Haze Defects Are a Solvable Problem (Part 2)

In Part 1 of this column (see “Haze Defects Limit Life of Photomasks ”), I covered the main types of haze defects that occur on reticles and examined mask lifetime with 193 and 248 nm exposure. This column examines the magnitude of the haze problem and estimates the efforts needed to get it under control.

“Haze defects are a solvable problem,” said Brian Grenon, consultant of Grenon Consulting Inc. (Colchester, Vt.). Grenon should know, since he was in charge of one of IBM's (East Fishkill, N.Y.) mask shops for almost 20 years, and now performs molecular analysis of optical surfaces, mostly reticles.

When reticles are exposed using 193 nm wavelength light, haze defects grow via a photochemical reaction and eventually require that the mask be “repelled” or repelliclized, which takes the mask out of commission while the pellicle is stripped, the mask cleaned, and a new pellicle is put on. Haze defects are either inorganic (such as ammonium sulfate and ammonium carbonate) or organic (hydrocarbons, carboxylic acid, cyanuric acid or other carbon-containing molecules). Sources of organics or sulfate-containing species are continuously being identified. 193 nm steppers now have SO₂ filters on board to reduce haze. Toppan Photomasks (Round Rock, Texas) uses a machine that was built in-house for radiation durability testing of new materials and products. “We need low residuals, cleaner reticles, low-outgassing materials and a clean reticle library environment,” said Franklin Kalk, CTO of Toppan Photomasks.

According to Grenon, there is not a fab or major IC manufacturer using 248 or 193 nm lithography that hasn't experienced the haze problem. “This is a systemic problem, and if you don't attack all the sources, it won't go away,” he said. “When you react quartz with the Piranha clean, you convert the surface to silica gel, which is a magnet for molecular contamination. And the energy required to remove these contaminants is about the order of the strength of the laser you're using, so you're essentially laser cleaning your mask. However, since the reticle is encapsulated in a pellicle, which is a semi-permeable membrane, the contaminants are trapped, and they tend to stick to the edges of the reticle first, but really affect all surfaces.”

Haze can even form on reticles that haven't yet been used by the fab. This is because sulfuric acid and hydrogen peroxide (Piranha clean) are used by most of the mask manufacturers in the qualification of the mask. “Reticles straight from the source can show haze, because they have been cleaned several times, often using Piranha solutions, which is the standard processes. The mask is exposed, developed, stripped and cleaned. This is followed by CD and overlay measurements, and a clean typically precedes the overlay measurement because the clamp can cause image placement errors,” Grenon said. “After that, the reticle is cleaned before the pellicle is put on. If there's a particle under the pellicle, it's stripped and cleaned again.” So there are six or seven routine cleans, even for a clean reticle. “Every time you clean, you form a silica gel monolayer,” Grenon said. “The issue is changing the way people clean.”

However, changing the reticle cleaning process is not trivial. Many mask manufacturers have gone to sulfate-free processing, which substantially reduces the chance of haze growth. Organic films are less of a problem because they can generally be cleaned off with hot water. But the inorganics pose more serious difficulties.

Identifying the optimal inspection frequency for masks is also key. Based on information from two major fabs and hundreds of reticles, the engineers at KLA-Tencor (San Jose) have arrived at some recommendations (Figure ). Critical mask layers must be inspected more often than non-critical, and lightfield masks tend to be more susceptible to contamination than darkfield masks (like vias), also dictating higher inspection frequency. KLA-Tencor's general guideline is to inspect reticles every eight or 15 days for critical and non-critical layers, respectively. The mask requalification approach is a combination of exposure limit and time elapsed.

1. The guideline for inspection strategy is every eight days for critical layers and 15 days for non-critical layers. (Source: KLA-Tencor)

Although inspection frequency is critical, Grenon said the most important issue is determining exactly when a reticle should be cleaned and repelled. If haze defects exist but are not printing, the fab wants to continue its use until haze starts to impact yield. “By changing the cleaning routines and optimizing inspections, the effects of haze can be minimized,” Grenon said.

Find more information on yield management.