Ceramic Heaters for Die Bonding Processes Dayton OH

Several trends and new requirements are impacting wafer processing and the manufacturing of semiconductor equipment components, such as heaters. Various reasons are driving the need for component improvements, such as an increase in the consumption of semiconductor devices, limited space in increasingly complex machines, and the ongoing push for increased productivity.

Increasing productivity in front-end wafer processing has been made possible, in part, by 300 mm wafers, which offer more devices per wafer. But when dealing with back-end assembly, the same economy of scale cannot be achieved because every individual device must be assembled, packaged and tested. Speed and thermal repeatability are two key techniques being used to improve back-end productivity in applications such as IC test, wire bonding and die bonding.

The continuous increase in the consumption of semiconductor devices and the emergence of new applications in optical components ? MEMS, LCD display, flip-chip, chip-on-glass and multichip modules (MCMs) ? has created a vast demand for faster throughput and better die bonding equipment for IC assembly and packaging. IC assembly applications may require a typical ramp rate of 100°C/sec to 500°C with a temperature uniformity of ±2°C and a cycle time of 7?15 sec. Similarly, IC chip testing stresses chips between -40°C to 125°C while monitoring electrical parameters requiring a faster cycle rate.

To manufacture ICs of all types, die bonding or die attach equipment typically connects the die to the die pad or die cavity of the package's support structure. The two most common processes for connecting the die to the die pad or substrate are either adhesive die attach or eutectic die attach.

To meet the demanding requirements in both of these processes, the temperature profile of the die attach material must be precisely controlled to ensure complete curing of the adhesive or melting of the eutectic materials. The heating device must be able to perform reliably with the following characteristics:

• provide a uniform temperature both during ramp-up and steady state
• heat up extremely fast
• dissipate heat quickly to allow for fast cooldown
• minimum dimension change during temperature cycle
• withstand compressive pressure during operation
• be highly finished with smooth and flat surface to enable heat transfer
• have mechanical features, such as grooves and holes, for vacuum passage and/or curved surfaces
• have rapid/short sensor response times for precise control of temperature profile
• operate under high-power density

Choosing the right material through FEM

In a collaborative design with a die bonding equipment manufacturer, we employed a finite element model (FEM) to tackle the stringent thermal performance required of these heating elements and understand and optimize critical material and performance variables (Fig. 1). The model was used to specify the power requirements for given heating rates, predict the effect that power densities have on the thermal stress of different materials, simulate the effect of thermal conductivity on temperature uniformity, and evaluate cooling behavior under different implementation schemes. The model not only helps establish the material requirement, but also helps fine-tune the heating element power distribution to achieve a uniform temperature.

From a thermo-mechanical point of view, thermal conductivity and the temperature coefficient of thermal expansion (CTE) are the two most important properties that dictate the performance of a candidate material for heaters in die bonding machines. To establish a semi-quantitative relationship between power density and stress, a model was created to predict the stress level under various power densities for two of the high-performance materials: alumina (Al2O3) and aluminum nitride (AlN). The maximum stress is found to be about 3× higher for a high CTE and low thermal conductivity material, such as alumina, vs. a high thermal material like AlN. It was also found that stress increases much faster with temperature in the case of alumina than that of AlN. It is clear that AlN is the preferred material choice to meet the fast ramp-up requirement of this application. AlN is especially suitable for applications requiring a clean, non-contaminating heat source. AlN heaters can operate up to 600°C.

Understanding thermal conductivity

The high thermal conductivity of AlN and an optimized circuit layout combine to produce good temperature uniformity across the heater surface. Thermal conductivity also plays a key role for achieving highly uniform temperature. It is possible to design a heater with extremely uniform surface temperature when a distributed power input pattern is optimized using highly thermal conductive heater matrix. Extremely high uniformity of surface temperature (steady state) can be designed by properly distributing the power within the heater. The cooler terminal side and non-symmetrical temperature pattern is a result of the presence of a heat sink and constraint of power input at the location.

Building an AlN heater

Based on the results of the theoretical analysis for heater performance and design, we developed a manufacturing process and proprietary composition to realize an AlN heater that meets the aggressive requirements in semiconductor die bonding and IC testing applications.

The basic structure of the high-performance ceramic heater consists of the AlN matrix, the heating element with distributed wattage based on FEM, which ensures temperature uniformity, and a high-power input capability and terminal.

The basic structural units were assembled in a green state and then sintered in a nitrogen furnace to allow densification to occur. The resultant AlN heater is a nearly full-density ceramic compact with little or no porosity, which, combined with uniformed grains, ensures high mechanical strength and thermal conductivity. The mechanical strength (ASTM type-A configuration) of an AlN processed heater has a mean of 371 MPa and Weibull modulus of 11.

Using a patent-pending process, a thermocouple was integrated into the assembly (Fig. 2). This process ensures the reliability of the heater/sensor interface because the heater is bonded into the assembly. This is important in ramping applications that require a high response rate. Figures 3 and 4 show the relative size of the heater and one of many potential configurations, respectively.

Using FEM to optimize performance

Custom designs can be rapidly accommodated, including those with complex topographies such as holes, notches and vacuum grooves. Using a FEM technique, the heater circuit is optimized and thermal performance is simulated prior to manufacture.

With a very high power-to-mass ratio, advanced ceramic heaters make it easier for devices to be constructed that are smaller and lighter weight without sacrificing performance. As a machinable heater, they can become an integral part of the design, thereby lowering part count, simplifying assemblies and ultimately improving performance. For example, clamshell designs provide a very efficient approach to heating small tubes for flowing liquids and gases, whereas flat rings or rectangular forms easily accommodate mounting applications.

This process of developing custom designs is completed by carefully considering the environment and defining the boundary conditions. The heating element pattern is optimized using the FEM technique. Infrared (IR) images of the AlN heater reveal excellent temperature uniformity of ±2°C at 400°C steady state.

In addition to uniform temperature distribution, the heater must provide a fast heat-up rate for the short die bonding cycle. Collected data indicates that an AlN heater takes about 10.5 seconds to reach 400°C when powered at 250 wsi power input. When power input is increased to 1000 wsi, a linear temperature profile with a heating rate approaching 150°C/sec is achieved and takes <3 sec to reach target temperature. Such a heating rate exceeds the typical 100°C/sec requirement for die bonding applications. Finally, a small overshoot of <5°C at 400°C can be easily achieved using a self-tuning proportional-integral-derivative (PID) controller, even at a 150°C/sec ramp rate.

Cooling the heater

AlN heaters can provide rapid heating of small parts and assemblies. For applications requiring cyclic heating and cooling, compressed air is one method for quick cooling that should be considered. Compressed air is widely available, does not typically create concerns related to leaks, accommodates the range of common heater operational temperatures, and can be easily integrated into production equipment because it requires relatively simple, small, lightweight components and does not complicate electrical isolation requirements.

When a system operating on a thermal cycle is appropriately designed to use forced air as a coolant, good performance can be achieved without the disadvantages of high initial cost and complexity associated with many other cooling technologies.