An exploration of where trace metals come from, the impact they have on the industry and what can be done to reduce the risks.

KNUT BEEKMANN, Precision Polymer Engineering (PPE), part of the IDEX Sealing Solutions Group, Blackburn, England

Triboelectricity is defined as a charge of (static) electricity generated by friction. The concept was first applied in the 1940s for electrostatic painting and is now widely used in photocopy machines. This phenomenon becomes a concern in wafer manufacturing processes since water is a polar molecule and deionized water (~18MOhm) is a good insulator [1, 2].

When working at the nanoscale of microchip production, even low levels of contamination have the capacity to alter the electrical characteristics of the device and affect the reliability of the end product. Operational hygiene has always been an issue due to the sensitivity of semiconductors to contaminants, but the threat of trace metal contamination specifically is significant. This is mainly true for front end processing but, due to the high mobility of many of these contaminants, it remains a threat at all stages of the manufacturing process flow.

Trace metal constituents of elastomer seals can be released as byproducts during erosion of the seal in aggressive plasma or chemical environments that are part of routine process tool operation. Contamination of semiconductor devices by trace metals adversely affects device performance and as linewidths decrease, the allowable levels of metal contamination reduce. This article explores where trace metals come from, the impact they have on the industry and what can be done to reduce the risks.

Background

Semiconductor microchips, which provide inexpensive, fast computing power for electronic devices, are made from millions or even billions of transistors. The transistor is fundamentally an electronic switch that contains no moving parts but uses an applied low voltage to the gate which in turn allows electrons to move from the source to the drain.

The overall chip making process involves many repeating steps to form the transistor at the front end, and subse- quent formation of the back end interconnect including multiple metal and dielectric levels and several etch steps in between. In the process of building these layers, many transistors are created and interconnected. When completed, a single wafer will contain hundreds of identical chips that must pass rigorous testing. The chip is then mounted onto a metal or plastic package that undergoes final testing, ready to be assembled into final products [1,2].

During routine operation, many components within the process tools and ancillary equipment will be subject to wear and abrasion, particularly those components within the process module that are directly exposed to harsh physical and chemical environments. The most critical locations are those where components are exposed to such environments and in proximity to the substrate being processed.

Equipment consumable items that can sometimes be overlooked are elastomer seals or O-rings. These materials have a certain lifetime proportional to the mechanical and chemical properties of the operating environment and the physical constraints of the groove and location. While an elastomer in a critical location may not actually determine the maintenance cycle of the process tool, byproducts and elastomer constituents will be released into the process environment during active operation. Therefore, whatever constitutes the elastomer can contaminate the wafer and this applies equally to the trace metals.

Trace metal contaminants fall broadly into two categories. Alkali metals which include elements such as sodium (Na), potassium (K) and lithium (Li) and heavy metals which include elements such as copper (Cu), iron (Fe), zinc (Zn), titanium (Ti) and chromium (Cr). The effects on the device of such contaminants vary depending on the type of the element. Sodium for example, can readily lose its outer electron to form an ion with charge +1. It can then readily diffuse through the oxide under the influence of an electric field even at room temperature, however; it cannot penetrate the silicon crystal lattice which means that a charge can accumulate at the silicon/silicon dioxide interface. This in turn leads to unpre- dictable voltage threshold shifts and correspondingly random digital outputs from logic circuits.

Additional failure mechanisms include current leakage through the dielectric and reduced dielectric breakdown voltage, degradation of time dependent dielectric breakdown (TDDB), or complete breakdown of the gate [3]. Gettering layers are also no guarantee of eliminating the issue. Phopho- silicate glass (PSG) and borophophosilicate glass (BPSG) layers are often used to getter sodium ions, however, the presence of moisture either through integral process steps or atmospheric absorption can facil- itate the release of trapped mobile ions in the getter [4]. Rather than accumulate at the semiconductor interface, heavy metals tend to diffuse through the semiconductor, where they effectively create energy states in the bandgap of the semiconductor causing changes in carrier lifetime or the diffusion length [5].

Consumer demands for faster, more powerful and portable technology with greater functionality is a key factor driving the semiconductor manufacturing industry.

Although the part of Moore’s law that refers to shrinking technology remains largely intact, the pressure on cost reduction is rising throughout the whole value chain [6]. Reduced device dimensions and gate thickness leads to devices that become more sensitive to a number of factors including trace metal contamination.

It is clear that such contamination leads to unstable device performance, yield loss, device degradation with increased risk of reliability failures, potentially costing the fab in lost time, loss of revenue and wafer production capacity.

Purity in elastomers

When choosing elastomer materials or seals for critical applications, device manufacturers must ensure that they select appropriate materials with ultra-low levels of trace metals, in order to avoid contamination and device degradation. Manufacturers must also decide on the material in accordance with the location in the tool and the chemistry involved. Critical locations where the elastomer is in contact with the chemistry or process media, where degradation takes place, and where the byproducts of this degradation can be transported to the wafer, require the highest quality seal material in order to avoid contaminating the device. The sealing product must precisely fit the characteristics of the operating equipment.

There is often a large choice of products for any one particular application and ‘semiconductor compatibility’ is often taken for granted especially in critical applications. However, not all elastomer materials are equal when it comes to the level of undesirable contaminants. For many device applications, it is no longer adequate to measure contamination at the parts per million (PPM) level. When analyzing trace metal levels in elastomer materials, vapor phase decomposition (VPD) combined with inductively coupled plasma mass spectrometry (ICPMS) yields data down to parts per billion [7]. A number of different elastomer materials have been analyzed by an independent test laboratory in order to quantitatively determine the amount of trace metal within each sample. The materials analyzed include the leading elastomer brands and the results are graphically represented in FIGURE 1. It should be particularly noted that In order to accommodate all the samples tested, a log scale was used.

The results show that the elastomers that achieved the lowest trace metal content of all materials tested were entirely organic perfluoroelastomers, or FFKMs. The cleanest fluoroelastomer or FKM material was found to be Nanofluor Y75N, again a fully organic highly fluori-nated elastomer. FIGURES 2, 3 and 4 below illustrate the individual levels for several of the key contaminants that should be avoided for three of the cleanest materials tested.

Conclusion

It is clear that the seal lifetime is not the only factor that should be considered when making elastomer choices for specific applications. Elastomer or seal wear in key tool locations during normal operation exposes the wafer to the degradation byproducts of the elastomer material, and therefore also the impurities contained within the elastomer, such as trace metals. FFKM elastomers are particularly suited to the most critical applications, and the harsh environments presented by higher temper- atures, aggressive wet chemical and plasma processes. The more aggressive the environment and the more sensitive the device, the greater is the need to consider the contaminating degradation byproducts of the system components.

Contamination ultimately results in loss of yield, increased cost, or loss of reputation. The use of high purity components becomes a preventative measure, guarding against costly transistor damage or increased risk of poor reliability. Elastomer materials that contain only ultra-low levels of metallic contaminants are ideal for manufacturers of devices at advanced technology nodes and all fabs wishing to minimize the risk of random changes to electrical characteristics and reliability failures.

For further information about how to integrate high performance elastomer seals into your production equipment, and to understand the benefits of customized sealing solutions please contact the author at [email protected].