Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2000-03-14
2002-01-29
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192150, C204S192220
Reexamination Certificate
active
06342133
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method of manufacturing semiconductor devices whereby a Ti layer and a TiN layer are deposited on integrated circuit substrates in a single PVD deposition chamber without using either a collimator or a shutter.
2. Statement of the Problem
In modern integrated circuit (“IC”) technology, a metallization comprising a Ti layer, a TiN layer and a top conductive layer is often provided on a surface of a semiconductor body. The Ti layer serves to obtain a good adhesion and a low contact resistance between the metallization and the semiconductor body. When a layer of Al or Al alloyed with a few percent of Si or Cu is used as the conductive top layer, the TiN layer serves as a barrier to prevent chemical reactions of the Al with the Ti layer and the semiconductor material situated underneath the barrier layer. When W is used as a conductive top layer, deposited by means of a usual chemical vapor deposition (“CVD”) process for which WF
6
is used, the TiN serves as a barrier to prevent chemical reactions between Ti and F which is formed during such a CVD process. Thus, a typical IC fabrication process includes forming a stack of layers represented by Al/TiN/Ti/SiO
x
/Si or W/TiN/Ti/SiO
x
/Si.
In addition, anti-reflective coatings are frequently used in semiconductor processing to reduce light reflectance on the surface of metallic layers. Aluminum is widely used as a metallization layer material due to its low melting point, high conductivity and low cost. However, one drawback of Al is that the surface of Al is highly reflective. This high surface reflectivity greatly hampers the imaging process necessary for lithography. During a lithographic process, a photoresist layer must be deposited on the Al surface based on a photographical pattern previously formed in a photo-imaging mask. The high reflectivity from the surface of Al renders this photographic transfer process extremely difficult. To reduce the high reflectivity of Al, an anti-reflective coating (“ARC”) layer of TiN can be deposited on the surface of Al. The TiN layer appears as a brown or golden tint which significantly reduces the reflectivity of Al from near 100% to approximately 20% at the wavelengths of visible light. This ARC deposition process is a very important step in semiconductor processes whenever aluminum or other highly reflective metal layer is used.
Thus, a typical stack arrangement on an IC semiconductor substrate may include a Ti contact layer on the semiconductor surface, a TiN barrier layer, an Al interconnect layer, and a TiN ARC layer for the purpose of reducing optical reflection. Such a stack may be represented by TiN/Al/TiN/Ti/SiO
x
/Si.
Various plasma vapor deposition (“PVD”) sputtering techniques known in the art for depositing TiN/Ti stacks may be categorized as either nitrided mode (“NM”) or non-nitrided mode (“NNM”) techniques. In the NM (nitrided mode), typically a titanium target is placed in a sputter chamber, and the TiN layers are deposited by sputtering titanium with a sputter gas in the presence of nitrogen. For example, in a typical PVD technique, argon (“Ar”) gas supports a plasma used in plasma sputtering while the N
2
-gas reacts with the sputtered Ti to form TiN. In a NM technique, the titanium target is inundated with nitrogen atoms, becoming “nitrided”, such that a coating of TiN forms on the surface of the titanium target. This decreases the overall deposition rate of the desired layer of TiN onto the IC substrate because the nitrogen in the titanium target interferes with the sputtering of titanium. Another disadvantage is that the titanium target used to deposit TiN cannot then be used to deposit Ti. As a result, separate deposition chambers are required for each stage involving deposition of Ti and TiN. For example, in a conventional process to make a TiN/Al/TiN/Ti/SiO
x
stack, a PVD four-chamber cluster system includes Ti targets in three chambers and an Al target in one chamber. The Ti contact layer is deposited by maintaining a partial pressure of Ar gas in the respective chamber, while the TiN layers are deposited by maintaining a partial pressure of Ar and N
2
gases in the respective process chambers.
It is known in the art to use a shutter that allows deposition of Ti and TiN in the same deposition chamber. A Ti layer is first deposited using a Ti target. Then, a TiN layer is deposited on the Ti layer using the same Ti target in a NM. During TiN deposition in NM, the Ti target becomes nitrided. Before depositing the Ti layer on the next wafer, the shutter is inserted between the target and the wafer, and the target is sputtered in the absence of nitrogen gas to sputter away the nitrided parts. The target is thereby returned to its clean, metallic state and is ready for the sputtering of pure Ti.
To reduce the inefficiency of using a shutter or separate chambers for Ti and TiN deposition, modifications in the sputtering sequence have been suggested in the prior art. U.S. Pat. No. 5,858,183, issued Jan. 12, 1999, to Wolters et al., and U.S. Pat. No. 5,738,917, issued Apr. 14, 1998, to Besser et al, disclose NM techniques in which an extra Ti layer is deposited on a TiN layer before deposition of the aluminum layer, resulting in a Al/Ti/TiN/Ti/substrate stack. During the deposition of a TiN layer in the nitrided mode (NM), a plasma is generated in a gas mixture comprising Ar and N
2
near the Ti target. A nitrided layer comprising nitrogen is created thereby on the Ti target during this deposition step. During an extra process step, the titanium target is sputtered in the absence N
2
gas, resulting in deposition of an extra Ti layer containing nitrogen atoms. The extra sputtering step is intended to sputter away the nitrogen in the nitrided titanium target, returning it to a pure Ti, or metallic, state. If sufficiently cleaned, the Ti target is ready to deposit only Ti atoms on the surface of the next wafer being processed. This approach reduces the number of chambers being used. This known method has the disadvantage, however, that the extra layer comprising free Ti is deposited on top of the TiN layer. If a conductive top layer of Al or Al alloyed with a few percents of Si or Cu is provided thereon, the Al and free Ti atoms react with one another, forming compounds with a comparatively high electrical resistance. As a result, the conductive Al layer must then be provided to a comparatively large thickness in order to ensure that conductor tracks having a comparatively low resistance can be formed in the layer structure thus created. If a W layer is deposited on the extra Ti layer comprising free Ti by means of a usual CVD process in which WF
6
is used, then the free Ti reacts with F formed during such a CVD process. This leads to the formation of TiF
3
, to which W has a bad adhesion.
A similar approach is disclosed in U.S. Pat. No. 5,607,776, issued Mar. 4, 1997 to Mueller et al., which teaches deposition of an extra Ti layer onto a TiN-ARC layer, resulting in a Ti/TiN/Al/TiN/Ti/substrate stack. Even though Mueller et al. teach a thin layer of Ti, the extra Ti layer is very reflective and reduces the effect of the TiN-ARC layer. Furthermore, these conventional methods have the disadvantage of adding a process step to an already slow NM technique.
As IC structures have become more compact, the need for low resistance metal interconnects between these structures has increased. Tungsten deposited by CVD and aluminum doped with Cu or Si have been used recently in the industry to provide these interconnections. In the up-to-date semiconductor device, interconnection holes (contact holes or vias), which are provided in the interlayer dielectric (“ILD”) layer between the circuit elements and the wiring, have become narrower and relatively deeper, and it is difficult to form tungsten or Al alloy in contact holes by a sputtering process. As a result, low pressure chemical vapor deposition (“LPCVD”) techniques having good step coverage have been adopted for filling contact holes with a tungsten (W)
Ashtiani Kaihan
Biberger Max
D'Couto Gerard Chris
Fai Lai Kwok
Lu Jean
Lathrop & Gage L.C.
McDonald Rodney G.
Novellus Systems Inc.
Swenson Thomas
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