Turning the heat down

Traditional silicon nitride deposition techniques are not suitable for sub-90nm devices because of the need for low thermal budgets. We report on a new low temperature silicon nitride deposition method developed by Aviza Technology in association with Air Liquide Electronics.

The more advanced an integrated circuit becomes, the more stringent are the demands for certain properties of a dielectric or insulating material. In addition, it is essential that the dielectric/insulating layer maintains its specific electrical, physical and chemical properties after incorporation in the device structure and during subsequent processing.

Due to temperature budget constraints and the accelerated decrease of feature sizes below 0.12µm, one can no longer rely on traditional techniques for creating the layer but has to search for alternatives.

Here, we take a look at a new low temperature method for depositing silicon nitride, a key component of dielectric layers.

Deposition of amorphous silicon nitride (a-SiNx:H) films has been one of the crucial technologies in the semiconductor industry. Owing to the wide range of material properties that can be deliberately tuned to the desired values, a-SiNx:H films have a vast number of functional applications: gate dielectrics in TFT, barrier layers to contact metals, dielectric layers for memory devices, masks for lithography, encapsulation layers and optical coatings with tunable refractive indices (1).

The traditional dichlorosilane (DCS) ammonia (NH3) process for silicon nitride film deposition temperatures requires temperatures above 630 degrees C. Newer bis tertiary-butylamino silane (BTBAS)/NH3 process temperatures are at 570 degrees C or above. Both of these current processes are not suitable for meeting today's modern thermal budget and contamination requirements for sub-90nm film formation.

For chemical vapour deposition (CVD) processes, the thermal budget can be decreased by the introduction of new precursor materials. Chemistries for low temperature deposition of dielectrics such as silicon nitride are typically complex and relatively unknown (2).

As part of an experiment to explore new better method for depositing silicon nitride, Aviza Technologies screened a large number of novel silicon nitride precursor materials on Air Liquide's laboratory reactors to select the most promising candidates for low temperature silicon nitride deposition.

Critical performance characteristics of the precursor evaluated during the selection included low temperature performance, minimal carbon and chlorine (Cl) incorporation in the film, chemical delivery difficulty and cost of high purity chemical in volume production.

Here, we take a look at these issues in more detail.

Low-temperature budget requirements:
The thermal budget becomes a serious problem at shrinking device geometries. Advanced devices below the 90nm node will be made with nickel silicide electrodes. Nickel silicide enables lower junction silicon consumption, lower sheet resistance and reduced agglomeration at required lower temperatures compared to cobalt silicide (3).

To prevent movement of the ultra shallow junctions (USJ) during a subsequent thermal cycle, the temperatures for process steps after USJ formation must be kept below 600 degrees C (2).

Chlorine and carbon contamination:
There is a need today to provide a CVD process to produce silicon nitride films with improved film characteristics without the accompanying generation of byproducts like ammonium chloride and without the incorporation of chlorine or carbon contaminants into the films (4). Chlorine can cause pitting or roughening of the underlying or adjacent silicon. Carbon impacts mainly the wet etch characteristics.

Plasma damage:
With aggressive device scaling and the wide use of plasma-assisted processes, the damage to devices caused by process-induced charging is receiving growing attention (5). Plasma-enhanced processes are not desirable for <65nm due to plasma induced damage (PID). PID is known to cause the "antenna effect" with the following negative implications:
¥ Increase of gate oxide leakage current.
¥ Increase of the threshold voltage of the transistors, and its variance.
¥ Degradation of the gate oxide lifetime.
¥ Degradation of the transconductance of transistors.
¥ Increase of the noise generated by the devices.
¥ Increase of hot-electron effects.

While it is known that plasma nonuniformity is the major cause for charging and that damage is the result of excess oxide current from this charging, many aspects of this mechanism for particular cases are still uncertain (6).

Precursor selection
After evaluating several chemicals, Aviza found that the SATIN chemical best met these performance characteristics.

Production process
The next step for the company was to incorporate the new precursor into the production process.

Aviza used its RVP500 hot wall batch system to deposit the silicon nitride films. Among several key features, this system includes a 50-wafer load in a new process chamber architecture that promotes efficient chemical delivery and evacuation.

Process chamber volume is minimised for quick gas exchange. Dual vertical injectors positioned at the wafer edge are used to establish the cross flow gas dynamics required to achieve uniform film thickness, film composition, and conformal step coverage within wafer, and wafer to wafer within the batch regardless of load size. The SATIN chemical is easily volatilised and the implementation of chemical delivery to the process tool was straightforward. Existing abatement and treatment systems were used to treat the RVP-500 chamber exhaust.

The results of this work can be seen through Figures 1 to 4.

Summary
Aviza Technology and Air Liquide have developed a low temperature silicon nitride deposition on the RVP-500 50-wafer batch tool using the unique chlorine and carbon free SATIN precursor.

The chamber design provides laminar flow across the wafers, resulting in excellent within-wafer, wafer-to-wafer, and run-to-run thickness uniformity and conformal step coverage in high aspect ratio trenches.

The SATIN precursor in conjunction with the RVP-500 chamber provides a solution to meet the cost of ownership and technical demands of current and future technology nodes for large-scale production.

Authors
Helmuth Treichel, Cole Porter, Karl Williams and Thierry Lazerand of Aviza Technology.
Benjamin Jurcik and Ashutosh Misra of Air Liquide Electronics

References
1 VI Belyi, LL Vasilyeva, AS Ginkvker, VA Gritsenko, SM Repinsky, SP Sinitsa, TP Smirnova and FL Edelman, Silicon nitride in electronics (Elsevier, Amsterdam, 1988) p34.
2 AE Braun, Semiconductor International, 3/1/2004
3 AE Braun, Semiconductor International, 3/1/2003
4 C Dussarrat, JM Girard, T Kimura, N Tamaoki and Y Sato, WO2004/030071, 04/2004
5 T Brozek, CR Viswanathan 1997 Semicond Sci Technol 12 1551-1558
6 S Ma, KC Saraswat, IEEE Electron Device Letters, Vol 18, No 10, October 1997