at the factors that may lead to Al corrosion after plasma etch and post etch clean. In particular, we have evaluated the surfaces of Al structures after NE14 and ST250 post etch cleaning using tools such as SEM and AFM. We focused on the three main aspects that may affect the Al corrosion: The Al plasma etch process, the extended cleaning in the organic solvent chemistries, and the Al corrosion as a result of water dilution in the organic solvent cleaners.

Plasma etching of SiO 2 with tapered sidewall for thin film encapsulation

We proposed the way to solve the problem of cracks in AlN cap layer for thin film encapsulation of MEMS. By development of sacrificial SiO2 etching with smooth tapered sidewall, the quality of subsequently deposited AlN cap layer is improved and reliable device sealing is achieved with a single cap layer.

Comparison of aluminum post etch cleaning on MEMS structures using formulated organic solvent cleaners

The formulated organic solvent cleaners for aluminum (Al) post etch residues removal have been available on the market for many years. They are used in large quantities in the fabrication of integrated circuits with aluminum interconnects. However, the effectiveness of these chemistries on the aluminum MEMS structures is less well known. In this study, we compared the effectiveness of four different formulated organic solvent chemistries for Al post etch residues removal for certain types of aluminum MEMS structures. The four different formulated solvent clean chemistries evaluated in this study were ST250 from Advanced Technology Materials Incorporated (ATMI), NE14 and ACT690S from Air Products (AP), and EKC265 from DuPont. Both ST250 and NE14 were implemented in a single wafer cleaner as they are typically used in a single wafer cleaning environment. ACT690S and EKC265 were implemented in a tank on a wet bench as they were formulated to work in total immersion environment. Short loop wafers of Al MEMS structures of several microns in sizes were etched in a DPS (Decoupled Plasma Source) metal etch chamber using Cl2/BCl3 plasma followed by H2O-based plasma photoresist strip in an ASP (Advanced Strip and Passivation) chamber on the Centura etch platform. These wafers were then cleaned in one of the four different solvent chemistries for comparison. We found that each organic solvent cleaner has its own advantages and disadvantages in cleaning efficiency, cost, as well as the post etch metal corrosion. For each and every organic solvent cleaner, the process conditions during cleaning must be optimized in order to achieve the best results for residues removal and corrosion prevention.