Aluminum Nitride-Based Ceramic Material Fits Power Semiconductor Applications

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

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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.

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.

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).