Notes of Semiconductor Technology, Wafer Fabrication And Testing June 2012 Abstract Physics of Semiconductor materials, p-n junction, MOS transistors, Silicon technology (fabrication, or front End), packaging and assemply (back end), 1 Contents 1 Physics of Semiconductor Materials 3 1.1 Genesis . 3 1.2 Band Model of Solids . 4 1.3 Crystal bonding model . 13 1.4 P-N junction . 15 1.5 Transistor . 16 2 Silicon Technology (fabrication) 26 3 Testing and packaging 52 4 Conclusion 55 2 1 Physics of Semiconductor Materials 1.1 Genesis At the basis of modern micro electronics, which is the basis on semiconduc- tors, devices there is Quantum Theory developed at the beginning of nineteen Century. In 1900, Planck came up with an equation that described the curve of radia- tion wavelengths at each temperature for a black body radiation 1. No matter what the material shape or shape of the container's wall, the results were the same; the shape of the graphs depended only by temperature. Planck posited that the surface of anything that was radiating heat and light such the walls in a black body device, contained vibrating molecules or harmonic oscilla- tors. These harmonic oscillators could absorb or emit energy only in the form of discrete packets or bundles. These packets or bundles of energy came only in xed amounts, determined by Planck's constant, rather than being divisible or having continuous range of values.This principle can be see again in the the photoelectric experiment, which showed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The remarkable fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron! This t in well with Planck's hypothesis that light in the black body radiation experiment could exist only in discrete bundles with energy. Photoelectric eect was known since the end of eighteen century but Einstein was the one who given the correct interpretation, and by the way his studies of this eect was the reason for the Nobel price the great physician got in 1925. In another words we can see the photoelectric eect in this way: when we increase the frequency of the light the emitted electrons sped out with much more energy. Then, when we increase the intensity of the light by using carbon arc light that could be made brighter by a factor of 1000. The brighter, more intense light had a lot more energy, so it seems logical that electrons emitted would have more energy and speed away faster, But that did not occur. More intense light produces more electrons, but the energy of each remained the same. This was something that the wave theory of light did not explain. Today we know that the behavior of the black body can be applied also at atomic scale range. Each electron of an atom can absorb or emit energy at specic frequency only at discrete values following the quite famous formula E1 − E2 = hν (1.1) where E1 − E2 is the energy jump of the electron in the atomic model of Bohr 1A black body is an idealized physical body that absorbs all incident electromagnetic radiation regardless of frequency or angle of incidence. It can be done with a small hole when electromagnetic radiation enters a cavity which walls reect inside and the radiation does not exit. From the external world is like if the external radiation has been completely absorbed by the hole, which represents the black body. A black body in thermal equilibrium (that is, a constant temperature) emits electromagnetic radiation called black-body radiation. 3 Figure 1.1: Black Body radiation as satellite of the atom nucleus,ν is the frequency of the radiation, and h is the Planck constant. When an electron absorb this packet of energy it emits an elementary particle called photon which exibits wave-particle duality (follows maxwel's equations as well otpical rules!) 1.2 Band Model of Solids Let consider electrons in an isolated atom. We know that an electron acted on by the Coulomb potential of an atomic nucleus may have only certain allowed energies ( Figure 1.3 ) 4 Figure 1.2: Bohr Atom and energy jump Figure 1.3: Allowed energy levels of an electron acted on by the Coulomb po- tential of an atomic nucleus 5 In particular, by quantum mechanics theory, the electron can occupy one of a series of energy levels below a reference energy taken as 0. 2 4 −Z m0q (1.2) En = 2 2 2 8oh n The electron carries charge −q so that q in 1.2 is a positive number. At low temperature, when more than one electron are associated with the atom, the electrons ll the allowed levels starting with the lowest energies. By the Pauli exclusion principle, at most two electrons (of opposite spins) may occupy a new energy level. Let us now consider the electron in the highest occupied energy level of an atom and neglect the lower lled levels. When two isolated atoms are separated by a large distance, the electron associated with each atom has the energyEn given by 1.2. If two atoms approach one another, however, the full atomic core of the rst atom exerts a force on the second electron.The allowed energy levels for this electron are modied due to this interaction (there is a change of potential). An energy level En, can contain at most two electrons of opposite spin. When we approach the two atom for the same energy level En cannot cohabit four electrons! When the two atoms are brought together to form one system only two electrons can be associated with the allowed energy level En. The two other should go in a dierent level quite close to En. In reality what happens is that the allowed energy level En of the isolated atoms splits into two levels with slightly dierent energies in order to retain space for a total of four electrons. This principle is valid for all electrons of Atoms not only the last two in the energy level En.As more atoms are added to form a crystalline structure, each of the original quantized levels of the isolated atom is split many times. When N atoms are included in the system, the original energy level En splits into N dierent allowed levels, forming the so called energy band which may contain at most 2N electrons (because of spin degeneracy). Since the number of atoms in a crystal is generally large-of the order of 1022cm−3- and the total extent of the energy band is of the order of few electron volts, the separation between N dierent energy levels within each band is much smaller than the thermal energy possessed by an electron at room temperature, and electron may easily jumps between levels. Thus, we may speak of a continuous band of allowed energies containing space for 2N electrons. This allowed band is bounded by maximum and minimum energies, and may be separated by a so-called forbidden-energy gaps, as showed in (Figure 1.4) or it may overlaps other bands. Electrons in one band can easily jumps in the levels of this band, but their thermal energy at room temperature does not allow them to jump the forbidden gap, so the name comes from this constraints. The fact if these bands overlaps each other, or form small gaps or very large gaps determines the proprieties of a given material as conductor, semiconductor or insulator. The allowed energy bands does not have the same depth. Electrons at high energy levels (far from the central nucleus of the atom) are less tightly bound and 6 Figure 1.4: Broadening of allowed energy levels into allowed energy bands sepa- rated by forbidden-energy gaps as more atoms inuence each electron in a solid; on the left (a) one-dimensional representation; on the right (b) two-dimensional diagram in which energy is plotted versus distance 7 Figure 1.5: Energy band diagram: N electrons lling half of the 2N allowed states, as might occur in a metal may wander farther from the atomic core. If the electron is less tightly conned, it comes closer to the adjacent atoms and is more strongly inuenced by them. This greater interaction causes a larger change in the energy levels and this is reected in a wider energy bands for higher energy electrons of the isolated atoms. We can see this in the Figure 1.4 where (a) is the one-dimensional sketch. However often is useful to use a two-dimensional picture (b) where the vertical axis still represents the electron energy while the horizontal axis represents position of the electron in the crystal. The formation of energy bands from discrete levels occurs whenever the atoms of any element are brought together to form a solid. So all solid in nature will have this energy bands representation. We told that the distances between these gaps cause dierent electrical proprieties of solids. Let's take one metal, the alkali. Each Alkali atom has a free valence electron in the outer shell. Let's take a piece of Alkali of N atoms. As we discussed before all these atoms when get closer to form the metal crystal structure split the allowed energy levels of electrons from N (isolated atom) to 2N (all atoms together).