|Diagram of die eject process. |
As the demand increases for smaller, cheaper, and more efficient devices, process technology must evolve to meet the challenges these devices present. Manufacturers must take special care when handling these devices, some of which include MEMs (microelectromechanical systems) devices, optical devices, GaAs (gallium-arsenide) die less than 50µm thick, devices with surface features such as vias or solder bumps, and small die down to 200µm square.
One process step for which sensitive die handling becomes critical is die sorting. Die sorting is the process of removing a die, package, or device from one carrier medium (commonly a diced wafer on tape frame) and placing them into another carrier medium (commonly waffle pack, Gel-Pak®, or JEDEC tray), essentially preparing the devices for the next step in the manufacturing sequence. During the sorting process die can be inspected for defects and sorted accordingly, or they can be sorted based on information contained in a wafer map file, output from a prober.
|Eject head with array of needles for large, thin die (shown on Royce Instruments, Inc.'s DE35-ST). |
Die sorting process solutions range from operators picking die manually with tweezers to fully-automated die sorters that can process entire wafers without operator input. With sensitive die, a manual process is less desirable due to a lack of precise process control. An automated system can provide solutions customized for each device type by using a specific tooling set and finely tuned process parameters, establishing a high level of repeatability. Most die sorters require an eject head (pepper pot) and pickup tool. The eject head contains needles which push the die from the tape while the pickup tool lifts the die off of the needles and places it into the appropriate output carrier. To minimize changeover time between different devices, the eject head and pickup tooling should be easily changed. This way, device specific tooling can be used with the least impact. Eject heads and pickup tools should be selected based on the die size and material.
Eject Head Design
Eject head designs can specify the number, type and position of eject needles. Multiple eject needles distribute force more evenly across the backside of the die than a single eject needle. The radius of the eject needles determines the area of the needle tip that contacts the die. A sharper needle is more likely to pierce through the tape, while a blunter needle will not. A larger needle tip radius (blunter needle) also leads to more contact area, meaning less eject pressure on the die. While avoiding any vias or solder bumps on the die underside, the ideal needle position is generally near the die corners to limit die overhang, but inward enough for the tape to be able to peel off the die. To facilitate this, vacuum holes are included in the eject head, so that the tape is securely held to the eject head surface during die eject. For small die less than 700µm square, the eject head surface is a sintered disk, allowing increased vacuum hold-down while keeping the die level. Larger die (greater than 5mm square) can use an eject head with surface vacuum channels to increase the vacuum hold down force. In thin die applications, the eject head setup becomes more critical. Typically for die thicknesses of 50µm or less, the number of eject needles is increased. Blunter eject needles (installed in the eject head to be coplanar) may also be used. In conjunction with this tooling, the eject needle height should be optimized to be as low as possible while still ejecting the die. This helps to ensure that a die is not cracked or damaged by the eject needles. With these thin die, UV tape is commonly used, as adhesion is minimized (less than blue Nitto tape) after the UV cure. It is important to ensure a wafer's tape has been properly UV exposed prior to attempting die sorting. It is recommended to UV cure the tape per the tape manufacturer's recommendations within 24 hours of mounting the wafer to the tape. This will allow for an easier die eject process for fragile die.
|Non-surface contact gripper mechanism; picking die using Royce Instruments, Inc.'s DE35-ST. |
In addition to the physical tooling, device specific process parameters can be stored in recipe files on the die sorting system for easy recall during device changeovers. These parameters include the height of the eject needles above the surface of the eject head, the speed with which the needles raise, and the height of the pickup tool for both the die pick and die place. Adjustment of these parameters works in conjunction with the tooling selection to ensure a quality die sort process.
For pickup tools, the material, size, and shape can be customized to meet the requirements of any die. The most common pickup tools are circular at the tip and use vacuum on the die surface for pickup. They are generally made from rubber, anti-static Vespel (hard plastic), or tungsten carbide. Rubber tips are low cost and softer than Vespel or tungsten carbide. Many types of rubber materials are available, including some with greater compliance for use with die with solder bumps on the surface (the pickup tip elastically conforms to the bumps).
Vespel offers good electrostatic discharge protection for devices and is more wear-resistant than rubber. Tungsten carbide can be precision machined to accommodate the smallest die (less than 200µm square) and other complex pickup tip geometries. Channel or "collet" style pickup tips, using vacuum, only contact two of the top edges of the die. These tools are specific to a particular die size and have a shallow radius so that the surface of the pickup tool does not touch the die surface. These tools can be used for devices with sensitive surfaces or optical lenses that cannot be touched.
However, for some devices, such as MEMs, a collet pickup tool does not suffice due to the limited clearance between the pickup tool surface and die surface. In these cases, a complete non-surface contact approach is needed. To meet this requirement, Royce Instruments, Inc. utilizes a unique gripper mechanism option on its die sorters' pick arm. This mechanism can hold two or three Vespel gripper fingers, which contact two sides of the die, but not the die surface (similar to a pair of tweezers). The grippers are pneumatically actuated with open and close position adjustment available. A vacuum regulator allows adjustment of the gripper closing force from zero to approximately 20 grams. Non-surface contact eject heads are used in conjunction with the grippers. These eject heads include a dull center needle which holds the die stable until the grippers close around it. This provides a reliable way to pickup MEMs devices without damaging the mechanical structures on the die surface.
This approach works well for even the thinnest of die. Typically the die is not gripped at the very tip of the gripper fingers, but part way up the grippers. This ensures that die of all thicknesses can be processed using this non-surface contact method.
Handling Wide Range of Die
Die sorters, when used with various combinations of the tooling presented here, allow the processing of a wide range of sensitive die, devices, and packages from all market segments. By moving from a manual to automatic process, the manufacturing engineer will no longer see problems such as excessive force on the die or die rotation, as a result of the die sorting process. Once a process has been established, all die will be reliably processed with the same high quality results.
To meet today's die sorting challenges, Royce Instruments, Inc. brings over 28 years of die sorting experience to its worldwide customer base. The company offers a semi-automatic and fully automatic system to process die as small as 150 to 200µm square, on wafers up to 300mm diameter. By utilizing a floating flexure design for the pickup arm, die surface contact force can be drastically reduced. This, along with other process options including the non-surface contact gripper-finger mechanism, facet inspection, underside inspection, die inversion, and wafer mapping, allow Royce to deliver successful die sorting process solutions.
Contact: Royce Instruments, Inc., 831 Latour Court, Suite C, Napa, CA 94558 707-255-9078 fax: 707-255-9079 E-mail: firstname.lastname@example.org Web: http://www.royceinstruments.com