Why are CPUs becoming smaller and smaller?

Question

It is a known fact that over time processors (or chips) are becoming smaller and smaller. Intel and AMD are in a race for the smallest standards (45nm, 32nm, 18nm, ..). But why is it so important to have the smallest elements on the smallest chip area?

Why not make a 90nm 5x5cm cpu? Why squeeze 6 cores into a 216mm2 area? It will be easier to dissipate the heat from larger area, manufacturing will require less precise (and thus cheaper) technology.

I can think of few reasons:

• less size means more chips could be made on single wafer (but wafers aren't very expensive, right?)
• smaller sizes are important for mobile gadgets (but everyday PCs still use tower boxes)
• small size is dictated by light-speed limit, the chip can't be larger than the distance an EM field can travel in 1 cycle (but thats approximately several cm at 3GHz)

So, why do chips need to become smaller and smaller?

Answer 1

It's like candy bars. They keep making them smaller at the same price to increase profit.

Seriosly though, there are good reasons for smaller chips. The first and foremost is that more chips can be fit onto a wafer. For large chips, the cost is all about what fraction of a wafer it uses. The cost to process a wafer is pretty much fixed, regardless of how many chips result from it.

Using less of the expensive wafer is only one part though. Yield is the other. All wafers have imperfections. Think of them as being small but randomly scattered about the wafer, and any IC that hits one of these imperfections is trash. When the wafer is covered by lots of small ICs, only a small fraction of the total are trash. As the IC size goes up the fraction of them that hit a imperfection goes up. As a unreal example that nonetheless points out the issue, consider the case where every wafer has one imperfection and is covered by one IC. The yield would be 0. If it were covered by 100 ICs, the yield would be 99%.

There's a lot more to yield than this, and this is greatly oversimplifying the issue, but these two effects do push towards smaller chips being more economical.

For really simple ICs, the packaging and testing cost dominates. In those cases, the features size is not so much a driving issue. This is also one reason we have seen a explosion of smaller and cheaper packages lately. Note that extreme small features size is being pushed by very large ICs, like main processors and GPUs.

Answer 2

As the process size gets smaller, power usage decreases.

Smaller transistor processes allow the use of lower voltages combined with the improvements in construction technique mean that a ~45nm processor can use less than half the power that a 90nm processor uses with similar transistor counts.

The reason for this is that as the transistor gate gets smaller, threshold voltage and gate capacitance (required drive current) gets lower.

It should be noted that as Olin pointed out this level of improvement doesn't continue to smaller process sizes as leakage current becomes very important.

One of your other points, the speed at which signals can travel around the chip:

At 3ghz the wavelength is 10cm, however the 1/10th wavelength is 1cm which is where you need to start considering transmission line effects for digital signals. Additionally remember that in the case of Intel processors some parts of the chip runs at twice the clock speed so 0.5cm becomes the important distance for transmission line effects. NOTE: they may be operating on both clock edges in this case, meaning the clock doesn't run at 6Ghz but some processes going on are moving data that fast and have to consider the effects.

Outside transmission line effects, you also have to consider clock synchronization. I don't actually know what the propagation velocity is inside a microprocessor, for unshielded copper wire its like 95% of the speed of light but for coax is like 60% the speed of light.

At 6Ghz the clock period is only 167 picoseconds the so high/low time is ~ 84 picoseconds. In vacuum, light can travel 1cm in 33.3 picosends. If the propagation velocity was 50% the speed of light then its more like 66.6 picoseconds to travel 1 cm. This combined with the propagation delays of the transistors and possibly other components means that the time the signal takes to move around even a small die at 3-6Ghz is significant for maintaining proper clock synchronization.

Answer 3

The main reason is the first one you mentioned. Wafers (what you call plates) are very expensive, so you want to get the most from them. Earlier wafers were 3 inch in diameter, today's are 12 inch, which not only gives you 16 times as much real estate, obviously, but you get even more dies out of them than that.
So it's clear that they would use this technology also for CPUs used in tower PCs, even if it doesn't look like it's necessary there. And don't forget that laptop PCs also have this kind of CPUs, and they are on a budget as far as space is concerned.
Speed is also a concern, at 3 GHz signals travel less than 10 cm per clock cycle. As a rule of thumb from 1/10th of that we have to take care of transmission line effects. And that's less than 1 cm.

edit
Smaller feature size also means less gate capacitance, and this allows for higher speed. Faster switching means less power consumption, since MOSFETs will go faster through their active region. In practice manufacturers take advantage of this to clock faster, so that in the end you won't see much of this power reduction.

Answer 4

POWER: less semiconductor contact area = less power

HEAT: less power = less heat

HIGHER YIELD: Smaller area means higher production yield/tray = more chips/Hr

MONEY: Smaller chips = less raw materials = lower cost

FUTURE TECHNOLOGY: Smaller designs afford real-estate for new technology expansion. Remember when math co-processors once lived on a separate chip.

PROGRESS: Because we can, we should to further progress. Someone will always discover better and brighter ways for existing techniques. If we don't, we will never progress

Answer 5

The CORE reason why CPUs keep getting smaller is simply that, in computing, smaller is more powerful:

To a first approximation, computation involves two basic actions: transmitting information from one place to another, and combining strands of information to produce new information. Since we are used to using electronics here, let's call the hardware for these actions 'wires' and 'switches'. For both of these, smaller is better:

Wires: Since the speed of transmission on a wire is essentially constant, then if you want to get information from one place (e.g switch) to another, you have to shorten the wire. (you may be able to achieve a faster speed, but eventually you hit the speed of light limit, at which point you are forced back to shortening).

Switches: A switch works by information from one or more input wires entering and suffusing the body of the switch, causing its internal state to transform so as to modulate the information on one or more output wires. It simply takes less time to suffuse the body of a smaller switch.