Power Electronics
Aluminum Nitride-Based Ceramic Material Fits Power Semiconductor Applications

Aluminum Nitride-Based Ceramic Material Fits Power Semiconductor Applications

Rising die junction temperature creates reliability and performance issues for power semiconductors. While higher thermal performance often comes at a substantial increased material cost, a new alumina nitride-based ceramic provides a solution that bridges the cost and thermal performance gap between 96% alumina and traditional alumina nitride.

It is well documented that rising die junction temperature creates a number of reliability and performance issues for power semiconductors. As an example, a Freescale White Paper on thermal management lists the following power semiconductor issues associated with high junction temperatures1:

·     Increase in leakage currents

·     Gate oxides degrade more quickly

·     Ionic impurities move more readily

·     Mechanical stresses increase

·     Diode forward voltage fails

·     MOSFET on-resistance increases

·     MOSFET threshold voltage falls

·     Bipolar transistor switching speeds slows

·     Bipolar transistor gains tend to fall

·     Breakdown voltages tend to increase

·     Transistor Safe Operating Areas decrease

 

Consequently, one of the major roles of packaging in power semiconductor applications is to effectively remove heat from the semiconductor device. This is also one of the key reasons that the highest power devices utilize higher cost ceramic packaging options such as Direct Bond Copper (DBC) and Direct Plated Copper (DPC)2. For this type of packaging technology, thick Cu (either plated or from Cu foil) is bonded to ceramic substrates fabricated from alumina (96%), aluminum nitride (AlN), zirconia toughened alumina (ZTA) or Si3N4. The thermal conductivity of these different ceramic materials are listed in Table 1, along with the relative ceramic costs3.

Ceramic

Thermal Conductivity
(W/m-K)

Ceramic Packaging Technology

Cost Factor

Al2O3(96%)

20 W/m-K

DBC or DPC

1

ZTA

25 W/m-K

DBC or DPC

>1

AlN (95%)

170 W/m-K

minimum

Direct Plated Copper

(DPC)

>8

Zirconia

140 W/m-K

Active metal braze

>8

Table 1. Performance of Ceramic Packaging Materials

As is clear from this table, which uses 96% alumina as the relative cost basis, higher thermal performance comes at a substantial increased material cost. We will describe a new ceramic solution, based on aluminum nitride, that will provide a solution that falls in between 96% alumina and traditional AlN from both a cost and thermal performance standpoint.

Aluminum Nitride Too Expensive

Aluminum Nitride (AlN) is an ideal choice for high thermal demand applications because of its combination of high thermal conductivity and mid-range CTE of 4.5 ppm/C; but AlN’s >8 times cost factor relative to aluminum oxide limits its application significantly. AlN is currently used in power semiconductor packaging, but only in situations where there is no other feasible alternatives. The high cost pressure in the power semiconductor market and the significant portion of total device cost that packaging entails, increases the pressure to minimize high cost AlN usage.

AlN’s high cost comes from a number of factors. Some of the most significant are listed in the Table 2.

Cost Factor

Comparison to Alumina

Comments

AlN ceramics are fabricated from AlN powder, and AlN powder cost for typical electronic grade material isextremely high

AlN powder is 15 to 25 times more expensive then alumina powder

Electronic applications almost exclusively utilize high cost “carbo-thermally reduced” AlN powder

AlN ceramics are processed at very high temperatures. High temperature furnaces increase capital costs and decrease furnace throughput

Alumina is processed at 1450°C to 1620°C, AlN is processed at 1825°C.

For alumina, continuous furnaces are available. For AlN, only batch graphite or refractory metal furnaces are available.

High powder processing costs due to reaction with H2O.

During early processing stages, ceramics are shaped by forming a ceramic, binder and solvent slurry. AlN reacts with water so the slurry must be non-aqueous, compared to aqueous processing for alumina.

Non-aqueous processing requires higher equipment costs due to explosion hazards and environmental concerns, and also require solvent recovery systems.

Table 2.  AlN Comparisons

 

New AlN Based Ceramic Material for High Thermal Demand Applications

A new material has been developed which fits in the cost/performance “gap” between Al2O3and conventional AlN. The main features of this new material, which is labeled “HBLED Grade AlN4” (due to its fit in the High Brightness LED market) are much lower powder and process costs, a thermal conductivity between alumina and conventional AlN and a white color which is highly reflective in the visible. This new material is an ideal material for power semiconductor applications where cost and thermal performance are also critical.

The new HBLED grade AlN has a thermal conductivity of 100 W/m-K which is five times higher than alumina, but 42% lower than conventional AlN. This is more than adequate for most power semiconductor applications. The mechanical, electrical and physical properties are very similar to conventional AlN.

One very critical factor is that this new AlN material utilizes a much lower cost AlN powder that is made by “direct nitridation” of aluminum metal. This powder is typically 60-75% less expensive than the traditional carbo-thermally reduced powder5used in electronics applications.

In addition, HBLED ALN is processed at 1700-1725 °C. In this temperature range, continuous furnaces are available which use alumina heat shields and Mo heating elements. Though more expensive than lower temperature alumina sintering furnaces, from a cost and throughput standpoint this is a significant improvement compared to conventional AlN processing in high temperature refractory metal or graphite batch furnaces.

Fig. 1 New low cost AIN grade, which is white and has tile dimensions of 4.5-in. square. It is shown with a traditional AlN substrate (which in this case is 2-in. square).

Fig. 1.     New HBLED grade AlN, which is white and has tile dimensions of 4.5-in. square. It is shown with a traditional AlN substrate (which in this case is 2-in. square).
Fig. 1. New HBLED grade AlN, which is white and has tile dimensions of 4.5-in. square. It is shown with a traditional AlN substrate (which in this case is 2-in. square).

 

What is Next?

We focused on a new AlN ceramic technology that results in a material that from a cost/performance standpoint bridges the current wide gap between high thermal conductivity, high cost AlN; and lower thermal performance, lower cost aluminum oxide.

For the focus applications of this technology, which includes power semiconductor and HBLED packaging, the 100W/m-K thermal performance is more than adequate. Due to the highly cost competitive nature of these applications, and the current high packaging costs for power devices, there is a strong fit for a new material with significantly lower cost structure. As this material is adapted more widely, it is expected that it will compete favorably for many applications that are now served exclusively by aluminum oxide.

 

References

1. Freescale White Paper: Thermal Analysis of Semiconductor Systems

2. DPC is a product line from Tong Hsing Electronic Industries, Taiwan and is used primarily for

power semiconductors and for HBLED applications. It consists of patterned, plated Cu on ceramic boards.

3. This is the relative cost of ceramic component alone, not including the cost to fabricate the package.

4. Because of the high thermal demand in the high brightness LED market, this material was originally designed for this application and that is the origin of the name.

5. Fabricated using the carbo-thermal reduction of aluminum oxide.

 

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