From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) These PowerPoint color diagrams can only be used by instructors if the 3 rd Edition has been adopted for his/her course. Permission is given to individuals who have purchased a copy of the third edition with CD-ROM Electronic Materials and Devices to use these slides in seminar, symposium and conference presentations provided that the book title, author and © McGraw-Hill are displayed under each diagram.

From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) General Conductivity  = conductivity q i = charge carried by the charge carrier species i (for electrons and holes q i = e) n i = concentration of the charge carrier  i = drift mobility of the charge carrier of species i Conductivity of the material depends on all the conduction mechanisms with each species of charge carrier The dominate conduction mechanism is often quite difficult to uniquely identify Conductivity will also change with temperature, composition, and ambient conditions

From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Temperature Dependence of Conductivity  = conductivity    constant    = activation energy for conductivity k = Boltzmann constant, T = temperature For many insulators, and in a majority of cases, the conductivity follows an Arrhenius-type temperature dependence:

From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Conductivity versus reciprocal temperature for various low-conductivity solids SOURCE: Data selectively combined from numerous sources. Fig 2.29 If above equation is plotted log(  ) v. 1/T, the result is a straight line with a negative slope that indicates the activation energy, E .

Thin Metal Films Resistivity of a material that is listed in tables and our analysis thus far, have been based on bulk material properties In certain applications, such as micro/nano electronics, metal films are widely used to provide “interconnects” to connect the circuit together Various methods are used to deposit thin films –We’ll briefly discuss two here From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

Intro to Evaporation For early semiconductor technologies, evaporation was the “method of choice” for metal deposition Evaporation has been primarily replaced by sputtering for 2 reasons: –Ability to cover surface topography – step coverage Evaporated films have bad step coverage –Hard to deposit well-controlled alloys which are typically used to form reliable contacts and/or metal lines Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press

Evaporator Wafers are loaded into a high vacuum chamber The “charge” (e.g., metal) to be deposited is loaded on a heated crucible that heats up the metal –Can be done by resistive heating and other means As phase transformation continues, the metal becomes a vapor Due to the low pressure in the chamber, the atoms in the vapor travel in a straight line until they strike a surface and accumulate as a film Alloying can be done with multiple crucibles along with “shutters” to control which crucible(s) is(are) depositing Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press Figure 12.1 A simple diffusion-pumped evaporator showing vacuum plumbing and the location of the charge- containing crucible and the wafers.

Evaporator Sources Figure 12.7 Resistive evaporator sources. (A) Simple sources including heating the charge itself and using a coil of refractory metal heater coil and a charge rod. (B) More standard thermal sources including a dimpled boat in resistive media. Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press Figure 12.8 Example of an inductively heated crucible used to create moderately charged temperatures

Shutter Example Since atoms of the vapor travel in a straight line, the shutter can block deposition Shutter rotates and can be controlled through it’s spin rate Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press

Alloying Figure Methods for evaporating multicomponent films include (A) single-source evaporation, (B) multisource simultaneous evaporation, and (C) multisource sequential evaporation. Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press

Shadowing and Step Coverage Shadowing and Step Coverage Problems Can Occur in Low Pressure Vacuum Deposition in which the Mean Free Path is Large Figure 6.5 IC Technology -Dr. W. Hu

Step Coverage Figure 12.5 (A) Time evolution of the evaporative coating of a feature with aspect ratio of 1.0, with little surface atom mobility (i.e., low substrate temperature) and no rotation. (B) Final profile of deposition on rotated and heated substrates. Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press Assuming incident atoms are immobile on the surface, the topography will cast a “shadow” and result in a discontinuous film This situation can be improved by rotating the wafer (planetary evaporator)

Electron Beam Evaporation IC Technology -Dr. W. Hu

Sputtering Figure 6.6 A dc sputtering system in which the target material acts as the cathode of a diode and the wafers are mounted on the system anode. Figure 6.7 Sputtering yield increases rapidly as ion energy is increased above the sputtering threshold (argon) IC Technology -Dr. W. Hu

Resistivity of Thin Films From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) (a)Grain boundaries cause scattering of the electron and therefore add to the Resistivity by the Matthiessen’s rule. (b) For a very grainy solid, the electron is scattered from grain boundary to grain boundary and the mean free path is approximately equal to the mean grain diameter. Fig 2.32 In a highly polycrystalline material, conduction electrons are more likely to be scattered by grain boundaries than other processes:

Scattering in Thin Metal Films Conduction electron is free within a grain, but usually scatters at the grain boundary Therefore, the mean free path, l grains is approximately equal to the average grain size, d If = l crystal is the mean free path of the conduction electrons in a single crystal (no grain boundaries), then From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

Scattering in Thin Metal Films (cont.) Resistivity is inversely proportional to the mean free path Therefore, the resistivity of the bulk single crystal  crystal  1/ and the resistivity of the polycrystalline sample is   1/ l Thus, From this, it is clear to see that smaller grain diameter d (i.e., more grainy films) will have a higher resistivity From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

Scattering in Thin Metal Films (cont.) For a more rigorous theory, a number of effects have to be considered More than one scattering event at a grain boundary may be necessary to totally randomize the velocity Therefore, there is a need to calculate the effective mean free path that accounts for how many collisions are needed to randomize the velocity From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

Scattering in Thin Metal Films (cont.) One possibility is that the electron may be totally reflected back at a grain boundary Suppose that the probability of reflection at a grain boundary is R, and if d is the average grain size (diameter),then the popular Mayadas-Shatkez formula is approximately given by: From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Is in the form of Matthiessen’s rule Indicates that the grain boundary scattering contribution  grains to the overall resistivity is (1.33  )  crystal

