Final Report

BMDO-STTR Phase 1 R&D

November 10, 1999

BMDO-STTR Contract No. DTRA01-99-M-0074
Contractor InPod,Inc.: 8601 Cross Park Drive, Suite 100, Austin Texas 78754
Corporate official: Greg Hollingshead; 512-970-3204
Subcontract to the University of Houston; (PI: Prof. A. Bensaoula ; Co-PI: Dr. N. Badi)

Title:

Multilayer Ceramic Capacitor Chip (MLC3’s) for High-Energy Density Storage
and High Frequency Power Switching Device Applications

Objectives:
The objective of this phase I research was to develop low-cost, high-energy density, multilayer boron nitride capacitors that operate in an extended range of temperatures.

The three tasks as defined in the phase I contract were:

Task 1: Processing and patterning
Task 2: Boron nitride capacitors fabrication
Task 3: Frequency and temperature characterization of the capacitor structures

During this phase 1 contract, efforts were devoted to demonstrate the feasibility of fabricating single and multilayer ceramic capacitor chips (MLC3’s) based on insulating thin boron nitride (BN) layers and conductive copper (Cu), aluminum (Al), and titanium nitride (TiN) as internal electrodes.
In task 1, we used plasma and chemical etching of BN films as well as standard processing techniques (photolithography, metallization) in the fabrication of the capacitor devices.
In task 2, we used ion and electron cyclotron resonance plasma (ECR) sources assisted physical vapor deposition (PVD) technique to generate the thin film materials.
In task 3, we performed breakdown voltage, capacitance, C-V, SIMS, SEM, and frequency/temperature measurements in the characterization of the BN capacitors.
In addition, business research and marketing strategy studies were conducted with the help of business school students. Their final report was enclosed as part of the phase II proposal. These studies have been directed into the following areas:

Overview of the thin film capacitors market;
PVD process and equipment information;
Economic industry trends;
Target capacitor manufacturers;
Niche applications for our BN capacitors.

Introduction
Multilayer ceramic capacitors (MLCCs) are the most reliable source for high-energy density storage banks. They also find use in high frequency switch mode power supplies. MLCCs have a large share in the capacitor market. Recently, much effort has been paid to achieve higher capacitance using MLCCs with smaller size. The realization of multilayer ceramic capacitors with higher capacitance and volumetric efficiencies is today the biggest challenge for MLCC’s manufacturers. These MLCCs are expected to extend the application field where electrolytic or plastic film capacitors are currently used.
The main limiting factor for MLCC development is that of thickness control and integrity of the dielectric layers as well as effective electrodes; the value of the dielectric constant being somewhat a secondary consideration. The objective being, smaller case sizes for a given capacitance value, higher capacitance value in a given case size, higher reliability and lower cost per unit. Three types of dielectric have come to dominate the market. Their characteristics in the form of percentage change in capacitance as a function of temperature are shown in Fig.1. One can see clearly the limited operating temperature and none of these types and especially Z5U capacitor will stand the heat inside the engine compartment of an automobile or in a military fighter aircraft. Besides, these capacitors display a high rate of aging.

Fig. 1. The change in capacitance as a function of temperature for the three most popular ceramic capacitor

The formal equations of capacitor are:

f: conversion factor
Metric system: f = 11.31: centimeter
English system "inch" f= 4.452

In our case, the energy stored U will be:

The energy density stored in a capacitor is the potential energy/volume

Where K is the relative dielectric constant of the material, A is the effective area of the internal electrode, d is the thickness of the dielectric layer, and E is the electric field. Parametrically, it is desirable to optimize K, A/d and E simultaneously. Practically, it has been easier to attack the problem from two approaches. The first of these is to engineer dielectric films with high K and E. This work has extended the energy density of "conventional" capacitors by an order of magnitude. The current trend is to optimize the A/d ratio in the expression for the capacitance. This will result in high energy density at low voltage.

The parameters of interest for such capacitors include:

Capacitance (C).
Temperature coefficient of capacitance (TCC).
Breakdown voltage (BDV).
Capacitance per unit volume or weight (volumetric or weight efficiency).
Dissipation factor (DF) or loss tangent.
Insulation resistance (IR).
For certain applications, radiation immunity.

Applications addressed with this Phase I research
Compact and miniature power sources with extended range of temperature can replace existing capacitors and will meet several new heavy-duty devices applications in the semiconductor industry (i.e. surface mount industry), military (i.e. explosives, fuses, safe-arm-fire devices, and explosive detonators, etc.), and space (i.e. compact power supplies, solar-powered equipment, etc.). They are well suited for pulse power applications such as ignition systems, lasers, x-ray generation, power supplies, electric vehicles, solar-powered equipment and physics research. Applications involving compact power density sources operating in harsh environments and compatible with Micro Electro Mechanical Systems (MEMS) are not excluded. They also find use in high frequency switch mode power supplies because they can be optimized to minimize both effective series resistance (ESR) and effective series inductance (ESL).

With the use of BN as the dielectric, we are able to expand the operating temperature limit to 800 ^oC or more. Secondly this technology allows the MLCC to be more compact and easily reach the <1000 Å size versus the 3 mm layer thickness limitation exhibited by currently used capacitors.