2. Calculation and Classification of Laser CoD
2-1. Conversion of Downtime into Cost
Figure 1 illustrates the method for converting the downtime into the cost. Based on the assumption that one scanner system processes 1,000 wafers per day (1000 WPD) at a cost of 1,500 yen per wafer, as shown in the graph on the left in figure 1, the number of wafers produced per day is reduced to 900 WPD if the laser uptime is 90% and, as a result, the cost of producing a wafer is increased from 1,500 yen to approx. 1,670 yen. This increase in the production cost is an extra cost of 170 yen per wafer while, in addition, an opportunity loss of 150,000 yen (1,500 yen x 100 wafers) is also generated since no wafers can be produced while the laser is down. The sum of these two kinds of cost, 303,000 yen (153,000 + 150,000), is defined as the Cost of Downtime (CoD) or downtime cost.
Figure 1: Cost of Downtime
2-2. Laser Downtime Categories
In order to reduce the laser downtime, the occasions that cause the laser to be down need to be analyzed. Laser downtime can be classified into the following four categories by a combination of 2 factors each vertically and horizontally, as shown in Table 1.
- Gas refill
|- Reqular parts replacement
- Calibration (manual)
- Software upgrade
- Investigation / Engineering
|Un-scheduled||- Errors||- Parts failure without prior
notification to customer
In table 1, the horizontal factor “Scheduled” indicates that a customer is notified because the laser is scheduled to be down, while “Unscheduled” indicates that a customer is not notified because the laser has not been scheduled to be down. Vertical factor “Online” indicates that the scanner is producing wafers or the laser is under control of the scanner, while “Offline” indicates that the scanner is not producing wafers or the laser is not under control of the scanner.
Therefore, the downtime can be classified into the following 4 categories:
Scheduled Online Downtime
This downtime has been scheduled, so that the downtime is under control of the scanner. A typical reason for downtime in this category is gas refill. A gas refill is a process that replaces the laser gas with new gas when the laser unit has reached a specified number of pulses or been in use for a specified period. One refill requires the laser to be down for approx. 15 to 20 minutes. This Scheduled Online Downtime is calculated based on the time required for a gas refill.
Scheduled Offline Downtime
This downtime has been scheduled by discussion with a customer to perform regular maintenance of the laser unit. Periodic replacement of consumables, adjustment required to maintain the laser performance, upgrading of the software, and measures to be taken against any trouble already known fall into this category. The Scheduled Offline Downtime is timed from the start of the maintenance work by Gigaphoton’s service personnel to the release of the laser unit to the customer.
Unscheduled Online Downtime
This is downtime generated when the laser unit could be recovered without any repair work performed by Gigaphoton’s service personnel among unscheduled downtime resulting from deterioration of the laser performance, troubles, etc. When the laser performance has deteriorated, the laser unit itself causes an error or the scanner detects it and generates an error. At this time, Gigaphoton’s service personnel do not repair the laser, but an error reset signal from the scanner or an error reset operation performed by the user may be able to recover the laser unit. This causes no downtime due to repair work performed by Gigaphoton’s service personnel, but it does cause downtime due to deterioration of the laser performance. The Unscheduled Online Downtime is calculated by sampling the downtimes caused by deterioration of the laser performance from the error log of the scanner and adding up these downtimes.
Unscheduled Offline Downtime
This is downtime caused by the need for Gigaphoton’s service personnel to repair the laser unit if the user or scanner system cannot recover the error or trouble caused by deterioration of by occurrence of trouble in the laser unit. Unscheduled Offline Downtime is timed from the receipt of the call from the user by Gigaphoton’s service personnel after deterioration of the laser performance has been detected to the release of the laser unit to the user after completion of the repair of the laser unit.