Laser Wafer Marking has traditionally been used for placing discrete marks on silicon wafers. As the industry advances, changing the requirements for wafer marks continue to change and become more complex. The two most important reasons for marks on Silicon wafers and individual die are traceability and identification there are options available when marking wafers. The purpose of this article is to explore some of those options for marks as follows:
Wafer Marking with Nd:YAG & Fiber Marking Systems
Nd:YAG was the first to be used for marking wafers. These system are very useful for marking wafers; however, as technology has developed and advances the Nd:YAG has generally fallen out of favor due to their inefficient operations, specifically:
- The YAG rods and Flash lamp are exposed to damage if water jackets break.
- The System generate tremendous heat and require water cooling.
- The lamp life is short requiring up to monthly maintenance.
- Beam alignment is difficult and time consuming.
Fiber marking can be problematic in terms of debris created or more properly stated control of debris and depth of marks. Systems in the 1064-1067 nm range can also be difficult to properly incorporate into clean rooms. The final issue, Fiber marking systems use for silicon wafer marks is the spot size with a potential of up to 70,000 useable die on each 300mm wafer. The requirements to mark each die spot sizes of 20µm or even smaller is important. The 450mm standards, which are starting to be introduced into the semiconductor industry, will create even more individual die per wafer. Due to the increased size of the 450mm wafer coupled with the reintroduction of notch free technology and even smaller exclusion zones, the ability to write smaller details and bar codes will continue to increase pressure to push to other technologies. The 450mm technology has been delayed due to the earth quakes and Tsunami damage in Japan and is expected to make a resurgence. Fiber marks may remain best suited for solar panel application where the surface is ablated or roughed up to increase conductivity. For large feature marking of opaque substrates and where proper hardware and software controls exist to eliminate debris.
Fiber marking systems are now much more widely used for wafer marks as most of the problems associated with Nd: YAG series are eliminated and all the efficiencies and effectiveness are maintained. Proper software controls, utilizing very high frequency, will provide for strong marks contrast allowing easy readability and also controlling depth penetration into the surface material. This sometimes referred to in the industry as soft marking.
Laser Wafer Marking and Other Systems
Marking with Excimer Lasers- Another interesting process, pioneered by Innolas Gmbh in Munich Germany, provides the ability to create markings or bar codes (ie: ECC200, Data Matrix, QR) on the wafer. These markings will survive the manufacturing process so the wafer can be read at each stage of the manufacturing process.
Wafers are made from different materials for different uses. Some materials such as sapphire, GaAs, Ge, Sin and other 3/5 materials can be marked well with Excimer system. The use of the Excimer is advantageous for the prevention of micro cracks on the surface as the light used is 193nm. The disadvantages of surface cracking caused by the Co2 series is eliminated.
Marking with Co2- Generally speaking WLSC does not recommend Co2 systems for wafer marking; however, Co2 can be used for some applications. Applications would be dependent on if the process can tolerate a thick wide beam width (from 200µm to 750µm) and the parameters can be controlled to prevent deep etching. This starts with ending point holes and heat transference.
Marking with 355nm and 532 nm Series-UV series at 355nm and Greens series, at 532nm are ideal for wafer marking. The UV and Green series provide for very small feature marks, potentially as small as 10µm on the 532nm series. This can be accomplished with little or no heat transfer into the product. The 355nm and 532nm allow for flowing of the surface material on the wafer to create clear clean easy to read marks. Due to the small feature size customers have the ability to write small characters. Even customers with large numbers of individual die can mark each die up to 70,000 1 mm x 1 mm die on a 300mm wafer. Generally, customers are marking identification details on the font side of the wafer. This identification has a standard driven by GEMSEMI standards controlling the location of marking (generally relative to the notch ) and size of marking, degree of offset from the notch, type of text (font/data matrix), and more. In lights out manufacturing, FOUPS are loaded with robots and all marking is verified to be readable and in the correct window (placement on the wafer) via fixed vision cameras.
Marking on Wafer Backside:
- Damage to chip circuits can occur from marking.
- Marking creates raised edges or ridges at the marking point. These can be controlled and minimized; however, they will still exist and polishing is required to remove the ridges. Polishing will also remove of the polished surface.
- Backside marks eliminates both of these issues, and still allows all the individual die of a wafer to be marked.
- Marks on the back side of wafer can be done for product identification, track and trace of the chip (die eventually cut from the wafer), to track the chip (after dicing) through the production process, and to identify chips that have failed electronic testing and remove them for the product flow prior to packaging.
- Individual die are generally marked after the wafer has been flipped. This is referred to as back side marking, as opposed to marks the wafer on the polished side. Polished side marks are not recommended for several reasons.
Laser Wafer Marking at Worldwide Laser Service Corporation
- Manual or operator load and unload of wafers typically uses for very small production run, prototype work, or development and research purposes.
- Semi-Automated where cassettes are placed by operators and wafers in an automated mode these systems can feature:
- Load, unload, and rejection cassettes
- Laser Wafer Marking cassette [or in some cases the wafer is marked at the pre-aligner]
- Machine vision for reading text and bar codes to verify the markings are:
- Correct information is printed [ie: each die properly marked for tracking and/or testing results in order to provide for proper packaging and manufacturing tracking results].
- Robot for scanning cassettes, mapping, picking up the wafer, unloading the cassette, placing wafer in the pre aligner moving the wafer to marking chuck or station, flipping the wafer 180 degrees for back side marking, and retuning the wafer to the correct station or cassette based on machine vision verification and reading results.
The engineering staff at WLSC has been involved with wafer marking for the past 35 years, starting with the Nd: YAG systems installed in wafer marking systems. WLSC provided parts, repairs, and service directly after founding the company 32 years ago in 1986. The next step was for WLSC is to provide wafer marking services and semi-conductor marking. We have installed systems for customers in the N. American market place, with emphasis concentrated on our base in Phoenix AZ. The third phase of development and capability expansion related to wafer marking was the design, building, and installation of WLSC wafer marking systems. We began shortly after opening our company headquarters located at 1340 W. San Pedro Street, Gilbert AZ 85233. Today Worldwide Laser Service Corporation offers several different models or configurations of Laser Wafer Marking systems.
We look forward to hearing from you and working with you and your company on all semiconductor and wafer marking requirements.