Precision gas delivery is at the core of semiconductor manufacturing. Many different gases – corrosives, reactives and inerts – in varying flow rates are delivered to process chambers to produce critical features on silicon wafers that are used in manufacturing logic and memory chips. These gases, collectively called electronic specialty gases (ESG), also find applications in the manufacture of solar cells and flat panels including liquid crystal displays (LCDs) and light emitting diodes (LEDs). There is not only a need to deliver these gases in precise quantities and with excellent repeatability but also with stringent purity levels.
As semiconductor technology transitions to advanced nodes below 5nm, the requirements for process gas accuracy, repeatability and contamination-free gas delivery have become paramount. Some of these gases are highly corrosive and some are highly reactive. Careful consideration must be given to compatibility of these gases with wetted surfaces used for gas delivery. Also, some specialty gases such as hydrides thermally decompose at elevated temperatures, frequently necessitating component replacement, and thereby impacting cost of ownership.
In this article, we will review the types of electronic specialty gases used in semiconductor manufacturing and analyze the criteria for delivering these gases contamination-free at the wafer level. A suite of SEMI Standards addresses the purity specifications for specialty gases used in semiconductor manufacturing. SEMI also maintains a comprehensive database of all gases used in semiconductor manufacturing that is constantly updated as more and more new gases are used for advanced process nodes.
Use of specialty gases in semiconductor manufacturing
Over 100 gases are used in semiconductor manufacturing today, primarily in dry etch (dielectric, conductor and polysilicon etch), chemical vapor deposition (both dielectric and metal deposition), diffusion and ion implantation. Examples of these gases are shown in Table 1. SEMI Standards maintains a comprehensive database of all gases used in semiconductor manufacturing in the SEMI AUX030 document, which includes a unique gas code, chemical formula and chemical name including trade names, to help with standardization of these gases to exacting specifications regardless of a fab location worldwide. In addition, the standards help avoid confusion when it comes to existence of isomers of gases with same chemical formula but different chemical properties or reaction chemistries.
Impurity contribution at a wafer level is the most important consideration when delivering specialty gases for semiconductor manufacturing. Impurities originating in specialty gases can lead to killer defects in semiconductor chips, thereby lowering the chip yields. Impurity contribution must be eliminated at manufacturing phase, delivery phase (delivery vehicles such as cylinders), gas delivery systems (material compatibility) and interactions in process chambers (chemistry, plasma). Ensuring highest level of purity at the wafer level requires comprehensive analysis and a system-level approach to contamination control.
ESG manufacturers go to great lengths to purify gases and often utilize purity-detection methods requiring cutting-edge technology. SEMI Standards specifies a suite of standards addressing the purity levels of specialty gases that cover most processing steps used in semiconductor manufacturing. These are maintained by the SEMI Standards North America Gases Technical Committee and they establish definitions, specifications, procedures, and analytical methods for the gases.
Suitability of analytical technique is predicated on ease of use, reliability, maintenance, precision, sensitivity, and availability. The Gases Technical Committee reviews the specifications for gas purity and suitability of analytical techniques every five years to keep current with increasingly stringent demands of advanced process nodes of semiconductor technology. SEMI C3.2 to C3.58 cover purity specifications for various specialty gases.
Another important consideration when establishing wafer level impurity is compatibility of specialty gases with materials used in the gas delivery systems, which deliver gases in precise quantities to process chambers that produce the semiconductor chips. Figure 2 shows gas delivery systems and how they fit in conjunction with other systems in a wafer fab. A gas-delivery system consists of multiple flow channels (gas sticks) and uses a variety of components to control and deliver the gases to process chambers including manual valves, pressure regulators, filters, pressure transducers, mass flow controllers and isolation valves.
