Advanced Ceramic (AlN) Heaters For Semiconductor Backend Packaging Applications

by Dr. Hongy Lin
Principal Scientist, Watlow Electric Manufacturing Company

As electronic devices become increasingly smaller, lighter, thinner and more powerful, the demand for more precise and faster packaging technology has increased. Despite the difference in various packages available in the industry, the technology of packaging processes share common steps.

In general, these steps include: die preparation, die attach, wire bonding and encapsulation. Among these steps, die attach is one of the most challenging processes, as it involves material compatibility, placement and dispensing accuracy, precision process temperature profile and a short cycle time.

A die bonder or die attach equipment is used to attach the die to the die pad or die cavity of the package’s support structure. A typical die bonder consists of a holding and indexing system, a system for dispensing die attach material and a pick and place system for ejecting the die from the wafer tape. The two most common processes for attaching the die to the die pad or substrate are adhesive die attach and eutectic die attach.

In adhesive die attach, adhesives such as epoxy, polyimide and Ag-filled glass frit are used to attach the die. Eutectic die attach uses a eutectic alloy. Au-Si eutectic, one commonly used alloy, has a liquidous temperature of 370°C and another alloy, Au-Sn, has a liquidous temperature of 280°C. Epoxy adhesive has a process temperature of 250°C. In both 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. In addition to fast heating, uniformity in temperature over the attaching material is crucial to minimize the defects at the bond line.

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 – has created a vast demand for faster throughput and better die bonding equipment for IC packaging. IC packaging requires a typical ramp rate of 100ºC per second to 4-500ºC /-2ºC and a cycle time of seven to 15 seconds. Similarly, IC chip testing, which stresses chips between -40 to 125ºC while monitoring electrical parameters, also requires a faster cycle rate.
To meet the demanding requirements in those applications, the heating device must be able to perform reliably with the following characteristics:

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

Thermo-mechanical Design of High Performance Heaters

To tackle the stringent thermal performance, the finite element model (FEM) was employed to understand and optimize critical material and performance variables. The model was used to simulate the effect of thermal conductivity on temperature uniformity, predict the effect that power densities have on the thermal stress of different materials, specify the power requirements for given heating rates and lastly, evaluate cooling behavior under different implementation schemes. The model not only assists in establishing the material requirement but also helps in fine tuning 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 performance. 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 and aluminum nitride, listed in Table 1. As shown in Figure 1, the maximum stress is found to be about three times higher for a high CTE and low thermal conductivity material such as alumina versus a high thermal material like aluminum nitride. It was also found that stress increases much faster with temperature in the case of alumina than that of aluminum nitride. It is clear that aluminum nitride is the preferred material choice to meet the fast ramp-up requirement.

Table 1: Thermal properties of alumina (Al2O3), aluminum nitride (AlN) and silicon nitride (Si3N4) at 25ºC.

Material

Thermal conduc-tivity (W/K·m) Thermal expansion coefficient (x10-6/ºC) Heat capacity (J/g·K) Density(g/cm3)

Al2O3

30

7.2

0.88

3.96

AlN

140

4.5

0.78

3.26

Si3N4

40

3.2

0.71

3.25

Table 2: Effect of thermal conductivity on temperature uniformity of a ceramic heater.

Materials

Si3N4

AlN

Thermal conductivity (W/Km)

40

140

Temperature uniformity (?T) at 400°C

7.4ºC

1.1ºC

Figure 1: Maximum stress and heater temperature after the heater is powered for 0.5 seconds at various power density.

Figure 2: Stress distribution across the trace layer powered for 0.5 seconds at 1300 watts. The heater temperature is approximately 188°C. Note the maximum stress occurs at the heater element layer due to the largest temperature gradient and CTE mismatch.

Thermal conductivity also plays a key role for achieving a highly uniform temperature. It is possible to design a heater with an extremely uniform surface temperature when a distributed power input pattern is optimized using a highly thermal conductive heater matrix. Table 2 indicates that the ?T (Tmax-Tmin) of 1.1ºC is achieved for aluminum nitride, while silicon nitride has a ΔT of 7.4ºC, about seven times larger. As shown in Figure 3, an 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 the constraint of power input at the location.

Figure 3: Uniform temperature in an aluminum nitride heater with high thermal conductivity.

The power requirement for various heater temperature profiles can be calculated when the environmental boundary conditions are well defined. However, to understand the temperature of the heater in operation, it is more important that the corresponding temperature on the part is known. Due to the presence of surface irregularity on the heater and part (die or collet), the heat transfer cannot be modeled using the assumption of a perfect interface because thermal conductivity of the air gap is less than that of the ceramic heater material. Therefore, the effect of the air gap needs to be considered in the model. Thus, a thorough understanding of surface conditions such as roughness and flatness is necessary in order to precisely predict the temperature of the part and the power required to drive the heater.

Using a surface profilometer and electron microscopy the surface characteristics of the heater were determined. It was found that the typical surface of a fine ground ceramic heater is very smooth as indicated by the SEM photo in Figure 4. Roughness (Ra) can be processed to around 0.6 microns or less as shown by the profile in Figure 5. However, the flatness of the surface is usually in the range of two to five microns. Thus, the presence of a discontinuous air gap in the micron range is unavoidable when a part is placed on the heater surface. Incorporating an air gap in the model for the heater design will provide a more accurate estimation of heating and cooling behavior as well as the power requirement.