Surface Scattering From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Conduction in thin films may be controlled by scattering from the surfaces Consider scattering of electrons from surfaces of a conductive film with thickness D Assume that scattering from the surface is an inelastic process The electron loses the gained velocity from the field Put another way, the direction of the electron after the scattering event is independent of the direction before the scattering process Again, it’s unlikely that one surface scattering will completely randomize the electron’s velocity Also, the mean free path, l surf, of the electron will depend on its direction right after the scattering process... Fig 2.33

Surface Scattering From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) The mean free path of the electron depends on the angle  after scattering. Fig 2.34 If the angle  after surface scattering is zero,(electron moves transversely to the film length), then l surf = D In general, the mean free path l surf will be D /(cos  ) If scattering is a truly random process, then the probability of being scattered in a direction back into the film (i.e., +y direction) would be 0.5 on average However, we know that the electron’s direction right after the surface scattering is not totally random because the electron cannot leave the film Therefore,  is between –  /2 and +  /2 and cannot be between –  and +  The electron’s velocity after the first surface scattering must have a y component along +y, and not along -y

Surface Scattering From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Fig 2.34 The electron can only acquire a velocity component along –y again after the second surface scattering. Therefore, it takes two colllisions to randomize the velocity, which means that the effective mean free path must be twice as long: 2 D /cos  So, to find the overall mean free path, l, for calculating resistivity, we must use Matthiessen’s rule If l is the mean free path of the conduction electrons in the bulk crystal (no surface scattering), then:

Surface Scattering Then, we need to average for all possible  values per scattering (i.e.,  from –  /2 to +  /2) Now, we can relate l to as: From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)  bulk  1/   1/ l

From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

Interconnections & Contacts

Fig 2.36 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)

Three levels of interconnects in a flash memory chip. Different levels are connected through vias. |SOURCE: Courtesy of Dr. Don Scansen, Semiconductor Insights, Kanata, Ontario, Canada

Fig 2.37 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) (a)A single line interconnect surrounded by dielectric insulation. (b)Interconnects crisscross each other. There are three levels of interconnect: M – 1, M, and M + 1 (c) An interconnect has vertical and horizontal capacitances C v and C H. a c b

Interconnections and Contacts 3 Basic Interconnection Levels –n + diffusion –Polysilicon –Aluminum Metallization Contacts –Al-n + –Al-Polysilicon –Al-p Substrate Contact Not Shown Figure 7.1 Portion of MOS integrated circuit (a) Top view (b) Cross section IC Technology -Dr. W. Hu

Resistivity of Metals Commonly Used Metals Aluminum Titanium Tungsten Copper Less Frequently Utilized Nickel Platinum Paladium IC Technology -Dr. W. Hu

Ohmic Contact Formation (a)Ideal Ohmic Contact (b)Rectifying Contact (similar to diode) (c)Practical Nonlinear “Ohmic” Contact IC Technology -Dr. W. Hu

Ohmic Contact Formation Figure 7.3 Aluminum to p-type silicon forms an ohmic contact similar to Fig. 7.2(a) [Remember Al is p-type dopant] Aluminum to n-type silicon can form a rectifying contact (Schottky barrier diode) similar to Fig. 7.3(b) Aluminum to n+ silicon yields a contact similar to Fig. 7.3c IC Technology -Dr. W. Hu

Al Spiking and Junction Penetration Silicon absorption into the aluminum results in aluminum spikes Spikes can short junctions or cause excess leakage Barrier metal deposited prior to metallization Sputter deposition of Al - 1% Si IC Technology -Dr. W. Hu

Alloying of Contacts IC Technology -Dr. W. Hu

Contact Resistance IC Technology -Dr. W. Hu

Schottky Contact Figure Band diagram for an ideal Schottky contact: before contact (right) and after contact (left). Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press

Current Flow for Metal/Semi Contacts Figure Two carrier transport mechanisms typically found in metal semiconductor contacts. Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press

Electromigration Figure Typical point of flux divergence in an interconnect material. Fabrication Engineering at the Micro and Nanoscale, 4/e Stephen A. Campbell Copyright © 2014 by Oxford University Press

From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Electromigration Fig 2.38

Electromigration (a) (b) High current density causes voids to form in interconnections “Electron wind” causes movement of metal atoms IC Technology -Dr. W. Hu

Electromigration Copper added to aluminum to improve lifetime (Al, 4% Cu, 1% Si) Heavier metals (e. g. Cu) have lower activation energy IC Technology -Dr. W. Hu

Diffused Interconnections n- and p-type diffusions can be used for local interconnections pn-junction diode must be kept in its reverse-biased (non- conducting) state All interconnections have a series resistance R and shunt capacitance C per unit length The RC time constant limits operating frequency n + and polysilicon lines R S ≥ 30  /square Figure 7.9 Lumped RC model for a small section of an n+ diffusion IC Technology -Dr. W. Hu

Silicides/Polycides/Salicides Silicides of noble and refractory metals can be used to reduce sheet resistance of polisilicon and diffused interconnections Provide shunting layer in parallel with original inteconnection Figure 7.12 IC Technology -Dr. W. Hu

Properties of Various Silicides IC Technology -Dr. W. Hu

Salicide Self-Aligned Silicide on silicon and polysilicon Often termed “Salicide” IC Technology -Dr. W. Hu

Silicide Contacts in Devices IC Technology -Dr. W. Hu