Intel plans to use glass substrates for its largest and most advanced chips later this decade. This should make it possible to build larger and more complex chips and to communicate with the chips faster.
Intel currently has four options for packaging, or the way a silicon is used die connected to the outside world. The best known of these are the FCBGA and FCLGA chips. Almost all consumer processors use this technology, in which the actual silicon dies are connected to a substrate of bonded glass fiber layers. That substrate is called an organic substrate by Intel and is very similar to a piece of printed circuit board. Laptop processors are usually on an FCBGA substrate and socketed desktop processors on an FCLGA substrate, shortly before flip-chip ball-grid array in flip-chip land grind array. The FCBGA chips are soldered directly to a motherboard and the FCLGA chips are socketed.
For more advanced chips, for example when several chiplets need to be combined with each other, Intel developed the EMIB package a few years ago, in which small pieces of organic substrate are replaced by pieces of silicon, in which much more advanced compounds such as through-silicin via’s and connections with higher density can be realized. Foveros packaging is a kind of further development of this, where powerful chiplets are placed on a simple silicon substrate that performs simpler tasks. The whole thing is put back on a package.
Organic substrate versus glass
The organic substrates are limited in their ability to connect chiplets or dies together at high speed. The ‘printed circuit boards’ cannot be made infinitely complex and the density of the number of connections, and therefore their number and bandwidth, is limited. The total footprint is also limited to 120 by 120mm. EMIB is also somewhat limited in terms of dimensions. Chips of 4.5 times the reticle size are currently possible, with an upgrade of 6 times that size in the offing. However, the dimensions of the connection connections, the so-called bumps, are much smaller than with organic substrates. These are still 55μm in size, but scale to 36μm, while with organic substrates they do not become much smaller than 100μm. Foveros has much smaller contact bumps from 36 to 25μm, while Foveros Direct even scales smaller than 10μm.
With the development of glass substrates, Intel wants to combine the advantages of all substrates and make larger chips with high-speed interconnects. It even wants to make larger chips with optical communication to the outside, using wave guides optical computing enable. The glass interconnects must also have better mechanical and optical properties and should be cheaper to implement and more economical.
One of the properties of glass is that its expansion coefficient can be influenced, causing it to expand or contract just as much as the silicon die to which it is connected. The very small interconnections would be severed if the die and the substrate respond differently to heat. There can also be many more holes, which are analogous to that through silicon via’s not through glass via’s or TGVs are then realized in organic substrates. Intel speaks of a ten times higher TGV density than with organic substrates, although the difference with silicon options such as EMIB or Foveros is much smaller.
Glass also has insulating properties that reduce energy and signal loss, allowing faster communication speeds. According to Intel, 480Gbit/s is possible with the TGVs. These TGVs could have a pitch of less than 100μm and features such as electronic connections in the glass could have a pitch of only 5μm and a line width of 5μm. For comparison: in an organic substrate this is approximately 10μm. Another important advantage of many and very small bumps is the possibility of having the substrate take over some functions of the chip. A very complex aspect of chip design is the power distribution layer, the metal layers that power all the transistors. With glass substrates, some of those metal layers could be left out and moved to the substrate.
In short, Intel predicts that the switch to glass as a substrate will provide benefits for the scaling of internal wiring and TGVs, silicone dies can be connected to more and finer bumps, high-speed communication becomes possible and the power supply is improved. Glass packages can be scaled up to eight times the reticle dimensions or up to 240 by 240mm, which is a desirable feature for sample chips in data centers, especially AI chips.
The team that also developed EMIB has been working on glass substrates for about ten years. A working test chip has now been delivered with TGVs with a diameter of 75μm and three metal layers on either side of the glass. Intel has a billion dollar research center and manufacturing line in Arizona and is working with suppliers and partners to build a complete ecosystem. The company expects to switch to glass for very large chips for AI and data center applications later this decade. Organic substrates such as those we now have in our PC or laptop will continue to be used for consumer products.
2023-09-18 13:00:00
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