The back-end process: Step 4 - Die attach
BY PETER BÜHLMANN AND DOMENICO TRUNCELLITO
In back-end semiconductor manufacturing, the die attach process is a critical step. In simple terms, die attach is picking a chip from a wafer and placing it onto a substrate or metal lead frame. The way the chip is bonded defines the die attach process - epoxy, soft solder, eutectic and flip chip are the most widely used techniques. Die attach seems to be a simple process step in the semiconductor manufacturing chain. However, the continuously escalating requirements of today's applications create difficult challenges in die bonding.
Typical Die Attach Process Flow
In high-volume production, die attach is performed on fully automated assembly equipment. The basic die attach steps, some of which are performed simultaneously, are:
- A robotic loader picks up a lead frame from a stack and places it on the input area of the workholder.
- The lead frame is moved from the input position to the dispense position. Depending on the required placement accuracy, mechanical or optical alignment points are used to define the dispense position. Epoxy is dispensed in a pattern and volume appropriate for the chip size.
- A sophisticated vision system inspects the lead frame, dispensing pattern and bond pads before the substrate is transported to the bonding position.
- In the meantime, a pattern recognition system locates a good die on the sawn wafer.
- A vacuum pick-up tool mounted on a bond head grabs the aligned die from the wafer and places it on the programmed and pre-dispensed bond position on the substrate.
- Appropriate bonding time and bonding force result in a strong bond, according to the specified process requirements. An additional optical inspection is performed to ensure that placement position and epoxy bleed-out requirements are met.
- Each bond pad on the lead frame or substrate goes through this process before it is unloaded into an output magazine.
The Most Common Processes
There are four major die attach processes used in semiconductor packaging.
Eutectic Die Attach: The eutectic die attach process is a well-established bonding technique, having been used in the early days of semiconductors for the first transistors and integrated cicuits (IC). It is still widely used for small signal products - so called "jelly beans" - manufactured by the millions every day. Currently, eutectic die attach has regained importance in the field of packaging optoelectronic components and high-power communication devices. Under high temperature, the silicon-gold combination forms the eutectic bond. A scrub motion during the bond process increases the strength and quality of the intermetallic connection between the chip and substrate. For large chips, additional gold in ribbon form is used. Because of the high temperature of the process (up to 450°C), a protective forming gas atmosphere (a nitrogen/ hydrogen combination) is needed to prevent oxidation of the lead frame.
Soft Solder Die Attach: This process uses a solder material to bond the die to the lead frame. The solder is introduced as a wire preform and melted onto the hot lead frame surface as a liquid solder dot. A chip is placed on the hot solder and as soon as the solder cools down, a solid connection is established. These solders are typically lead- and tin-based alloys. A controlled temperature profile is required to define the liquidus/ solidus transition. Again, a protective forming gas atmosphere is required to prevent oxidation of the lead frame. Soft solder applications are typically used in automotive and high-power devices.
Flip Chip Bonding: A process similar to soft solder die attach is used in flip chip bonding. Here, the chip is flipped before being attached, and solder bumps between the chip and substrate serve as both an electrical and mechanical interconnection. Drivers for this process are high I/O counts and increased electrical performance requirements for high-speed, high-frequency applications.
Tape and Other Die Attach Processes: For some special applications, such as large memory devices, different tapes are used as the adhesive (epoxy, polyimides and thermoset/thermoplastic materials). The latest trend is to apply the adhesive directly to the backside of a wafer instead of using tape as an adhesive carrier. Wafer backside coating provides clear advantages for thin die and stacked die applications, because the adhesive is spread evenly on the bottom of the chip and it is always in the right position.
Epoxy Die Attach: Epoxy die attach is the most commonly used process, and the term generally encompasses the use of other adhesives, such as polyimide- or silicone-based materials. The adhesive is dispensed in paste form on the bond pad of the lead frame or substrate before die placement. Typical applications using epoxy cover a wide range of devices, from simple transistors to high power processors, memory and ASICs.
Today's packaging technologies create challenges for epoxy die attach processes. The key criteria for success (such as productivity and material cost) are not as crucial in advanced packaging. The challenges in this segment lie within the process and its execution.
