A system is described that mitigates unintended oxide growth for bare wafers while in-process storage and potentially post process at tools using nitrogen purge. 

BY SURESH BILIGIRI, Rorze Automation, Fremont, CA 

Unintended oxide growth on wafers while in storage or while in process is an important cause of excess process variability that can lead to poor yield and product quality.

To understand and eliminate the undesired oxide growth on wafers while in storage or between processes, we evaluated and compared wafers storied in a nitrogen-based environment to wafers stored in normal cleanroom environment. Important issues to consider:

• A typical wafer goes through a clean cycle prior to wafer moves.
• There are waste chemical handling costs involved in the handling, treatment and disposal of waste chemicals.
• If the oxide growth are uncertain, then metrology of these wafer before wafer move has to be performed and this adds more cost to the process.
• If the oxide growth has exceeded the specification, then a second re-cleaning cost is added to the process.
• Due to stringent environmental controls and corporate responsibility in green initiatives the cost of handling the waste chemicals will have an impact of greater than twice the first cleaning.
• Each time a wafer is handled, the risk of loss increases causing an impact on yield.
• In a high demand situation where the fab utilization is approaching the high numbers, the re-clean adds costs and a negatively impacts production volumes.

In order to assess the impact of the unintended oxide growth, tests were conducted at a semiconductor manufacturing fab using a standard bare wafer stocker (BWS600 by Rorze) and a nitrogen purge type stocker (BWS1600 N2 by Rorze). The wafers were removed from each to test for oxide growth and returned to storage after measuring. The tests were repeated with the same set of wafers and data is shared here.

The Rorze BWS1600 N2 consists of wafer PODS (about the size of a FOUP) that are stacked on a carousal. Each POD with 25 wafers is purged with N2 continually. In order to minimize the use of nitrogen and to create a very low O2 level, the POD has independent access door for each wafer slot on the POD (9mm door). This method (Rorze patent pending) offers N2 environment for wafers inside even during the wafer transfer from and to the POD with minimum ambient air interaction. The system and the storage method is shown in FIGURE 1.

FIGURE 1. The design is executed for minimal consumption of N2 as well as to mitigate the hazards of excess N2 in the fab.

FIGURES 2 and 3 shows the N2 and O2 purging sequence data. Note that:

1. When the shutter is open, O2 density will be increased because of mixture of air of mini- environment (FFU) is forced into the POD/ container environment.
2. When the shutter is open, N2 gas supply volume will be increased from 5 L/min to 20 L/min from the POD that helps to reduce O2 density inside the POD.
3. At any given time, when the shutter is open, even with the strong FFU flow as the wafer on end-effector directs flow from FFU the N2 concentration in the POD does not go above 5000 PPM

FIGURE 2. O2 density during wafer handling.

FIGURE 3. N2/oxygen density data during storage conditions.

The results of the oxide growth measurements on wafers stored in N2 environment (Rorze BWS1600 N2) were obtained on a regular frequency (Day 1, 3, 6, 7 10, 14 & 21).

Measurements were made using “Rudolph S3000A” metrology thicknesstool.Toensure the effect is uniform across the stored area, wafers were placed in different PODS inside (C8 is at top on the carousal close to FFU and C1 is farthest from FFU at the bottom).

An identical test method was executed by storing wafers in a bare wafer storage unit where wafer was exposed to the fab environment but in a clean storage area. (Rorze BWS600)

Results of the tests are shown in FIGURE 4. Wafers were set at different locations in the stocker to test the influence of storage location on rate of oxide growth. We found no noticeable difference in oxide growth for different cassettes. Even after 21 days with intermittent extraction to monitor growth (every three days), all wafers had less than 1.7 Anstrom thickness oxide growth. The impact of oxide growth without intermittent exposure could be much smaller.

FIGURE 4. Oxide growth is small even after 21 days.

Cost analysis

As per an earlier Sematech model that takes into account the cost of materials, capital tool costs, uptime in the fab etc. for wafer clean per wafer pass following were the costs estimated and noted below. (Data reference provide by Mr. Rob Randhawa, Founder & CEO of Planar Semiconductor)

• Single wafer cleaning cost per wafer using DI water based cleaning only: $1.90 per wafer – 300mm wafer
• Single wafer cleaning cost per wafer using standard chemicals without the IPA: $2.30 per wafer – 300mm wafer
• Single wafer cleaning cost per wafer using standard chemicals + IPA: $ 3.60 per wafer – 300mm wafer – This includes the reprocess cost of IPA

Based on the initial results we see, there is a substantial benefit to employ this technology that will help to make strides in continuing to help on cost controls while the technology node advances. The opportunity to eliminate the risk of oxide growth can potentially go beyond the wafer clean and to “in-process storage” where a wafer lot is in queue for the next step. There is a risk of delay where the wafer could continue to gain oxide growth resulting in potential yield loss and a domino effect of reduced productivity and a risk of not meeting demand.

As technology proceeds to smaller nodes, the tolerance for variations within atomic layers are not acceptable as it will impact performance and yield, necessitating such products and technologies to keep the cost down.

Tools such as the N2 purged bare wafer stocker can save anywhere from about $900,000 to about $1.7 million per year and easily pay off the cost of the system within a year or two.

Acknowledgements

Rob Randhawa, Founder and CEO of Planar semicon- ductor for sharing wafer cleaning costs, K.Sakata, Design Engineering Manager, at Rorze Corporation for technical details of the BWS1600/BWS3200 N2 purge system.