Wafer Laser Marking Systems
Wafer Laser Marking systems are used to place identification marks on silicon. Wafers are generally marked with depths not to exceed 2.5µm (sometimes less) and width of features between 20µm and 45µm. The shoulders or peaks around the features should not to exceed 4µm. Today there are two major methods for marking silicon wafers. These are beam and ink pad printing. In this article we will discuss the various options and considerations necessary for this process as related to wafer marking systems.
How does back end processing affect Wafer Laser Marking Systems:
Identification of the device so it can be tracked and controlled at all stages of the manufacturing process. Separating good components from bad components, to prevent any units with faulty electronics or circuitry from entering the end product and causing devise failure. When marks are placed on the polished surface of the wafer (generally in exclusion zone, where exact placement is driven by SEMI standards), the wafer marking feature depth and width must be tightly controlled in order to avoid cracking the wafer. If the wafer is cracked (micro cracks), defects are generally recognized immediately by initial testing.
Another consideration is the absorption of energy into the silicon, which can create premature component failure and shorten life of the device for end users. Generally this type of problem will not be discovered in the manufacturing process. For these reasons, the SI on the wafer is not actually marked or etched but rather surface material (SI) is melted and reflowed by the beam creating readable character and bar code configurations.
SOI devices are not marked on the chip but usually on the frame which is metal or ceramic and the electronic devise is not subject to damage from the marking, in these cases generally such as discrete electronic components the emphasis is on cycle time and speed of process.The major issues with marking the individual products is creating damage to the electronics and circuitry embedded on the wafer. Back side marking is the best alternative as opposed to marks on the polished surface of the wafer.
Recent developments are driving the miniaturization of electronic devises. The trend is expected not only to continue, but to accelerate. Coupled with the miniaturization trend is the elimination of ceramic. Gold, and other metals that covered the electronic devises so that today the devices are covered in plastic package or there is no cover at all and the backside of the silicon chip is visible and exposed and the wafer marking done on the back side is visible. Marking the back side of wafers is done for to principal or primary reasons, specifically;
Does laser spot size affect Wafer Laser Marking?
The IR systems ,which previously dominated this market, are increasing falling out of favor due to high operating costs, extensive maintenance requirements, and propensity to create damage to SI wafers and chips.
Spot size is defined as the radius of the beam, that hits the product being marked. If the system has 6 watts of power output and that laser has a spot size of 20µm, this yields a certain energy density. However, it is that same 6ws of power that is in a spot size of 40 µm the energy density, which is 50% of the previous example. Why that is important is because for the 40µm spot size. To do the same mark quality as the 20µm, spot size a more powerful or higher wattage is required instead of 6 watts 40 watts might be required and more expensive. This is important because, in the memory devices business, prices are low and costs related to manufacturing must be watched and controlled very carefully. Thus 355nm UV series and 532nm Green series are increasingly the choice for wafer marking systems.
Does Beam divergence and beam quality affect Wafer Laser marking?
Beam divergence is how much the beam open up or spread out as the beam travels away from the system. This factor, along with power stability, has had a dramatic effect on the quality of the mark, the speed of the mark, and the power levels required to achieve the mark. If power is not stable or the beam has great divergence, this creates changes in power levels as the beam is applied to the wafer damage to electronics can occur and achieving the tight tolerances for marking. The effect of beam divergence and beam quality become even more important as SI is highly sensitive surface for beam.
How to achieve the best beam divergence and beam quality for Marking:
Given that beam quality and divergence are critical components for successful marking on silicon wafers, what is the best way to assure that the system is designed and built with the best options to meet those specification? It is interesting, in this discussion, to note that originally all wafer marking systems were designed with Nd: YAG series and later Diode pumped YAG lasers. Though, 355nm UV series are best suited for silicon wafer marking. The origin of 355nm UV Series are diode pumped YAG systems. The 355nm series provides the best beam stability and light absorption for SI wafer marking. The 355nm UV series starts as a diode pumped YAG series emitting light in the 1064nm to 1067nm wavelength. Then the light is passed through two crystals. The first crystal changes the light to 532nm and the second to 355nm. The crystals are the limiting factor affecting functional beam lifetime, generally lasting for 8,000 to 12,000 hours of operations. Depending on the configuration of the wafer marking system and the choice of generator, the tube is either replaced after 8,000 to 12,000 hour of actual beam fire time (not on time but actual pulse emission time) or in some configurations the crystals can be turned extending the tube life time to 25,000+ hours. As a final consideration in this category, 355nm beam stability, divergence, and power, and affected by the beam being side pumped or end pumped. Contact WLSC to learn more about these options.
Wafer Laser Marking conclusion
- 355nm UV Series offer the best solutions and options for wafer marking operations. 532nm systems can also be used in these operations and applications very successfully. The 355nm UV series and the 532nm green series are also excellent tools for many semi-conductor and electronics packages marking operations.
- WLSC Corporation and Wafer Marking
- 355nm UV Series up to 60 watts
- 532nm Green Series up to 90 watts
- WLSC software division provides production host interface packages for remote or lights out wafer marking completely compliant with GEMSECS specification and SEMI standards for 150mm and 300mm wafer marking systems. WLSC office and manufacturing facility located in Gilbert AZ 85233 is placed to service our extensive customer base in Canada, United States, Mexico, and Central/South America. We look forward to hearing from you and working together on your wafer marking system and semiconductor system requirements.
Contact WLSC Today
Worldwide Laser Service Corporation, WLSC, has been involved in wafer marking operations for 32 years. Since our founding in 1986, and even before that, the founding officers of WLSC were involved with the service and support of Nd: YAG wafer marking systems. The level of knowledge and depth of experience at WLSC related to these systems is extensive. As technology moved on and 355nm UV series were becoming commonly available to industrial applications, the system engineers and integration specialists at WLSC were leaders in introducing 355nm UV series to medical, pharmaceutical, semiconductor and silicon wafer industries and applications. WLSC has continued to add systems with additional features and functions related to wafer marking systems and semiconductor operations.