(A) (B)

Figure 4: (A) SEM micrograph showing the smooth surface finish of polished surface (Ra= 0.1 microns), (B) fine ground surface with Ra of 0.6 microns.

Figure 5: Surface profile of the aluminum nitride heater after grinding. Surface flatness is around two microns.

To more accurately study the effect of the gap, a FEA model incorporating an air gap was created and the temperature response on the die was calculated. The result shown in Figure 6 indicates a significant difference in the temperature response between the collet and heater, which is around a 30ºC delay, when a gap with four micron spacing is present. The delay in part temperature response (collet in this case) translates into either a higher heater temperature, or greater power to compensate for the resistance due to the air gap.

Figure 6: Temperature on the part (collet) is significantly lowered when a micro-air gap is present between the part and heater.

One challenging aspect of designing the die bonder heater lies in the fast cooling. Even if the heater has high thermal conductivity, its cooling rate is long when only natural convection is involved. It is not surprising that a heater could take more than 250 seconds for it to cool from 400°C to 50°C, as shown in Figure 7. The cooling time is significantly reduced (~55 seconds) when forced air (20 m/s) is applied onto both the heater surfaces for cooling purposes. The time can be further reduced to 8.7 seconds when assisted with water flow through the channel inside the steel block attached to the heater bottom, as shown in Figure 8. As predicted by the cooling model, a heater assembly design for achieving 10-15 second cycle time is quite feasible when water cooling can be designed into the system.

Figure 7: Cooling curve for an aluminum nitride heater under various cooling conditions.

Because of the unique combination of material properties such as large thermal conductivity > 140 W/Km, small coefficient of thermal expansion of 4.5×10-6/ºC, high dielectric strength of 15KV/mm, high electric resistivity of 10×1014 ohm-cm and large elastic modulus of 330 GPa, aluminum nitride ceramic is an ideal candidate for a heater matrix among the high performance ceramic materials.


Figure 8: Water cooled heater assembly for fast process cycle application.

Manufacturing of an Aluminum Nitride Ceramic Heater

Based on the results of the theoretical analysis for heater performance and design, Watlow has developed a manufacturing process and proprietary composition to realize an aluminum nitride 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 aluminum nitride matrix, the heating element with distributed wattage based on FEA to ensure the temperature uniformity, as well as 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 take place. The resultant aluminum nitride heater is a nearly full density ceramic compact with little or no porosity. Figure 10 shows the microstructure of the aluminum nitride heater with a uniform grain size and minimum porosity all of which combine to ensure high mechanical strength and thermal conductivity.

Figure 10: SEM photo of an aluminum nitride ceramic heater microstructure showing typical grain size around two to five microns.

The mechanical strength (ASTM type A configuration) of an aluminum nitride processed heater has a mean of 371 MPa and Weibull modulus of 11. The modulus of rapture (MOR) data and two parameter Weibull plot are presented in the Figure 11.

Figure 11: Weibull plot of mechanical strength of an aluminum nitride heater matrix.

Following careful consideration of the environment and defining of the boundary conditions, the heating element pattern is optimized using the FEA technique. As shown in the infrared image of the heater (Figure 12), the resultant aluminum nitride heater has an excellent temperature uniformity of /-2°C at 400°C steady state.

Figure 12: Aluminum nitride heater with ?T of 4ºC at region of interest.

In addition to uniform temperature distribution, the heater must provide a fast heat-up rate for the short die bonding cycle. Figure 13 demonstrates the heating profile of a heater under various power density inputs using a Watlow SERIES F4 temperature controller. The data indicates that it takes about 10.5 seconds to reach 400ºC when powered at 250 watts per square inch power input. When power input is increased to 1000 wsi, a linear temperature profile with a heating rate approaching 150°C per second is achieved and takes less than three seconds to reach target temperature. Such a heating rate has well exceeded a typical 100ºC per second requirement and is well suited for die bonding applications. It is noted that a small overshoot of less than 5ºC at 400ºC can be easily achieved using a self-tuning PID controller even at 150ºC per second ramp rate.

Figure 13: Heating rate is proportional to the input power density.

To validate the reliability of the heater, a series of heaters with dimensions of 55 mm x 10 mm x 1.5 mm were produced and tested by cycling between 100ºC and 700ºC at power of 1000 wsi. The Weibull analysis (Figure 14) indicates that the MTBF life expectancy of the aluminum nitride heater is approximately 460,000 cycles.

Figure 14: Weibull plot for an aluminum nitride heater life test.

Conclusion

A high performance aluminum nitride ceramic heater offers significant advantages in terms of fast ramping and cooling as well as temperature uniformity and is ideal for the most demanding die bonding and flip-chip operations.

Dr. Hongy Lin is a Principal Scientist and advanced ceramic heater development team leader at Watlow. He has worked in the advanced ceramic industry for more than 20 years, including experience in structural, glass and ceramic heater applications. Dr. Lin holds a Ph.D. in ceramic engineering from the University of Missouri-Rolla.

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