Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
1999-10-14
2001-11-27
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S753000, C257S767000, C257S771000, C257S773000
Reexamination Certificate
active
06323553
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to integrated circuit metallization structures and methods.
BACKGROUND: ASPECT RATIO
In modern integrated circuit fabrication, it is increasingly necessary to fill vias and contact holes which have a high “aspect ratio”. This means a ratio of height to width which is 2:1 or more, and, as technology progresses, may be as high as 10:1 or more in future generations. Completely filling such holes with metal at an acceptably low temperature is very difficult, particularly for metals (such as aluminum) which do not have a good low-temperature CVD process.
Currently there are two types of processes which appear attractive for filling such holes: reflow methods and extrusion cavity filling methods. Reflow methods apply a high temperature to help newly-arrived atoms to move around on the metal surface; extrusion cavity filling methods (like the “ForceFill” (TM) process) apply physical pressure to force a soft layer of as-deposited material into the hole. However, both of these processes require high temperatures, which are incompatible with many materials, such as organics, polymers, and low-temperature dielectrics.
BACKGROUND: ALUMINUM METALLIZATION
One of the best characterized metallization systems for integrated circuits is aluminum. The interrelations of aluminum, silicon, and silicon oxides have been very extensively studied and characterized, and a huge wealth of practical experience has been accumulated, over the last 30 years, in dealing with this material system. Thus, while there has been much discussion recently of using alternative metallization systems, such as tungsten or copper, the aluminum metallization system is still extremely attractive.
For aluminum reflow to be compatible with low dielectric constant materials, the reflow temperature must be less than 450 degrees C. Furthermore, reflow processes normally require a good diffusion barrier for aluminum to be used at contact level; but the use of diffusion barriers will add to the metal stack height, thus increasing the line-to-line capacitance and the contact resistance. Furthermore, the speed of the chip will be reduced as well as the effective contact size.
Temperature constraints during metallization processing are important. For example, high temperatures (above 450 degrees C.) are incompatible with the low-permittivity materials which are desired for use in modern processes. Many of these materials are organic, and cannot withstand very high temperatures.
One constraint on aluminum metallization is imposed by the phenomenon of junction spiking. When pure aluminum is heated (even below its melting point) it can dissolve silicon which is in contact with it. Thus if pure aluminum meets monocrystalline silicon at a contact hole, the aluminum may grow dendritically into the silicon during thermal cycling. Such aluminum growths can penetrate junctions in the silicon, and thus destroy the desired electronic device. The conventional solution to this problem is to use aluminum metallization which is alloyed with a small fraction of silicon, e.g. 1% atomic. This fraction of silicon reduces the aluminum's tendency to dissolve silicon. However, the use of silicon alloys brings in some further problems.
Another constraint in the aluminum-silicon system is posed by precipitation. Thermal cycling of silicon-alloyed aluminum can result in the formation of monocrystalline silicon precipitates within the aluminum, or at the interface from the aluminum to the silicon or to the diffusion barrier. Such precipitates may be difficult to clear during the aluminum etching and/or may degrade the net resistance of the contact.
Another concern with aluminum metallization is electromigration: a pure aluminum line may gradually thin out, in service, in locations of high current density. However, the addition of copper greatly reduces this tendency.
Thus typical aluminum alloys use silicon (typically one half percent to 1% atomic) or copper (typically one-half % atomic) or both as alloying agents. Efforts have been made to find other satisfactory aluminum alloy compositions; see e.g. Kikuta and Kikkawa, “Electromigration characteristics for Al—Ge—Cu,” 143 J. Electrochem. Soc. 1088 (1996), which is hereby incorporated by reference.
Reduced Temperature Contact/Via Filling
The present application discloses a new metallization technology in which a melting-point depressant is locally introduced into the metal layer where it contacts the sidewalls of contact/via holes. This is preferably accomplished by a thin wetting layer which is only exposed on the sidewalls of contact/via holes. The wetting layer lowers the melting point and yield stress of the primary metallization layer at the points where this will most help the primary metallization layer to fill the contact/via hole. Since the concentration of the melting-point depressant is localized near contact/via holes, problems of precipitation or etch residues are minimized. This is particularly advantageous with aluminum metallization, but is also applicable to other metallization systems.
The advantages provided by the innovative structures and processes of the present invention include at least the following:
manufacturable;
no hardware modification needed;
effective in lowering the process temperature;
effective in preventing junction spiking;
little or no silicon/germanium precipitation;
ultra-thin protection layer can be formed; and
good extendibility to future generation of devices.
REFERENCES:
patent: 5355020 (1994-10-01), Lee et al.
patent: 5594278 (1997-01-01), Uchiyama
patent: 406291082-A (1994-10-01), None
Hong Qi-Zhong
Hsu Wei-Yung
Brady III Wade James
Loke Steven
Telecky , Jr. Frederick J.
Texas Instrument Incorporated
Valetti Mark A.
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