there are some many shapes of heat spreaders available in the market such as "star" shape etc. This type of heat spreader is used to attach to IC's die. The question is: Why people made heat spreader with such a funny shape? And what's the main reason behind it? Thanks

The shapes are derived from the desire to maximize surface area relative to the volume of metal. Ideally you want the temperature at the surface of the heat sink to follow as closely as possible the temperature of the IC. Since a large volume of heatsink material would introduce a significant lag in the time response (look at the time-dependent conduction equation) then a small volume of material is desired. But in order to maximize convective heat transfer at the surface, surface area needs to be as large as possible. Thus you end up with heat sinks with lots of fins, pins, and whatever else can be reasonably manufactured.

I don't think it's an issue of the transient behaviour of the heatsink that drives its design. The increase in surface area is key, heatsinks are simply 'area extenders'. By increasing the surface area you have to be careful that this positive effect is not compromised by the thermal resistance of the heatsink itself. Metrics such as the spreading resistance in the base (when the heat source area is smaller than the heatsink base area, fin efficiency and heatsink mass (for cost and mechanical concerns) should be considered.

www.coolingzone.com has lots of stuff about heatsinks, worth checking out.