AMC increasingly affects process yield
By Scott Anderson, Air Liquide-Balazs Analytical Services
As integrated circuits and disk drives shrink and LCDs grow larger, it is critical that AMC be properly controlled and monitored in electronics device manufacturing
The issue of particle control with the use of HEPA and ULPA filters has long been an essential part of the fabrication process for integrated device semiconductors as well as the disk drive and liquid crystal display industries. Although very efficient for removing particles down to a critical size of 0.05µm, filter technology is not as efficient for removing molecular contamination with average dimensions of 2Å to 30Å. The presence of such airborne molecular contamination (AMC) within the processing areas has played a more significant role as device geometries for integrated circuits and disk drives have shrunk and glass panels for liquid crystal displays have grown much larger. As these manufacturing trends have evolved, ever-shrinking concentrations of AMC have been shown to play a critical role in yield processes for all of these fabrication processes.
Overview of contaminant sources and yield effects
In recognition of growing concerns about AMC effects, a SEMI working group established the F21-95 standard in 1995. Now known as F21-1102, this standard defined the four classes of AMC as molecular acids (MA), molecular bases (MB), molecular condensables (MC or Organics) and molecular dopants (MD). Since the creation of the F21-95 standard, a number of studies and tests have been performed to help monitor and, in effect, limit the amount of AMC within various process disciplines. In this brief review article, we will discuss the sources of AMC and the effects that AMC can produce to limit process yield.
Molecular acids (MA)
Molecular acids in the form of atmospheric contaminants are comprised primarily of hydrofluoric acid, hydrochloric acid, sulfuric acid and nitric acid. The primary sources for each of these MA are the process chemicals used in the manufacturing areas. Acid contamination can spread throughout manufacturing by poor airflow design and recirculation of acid fumes into other process areas. Another source of molecular acids can be outside pollution. The use of chemical filters in process sensitive areas or at the air intake (primarily for removal of organics, NH3, NOx and SOx compounds) has a considerable benefit for removing MA and limiting associated problems.
At concentration levels of parts per billion molar (ppbM), the presence of MA can cause yield problems. These effects include the corrosion of aluminum and copper films; hazing of surfaces-both for products and process tools; as well as a host of electrical faults at the chip level. The interaction of acids and molecular bases in air can produce small-sized particles that can fall onto product surfaces. Important to all silicon-based processing, hydrofluoric acid (HF) is an especially critical MA due to its widespread usage and its deleterious nature. The aggressive nature of HF with SiO2 is critical for the thinner gate oxides now utilized in integrated device manufacturing. An ancillary issue is the presence of HF attacking the borosilicate glass in HEPA filters thereby releasing boron as an airborne contaminant and causing unwanted p-doping of silicon-based processes.
Molecular bases (MB)
Molecular bases include ammonia, amines (including trimethylamine from exchange resins, morpholine from humidifer systems and amines present in photoresist strippers) and amides (e.g., NMP). Ammonia is by far the MB found most often due to its presence in process chemicals (via NH4OH), photoresist chemicals (via HMDS), and its widespread use as an electronic specialty gas (especially for TFT-LCD manufacturing). Similar to MA, careful control of MB via proper air handling is critical for limiting the presence of MB in critical process areas.
The effects of MB are in some cases similar to those of MA in that aluminum or copper corrosion and salt formation can occur as a result of combination with MA in air. In addition, a time-dependent haze due to MB can occur on wafers, disks, and displays. Base-specific yield effects include T-topping of chemically-amplified deep UV (DUV) photoresists. Moreover, lithography processes are susceptible to MB effects due to the excess of ammonia byproducts in lithography chemicals and lithography process areas.
Molecular condensables (MC)
Molecular condensables in air can come from a number of sources found throughout the processing areas. Common examples and sources are shown in Table 1. These organic compounds have boiling points typically greater than 150°C and can adsorb and irreversibly bind to product and tool surfaces.
Beyond establishing proper air handling for MC control, it is imperative that materials used in cleanrooms, components used in air handling systems, and those materials that come into contact with the manufactured product all be tested for outgassing potential MC contamination.
Yield problems caused by MC are well documented and occur at a host of different process steps. Although too exhaustive for this summary article, several examples include phthalates, silicones, and plasticizers that can desorb and cause delamination problems for thin films and photoresists in semiconductor and disk drive manufacturing. Phthalates have been shown to affect gate oxide integrity and can also decompose to form silicon carbide. Finally, optic and mask hazing due to silicones, phosphates, and sulfur compounds has become more problematic with the shift to lower wavelength/higher energy systems (193 nm).