SEMI Standards F105 (Guide for Metallic Material Compatibility for Gas Delivery Systems) addresses the material compatibility for all gas-contacted flow paths of these components and piping interconnects. SEMI F105 covers all of the gases listed in Table 1. Material compatibility is a complex function of contaminants in the gas-delivery system (such as moisture, hydrocarbons, oxygen, etc.), reaction kinetics and surface chemistry, which can accelerate a chemical reaction with material used to deliver the gases. For example, Hydrogen Bromide, a common dry etch gas used in conductor etching, attacks stainless steel in the presence of a moisture exceeding 500 ppb2, 3.
Therefore, it is imperative that process equipment engineers implement an excellent purge routine to keep the moisture content in gas delivery systems at low ppb levels. SEMI Standards F58 (Test Method for Determination of Moisture Dry-Down Characteristics of Surface-Mounted and Conventional Gas Delivery Systems by Atmospheric Pressure Ionization Mass Spectrometry (APIMS)) and SEMI F112 (Test Method for Determination of Moisture Dry-Down Characteristics of Surface-Mounted and Conventional Gas Delivery Systems by Cavity Ring Down Spectroscopy (CRDS)) address instrumentation and methods to measure moisture down to ppb levels.
Some specialty gases, such as silane, commonly used in chemical vapor deposition processes, react aggressively with air or oxygen and moisture, resulting in the formation of silica dust in the fluid flow paths of the gas delivery systems, necessitating expensive equipment downtime and component replacement. SEMI F1 (Specification for Leak Integrity of High-Purity Gas Piping Systems and Components) addresses the required level of leak integrity to prevent air or moisture ingress into the gas delivery systems so that such catastrophic reactions can be prevented.
Optimum design of gas delivery system that helps to avoid dead space (which in turn, acts as a reservoir for contaminants such as moisture or oxygen) is also of critical importance for contamination-free manufacturing of semiconductors. Some specialty gases such as diborane, a very important gas used to adjust the resistivity of tungsten (W)/tungsten nitride (WN) barrier films in contact vias of DRAM and logic chips, tends to decompose at moderately elevated temperatures4. This often happens inside the heated thermal sensors of thermal mass flow controllers (MFCs). Such thermal decomposition would necessitate periodic replacement of MFCs and can be disruptive and result in expensive equipment downtime. A way to address such problems is to use a thermal MFC that incorporates a special thermal sensor that operates at lower temperatures or use a newer mass flow controller technology called pressure-based MFCs that do not use any heated thermal sensor at all.
As the process nodes continued to shrink and chip manufacturing became more complex, demand for electronic specialty gases has dramatically increased. The need to deliver these specialty gases, both corrosive and reactive, without contamination to process chambers has become critical. Many aspects of gas delivery affect purity levels of these gases that influence chemical reactions on a wafer level. Contamination arising from gas delivery can cause killer defects on chips, which in turn can adversely impact chip yield.
The industry experts in the SEMI International Standards Program have drafted numerous standards specifically addressing requirements and specifications for many of these gases, analytical methodology for impurity assessment as well as compatibility of gases with metallic materials used in gas delivery systems. The SEMI Standards community is constantly looking to develop standards that meet the stringent requirements of the dynamic semiconductor industry.
About the authors
Mohamed Saleem is CTO at Brooks Instrument (Hatfield, PA), and Laura Nguyen is Senior Coordinator at SEMI Standards (Milpitas, CA).
1. SEMI Standards, SEMI F105, “Guide for Metallic Material Compatibility for Gas Delivery Systems”, www.semi.org/standards
2. S.M. Fine, R.M. Rynders and J.R. Stets, “The Role of Moisture in the Corrosion of HBr Has Delivery Systems”, J. Electrochem. Soc., Vol. 142, No.4, April 1995.
3. S. Krishnan and M. Saleem, “A Feasibility Study of Chromium-Rich Oxide Passivated Stainless Steel Tubing for Gas Delivery Systems”, Semiconductor FabTech, 10th Edition, 2006
4. K. Ashtiani, J. Collins, J. Gao, X. Liu and K. Levy, “Pulsed Nucleation Layer of Tungsten Nitride Barrier Film and Its Application in DRAM and Logic Manufacturing”, SEMI Technical Symposium, Semicon Korea 2006.