The Reactive Ion Etching of Molybdenum and Bilayer Metallization Systems Containing Molybdenum

Abstract

Refractory metals such as molybdenum (Mo) are seeing increased usage in VLSI circuit designs, primarily due to their thermal stability. Properties desirable for a VLSI metallization system with contact to silicon include high metal conductivity, good thermal stability, absence of spiking, minimal electromigration, corrosion resistance, good adhesion, and low contact resistance. It is very difficult to fulfill all of these requirements with just a single molybdenum layer. Other metals such as Cr, Ti, A1, or TiW can be used as thin underlayers to obtain the properties desired. We will describe processes for the reactive ion etching of Mo, and recent work in which chloro/fluoro gas mixtures such as CC12F2/O2, SF6/C12, and SF6/C12/ 02 have been used to etch bilayer metallization systems containing Mo, particularly Mo/TiW. In this study, the effects of different cathode cover plate materials, plasma chemistries, and other processing parameters on the etch rate, selectivities, and sidewall profiles were investigated. An analysis of plasma etch chemistry, using the results of emission spectroscopy, is presented. The methods used for the metal deposition and processing of Mo are also shown to be key factors in determining etching characteristics.Refractory metals and metal silicides have emerged as important candidates for use in the fabrication of VLSI circuits (1-7). As early as 1971, Brown et al. (8-10) described the use of Mo for fabricating self-aligned, high-speed MOSFET's. Polysilicon has a number of properties that make it useful for gate or interconnect applications, but when compared to refractory metals (Mo, W) (11), its high resistivity can limit the performance of gate-level interconnections, especially as it affects the speed of smallgeometry, densely packed circuits (12-13). Molybdenum or tungsten, on the other hand, have resistivities up to two orders of magnitude lower than that of heavily doped polysilicon.In multilevel metal structures (14, 15), the choice of materials used for the lower levels of interconnections depends not only on electrical and metallurgical requirements, but also on the ability of the metal to withstand the high temperatures that it might be subjected to in subsequent processing steps. Aluminum or aluminum alloys, traditionally used interconnect materials, have relatively poor high-temperature capabilities. They react with silicon at only a few hundred degrees (16) and can form a eutectic with Si at temperatures above 550~ that can cause complete circuit failure (17, 18). Therefore barrier layers, such as platinum silicide or titanium-tungsten alloys, must be used to limit such reactions. Furthermore, aluminum alloys preclude the use of processing temperatures above 450~ after their deposition. The long-term stability of finished devices may also be affected because of electromigration.Molybdenum, which has a melting point of 2610~ and reacts with silicon at much higher temperatures than aluminum, can remain stable during subsequent anneals, diffusions, and depositions. It can be ...

Dry Etching of Electronic Oxides, Polymers, and Semiconductors

“…The absence of ion bombardment during photoresist stripping means there is no ion‐induced damage 52–57. Moreover, the addition of a few percent of CF 4 or SF 6 to the O 2 flow can substantially increase the resist removal rate.…”

“…Strip rates as high as 1 μm · min −1 in O 2 /CF 4 can be realized. Furthermore, elevating the sample temperature to 200 °C and employing microwave excitation can push the etch rate up to as much as 8 μm · min −1 52…”

Bulk molybdenum field emitters by inductively coupled plasma etching

“…Here the lateral etch rate was approximately six times less than that in the vertical direction. Others have exploited the plasma etching chemistry of Mo using CCl2F2/O2, SF6/Cl2 and SF6/Cl2/O2 [22]. Molybdenum oxychlorides are formed when Mo is exposed to mixtures of CCl4/Cl2 and O2.…”

“…Oxychlorides have a higher saturated vapour pressure than molybdenum chlorides and thus become volatile at elevated temperatures. Various reports have also shown that the Mo etch rate is dominated by low energetic, yet highly chemically active species [21,22]. More recently, deep reactive ion etching of Mo using SF6 was reported for high aspect ratio structures, 3 with mesa larger than 10 µm produced [23,24].…”

“…Others have exploited the plasma etching chemistry of Mo using CCl 2 F 2 /O 2 , SF 6 /Cl 2 and SF 6 /Cl 2 /O 2 . 22 Molybdenum oxychlorides are formed when Mo is exposed to mixtures of CCl 4 /Cl 2 and O 2 . Oxychlorides have a higher saturated vapour pressure than molybdenum chlorides and thus become volatile at elevated temperatures.…”