The major market drivers for stacked die applications are reduced space, weight savings and enhanced electrical performance of the devices (which are mainly used in portable consumer products). Stacking of chips, in which two or more ICs of different types are placed at the same coordinates in the x-y plane, is an alternative to silicon integration. The memory industry has discovered this opportunity to minimize package size and cost by stacking one die on top of the other in a single package. Such a system usually consumes less power and features higher speed than separate components. Stacked die applications provide flexibility in combining different devices without touching the design level of the silicon. Time to market can be drastically reduced. Additionally, the functionality of the device can be doubled or tripled in the same package size.
The vertically integrated system in a package has a much higher package integration ratio compared to the single die solution. In addition, the electrical performance and reliability of stacked die is improved because only one package has to be tested, and established IC assembly methods can be used.
Challenges of Stacked Die
To stay within standard package heights, the stacked chips need to be thinned. The backgrinding process is used to reduce the die thickness to the range of 50 - 125 µm. As a result, wafer handling needs special attention. Thus, a gentle and controlled die pick-up procedure is needed. Thin large die have a tendency to warp, which causes problems during the die bond process. Special tooling and techniques are required to keep the die flat on the substrate.
For the stacked die process, high die placement accuracy is essential to mount the upper chip accurately onto the lower one. Inaccurate die placement can lead to electrical failure (wire shorts) and impacts the epoxy bleed out. Since another cause of epoxy bleed-out is poor dispense quality, the position and volume of the dispensed epoxy needs to be consistent. Excessive epoxy may cover the wire bonding pads, preventing a proper interconnection. Using backside-coated wafers eliminates this problem since the adhesive is evenly spread on the backside of the top die and no epoxy/die offset can occur. This benefit makes the backside wafer process very attractive for stacked die. It is important to consider, however, that this process needs immediate curing, which may reduce productivity. Depending on the process applied (paste form adhesive), a curing step is required before the second die can be attached. However, curing often warps the substrate. Special downhold methods are required to keep the substrate in place for an accurate second die placement. For wire bonding applications, the substrate design rules and the loop height have to be carefully monitored to prevent a wire from shorting between the two die.
Small Thin Die on High BLT
The main drivers for small die applications on a high bondline thickness (BLT) are high-power RF amplifiers in small packages for wireless applications. BLT is the thickness of the resulting adhesive between substrate and chip after die placement. For high-power applications, a relatively high and constant BLT is essential for long and reliable performance of the device. Precise dispensing (amount and location of the epoxy) and a repeatable small bond force of 5 to 10 grams guarantee a high and constant BLT. The die attach equipment must be capable of compensating for the substrate thickness variations, which is very common, to achieve a consistent BLT. Today, with current high-end die bonding equipment, a standard deviation of 1 µm for a target BLT of 18 µm using a 1 mm square die is achievable.
The technology trends in the back-end industry can be summarized as "more functionality in a smaller space." Future devices will contain - in the same package - chips bonded with traditional techniques in combination with the flip chip process. Passive components may be added in the same package to increase functionality. Vertical or horizontal package integration seems to be the logical evolution. Complicated packages with seven-stacked die beside a flip chip device and a few passive components packaged in the same small unit could soon be real. Space, flexibility and cost considerations will push the limits of package design and die attach technology.
In the future, high-volume processing of very thin die (50 - 100 µm) will challenge the chip assembly industry. Packaging equipment needs to be optimized to meet the new requirements. Die pick-up, bonding process and epoxy dispensing technologies need to be fine-tuned.
In contrast to small die, there is also a trend in the industry toward very large die used in microprocessors and digital signal processors. Attaching large die requires different methods. Void-free epoxy dispensing for a large area is crucial. An increased and controlled bond force is a prerequisite for consistent chip planarity and BLT.
All of these new technology trends will only take off and survive in high-volume production if economic aspects are considered. In the area of die attach, increasing productivity without compromising placement accuracy will be key to meet adequate cost of ownership. It is essential that equipment manufacturers, material suppliers and packaging designers collaborate closely to meet future packaging requirements.
Peter BÜhlmann, technical manager die bonder, and Domenico Truncellito, senior communications manager, can be contacted at ESEC (USA), 1407 West Drivers Way, Tempe, AZ 85284; 480-893-6990; Fax: 480-893-6793; E-mail: email@example.com; firstname.lastname@example.org.