Molecular dopants (MD)
The two dopants of dominant interest and most widespread use are boron and phosphorus compounds. Various implant molecules and chemical vapor deposition (CVD) compounds are sources for both boron and phosphorus. However, two inherent sources of boron and phosphorus are materials used within the fab. The borosilicate glass present in HEPA and ULPA filters is a source of boron compounds. Some trace outgassing of boron over time can occur naturally, but improved control of this boron contamination source can be accomplished by a) strict control of hydrofluoric acid vapors in the air handling system and b) substituting filter material with a boron-free alternative. Phosphorus compounds are also found in and around filtration systems as the flame-retardants in potting compounds. These compounds can contain organophosphates that can outgas and condense on product surfaces.
The yield issues for MD are unwanted n-doping (phosphorus) and p-doping (boron) of silicon. These effects are known to be problematic at levels around 10 pptM, and products will become more sensitive as thinner junctions in advanced devices will produce inherently higher dopant concentrations.
Benefits of controlling airborne molecular contamination
The benefits of controlling AMC are numerous and far outweigh the cost of implementing control methods and performing baseline studies when considering the impact AMC may have on process success. Yield declines due to AMC have been well documented, and complete factory shutdowns, costing millions of dollars, do occur.
It is important to note some of the benefits of controlling AMC:
• Improved yield and process control
• Improved product reliability
• Ability to “copy exact” processes between factories, enabling new factories and processes to ramp up more quickly
• AMC data information allows the engineer to set rational, cost-effective specifications
• Establishing AMC data and specifications by process allows quicker recoveries from contamination events
AMC is, by definition, contamination species found in air. In this article, we have summarized the sources and general yield effects of AMC within the electronics fabrication industries. An important extension of AMC are the contamination species that eventually find their way to the silicon wafer, hard disk drive or liquid crystal display. This surface molecular contamination (SMC) can transfer from airborne species directly to contamination on the product surface or may be transferred indirectly via wafer handling or robotics from process tools or material contaminated by SMC. SMC is important to monitor and control as the eventual process problem and yield issue is one that occurs on the manufactured surfaces, and is not simply an airborne issue. Roadmap definitions and contamination levels were first defined in the 2003 International Technology Roadmap for Semiconductors for surface molecular organics (SMOrg), surface molecular dopants (SMD), and surface molecular metals (SMM). These surface related issues will only become more important as technology generations continue to advance.
The effect of haze or spots on a processed surface, for example, could be due to a number of issues. These include elevated concentrations of silica or TOC in rinse DI water. Alternatively, MA, MB, or MC in air could cause this hazing via MA or MB corrosion of the surface; MA and MB reaction to form particles on the surface; or MC condensation. Each of these gas phase pollutants can be transported to processing areas via insufficient or inefficient air handling. Still many other sources could be investigated and turn out to be the root cause of a simple effect of hazing or spotting on the processed surface, be it silicon wafers, hard disk drives, or liquid crystal displays. As shown by just this basic example, there are many opportunities for AMC to contribute to process yield issues in the modern fabrication environment.
Many yield incidents involving AMC and SMC can only be diagnosed with a thorough understanding of the chemicals, gases, materials and air handling systems in use. These parameters can act independently or in conjunction to promote contamination and process problems. Thus, it is vital that AMC (and SMC) be properly controlled and monitored, especially as manufacturing of electronic devices moves toward smaller features (semiconductors and disk drives) as well as larger areas (TFT-LCD).
A properly defined monitoring program can be limited in scope, and yet still provide important information for establishing baseline data of molecular contamination. It is this baseline information that is important for understanding normal levels of contamination and also allows a comparison with AMC data found during a troubleshooting event. Without baseline data, no comparison can be made and events that harm product yield are much more difficult to find, understand, and eventually correct.
Scott Anderson is the Director of Research & Development for Air Liquide-Balazs Analytical Services. He received his BS in chemistry from North Carolina State University and his Ph.D. in analytical chemistry from the University of Texas at Austin. His research efforts at Balazs focus on analytical method development and solving contamination problems for the electronics industry. He can be reached at Scott.Anderson@airliquide.com.