Very well written, to the point. Mahendra -----Original Message----- From: EmbeddedNet [mailto:[log in to unmask]] On Behalf Of Richard Ulrich Sent: Tuesday, March 07, 2006 6:10 AM To: [log in to unmask] Subject: [EM] Circuitree Embedded Passive Column (DRAFT) Here's a draft of my next column on embedded passives. Any comments would be appreciated. - Rick for "Embedded Passive Update" column, CircuiTree, May 2006 High-Capacitance Thick Film Dielectrics? Rick Ulrich In my last column, two months ago, I bemoaned the sad state of affairs with screen-printable thick film dielectrics. Even when packed with 95%+ by weight with randomly-dispersed high-k barium titanate and printed to only 10 microns thick, the overall k is a mere 30 and delivers only about 3 nF/cm2. One way of increasing the capacitance of this approach would be to somehow stack the high-k phase so that it is vertically continuous over as wide a lateral area as possible. Some kind of self-assembly or self-orientation would be necessary, and I know of work going on in that direction. But there's another trick that's been receiving new attention lately that may be able to significantly increase the capacitance density of screen-printable thick films. The concept is pretty simple: load the polymer up with conductive metal particles, as opposed to the insulating high-k particles that I described in the previous column. Rao Tummala, C. P. Wong, and their team at Georgia Tech have been exploring this lately and have managed to get effective dielectric constants of over 100 by dispersing Ag and Al particles in epoxy. To understand how this works, think about drilling a vertical hole one molecule wide all the way through the dielectric. The materials and their configurations encountered along the way are what the field sees. In the case of randomly-dispersed 10 micron k = 1000 barium titanate particles scattered in k = 3 epoxy, this hole would pass through alternating regions of high and low k material, which the field would see as capacitors in series. The result is an overall k in between the two, but much closer to the low end at about 30 since adding caps in series lowers the overall value of the set. But, if the dispersed particles are instead conductive, then all of the voltage drop is over the epoxy phase alone. The hole might pass through 10 microns of total applied film thickness, but only a small fraction of that will be actual dielectric, depending the concentration of conductive particles that were mixed in. In essence, this is a clever way to make a very thin polymer film that is easily screen printed. As you approach the percolation point, the minimum concentration of metal particles that causes them to start touching each other, the effective film thickness is very small and high specific capacitance can be obtained. Using 80 wt% of 3.0 micron Al in epoxy, they've managed to get an effective k of 109 and a dissipation factor of only 2%. I say "effective k" because the material that is actually acting as the dielectric has the same k it had in bulk form, about 3, but is spread very thinly. The value of 109 is based on the entire film thickness so this is the k it acts like. Printed to a total thickness of 10 microns gives a specific capacitance of almost 10 nF/cm2, a factor of three better than high-k dispersed in epoxy and better than 30 times what you'd get from unfilled epoxy. This latter number also represents the ratio of the printed film thickness to the effective thickness. The downside to this method is that you have to be very close to the percolation point to get these high values and, in this vicinity, the effective k and the dissipation factor change rapidly with particle loading, making tolerance a major challenge. Once the particles reach percolation and actually begin touching, the dissipation factor skyrockets since the film becomes leaky. Much past this, it would not function as a dielectric at all, so you need to be just a fraction of a percent under percolation for best results. But Georgia Tech is working on getting around this issue by using metal particles that have an oxide coating so that even when they touch, there is no electrical continuity. The primary application for this would be decoupling, since tolerance is not a major issue as long as a certain minimum amount of capacitance is achieved. Also, dissipation factor is not nearly so much a detriment in decoupling as it is in filters and A/C converters. In fact, some folks claim that a high dissipation factor is beneficial to decoupling since it helps remove high-frequency noise on the power/ground planes. Their results will soon be published in IEEE Transactions of Advanced Packaging along with a considerable amount of information on the processability of this sort of material. Much work remains to be done, but it's an interesting and promising approach that could eventually give tens of nF/cm2 with a screen-printable film. ****************************************************** Dr. Richard Ulrich Professor Dept. of Chemical Engineering 3202 Bell Center Univ. of Arkansas Fayetteville, AR 72701 [log in to unmask] (479) 575-5645 ******************************************************