The concept of layers helps in reducing the size of transistors.Every year transistors' number go on increasing in a microprocessor chip. Where does this increase come from?it comes- Chips gets larger and the transistors get more and more smaller. Transistors shrink in size beacuse of their ability to control photolithography and the technology of process advances.

Chip is not actually made up of all transistors only. It contains wires also. The wires are often wider than the transistors because of the harder deposition of aluminium and because the wires are on top of other layers.where these have to face an uneven surface. Aluminium is subject to a phenomenon called "metal migration" in which an  electric current actually draws metal atoms along with the electron flow and the metal thus can break or separate at weak points. As chips grow larger and device counts increases, the length of wires and their number increases. Modern chips are built up on the system of wires. This lead to the increase in the number of layers of wires on a chip to increase density. Copper was never used in place of Aluminium as it was harder to work with. It is also softer than Aluminium so it is harder to control its deposition.

the metal is the thickest layer in the chip. It adds a great deal of vertical relief to the chip surface. It is important to fill in the valleys with an insulator which is than polished flat. Then the insulator must be made smooth to varying depths to create contact points for the next metal layer. As the sizes of transistors shrink these can also be more easily switched because the distance between the drain and source is reduced and the capacitance of the gate is also reduced. Thus a channel can be easily opened or closed. In addition, wires length reduces, so the time for a signal to propagate locally is reduced.

Another reason for reducing size of transistors is something called  short channel effect. When the gate region of a transistor gets too small, electrons can tunnel through it whether it is off or on. This requires the correct adjustment of doping to increase the off resistance. In other words to build a tiny transistor that can be turned off, we have to dope it upto the point where it cannot be turned on.The solution to this problem was to place gates on both sides of the channel and charge them at the same time. the joint fields create a conductive channel, and when these are removed, the transistor turns off. So these were some of the reasons transisotrs go on shrinking. And the concept of layers help in decreasing the size of transistor and increasing the speed of chip or microprocessor