Materials for Microelectronics and MEMS mems materials.ppt
Materials for Use in Microelectronics & MEMS Devices Microelectronic device fabrication uses a selected variety of materials that include metals or conductors (more generally), semiconductors, and insulators. Since MEMS technology also employs microelectronic fabrication methods, many MEMS devices use the very same materials, or same types of materials. A primary characteristic of materials that defines where and how they’re used is their electrical conduction. Material Resistivity (-m) metals 10-8 semiconductors ~ 0.001 1000 insulators 1016
Commonly-Used Materials • Conductors of interest: • Al and several alloys (Al:Si and Al:Si:Cu) • Poly silicon (doped to control its resistivity) • Cu • Other metals in special cases (Au). • Semiconductors: • Si (by far the most important for microelectronics) • Other group IV elements (Ge, Si:Ge) • Other compound semiconductors (III-V like GaAs, AlGaAs & II-VI like ZnS, ZnSe, BeSeTe) • Common Insulators & Dielectrics: • Simple insulators like SiO2 and Si3N4 • Rare-earth oxides (HfO2) • Other insulating materials (ZnO, CaF2) and multifunctional oxides
Standard Device Fabrication Sequence Recall the primary processes that are used in microelectronics include film deposition photolithography to make a patterned mask on the film etch of the film (pattern transfer) by wet or dry etch methods other processes (e.g. doping, anneals, etc…) repeat the sequence… photolithography film deposition etch other processes Note that MEMS devices are usually much larger (~100 µm) and thicker (few µm) than microelectronic devices (usually < 1 µm and << 1 µm).
Device Fab using Photolithography & Pattern Transfer by Etching All the patterns (devices features) to make microelectronic or MEMS devices are made using the photolithographic & etching processes. 8. Selectively process the substrate in the opening. 7. Strip the remaining photoresist. 6. Etch film using resist as a mask. 3. Align photomask to wafer features. 5. Develop photoresist. 4. Expose photoresist through mask. 1. Spin photoresist onto wafer. 2. Softbake photoresist to dry it.
Chemical Behavior within the Periodic Table Here, various types of chemical behavior are identified. The arrangement of the periodic table by chemistry is clearly evident here.
Electrical Behavior within the Periodic Table Here the table has been changed to highlight the electrical behavior of the elements. Group IV semiconductors technologically important or precious metals C some magnetic materials Al Si P Ge Ni Cu Ga As Fe Co Ag In Sn Sb Pb Au Pt Group III-V compound semiconductors (e.g. GaAs, InP)
Atomic Arrangement in Solids Solids come in a variety of atomic arrangements. These can be classified into a few distinct groups. • crystal—all the atoms are properly arranged with respect to their neighbors; a crystal lattice. • polycrystal—atoms in one region are aligned correctly, but the regions aren’t aligned to one another; separated by crystal or grain boundaries. • amorphous—the atoms are arranged randomly; no long-range or short-range order. Crystal samples: quartz, Si, calcite, amethyst
Cubic Crystal Structures Many crystals are formed by placing atoms in a cubic structure (of one sort or another). These structures are built by placing atoms at various locations on or inside the cube. z y The simplest crystal structure is called simple cubic (SC). Atoms sit at the 8 cube corners. The next crystal structure still has the 8 corner atoms and one additional atom at the center of the cube. This structure is called the body-centered cubic (BCC) structure. The next structure in complexity still has the 8 corner atoms, but no center atom. Instead, there are atoms at the center of each cube face. It is called face-centered cubic (FCC). x
Lattice Constants In a simple cubic crystal, a larger piece of matter is formed by stacking these cubic building blocks in 3 dimensions. This building block is called a conventionalcell. z y a x The length of the cell side is called the latticeparameter and is denoted by a. It is measured in either Ångstrőms or nanometers.
Hexagonal Crystals If the atoms are arranged in a hexagonal form, a hexagonal close-packed crystal (hcp) is formed. Hexagonal crystal viewed from the side. c b a b a The lattice parameters here are called a & b (in the hexagonal plane). In this crystal they are equal (a = b). The distance to the next hexagonal plane is called c and is usually different.
The 14 Bravais Lattices There are a number of other 3-dimensional crystal lattices beyond the cubic structures. They are called the Bravais lattices and there are a total of 14 of them. Oxford University
Silicon & the Other Group IV Semiconductors Carbon, silicon, and germanium are in the periodic table in Group IV. This means that these atoms have 4 e- in their outermost orbits. For example, Si has 14 electrons distributed as: 2 (lowest) + 8 (next) + 4 (outermost) To form a Si crystal, one Si atom likes to bond with 4 nearest-neighbors. Each Si atom tries to do this. The bonding uses these 4 outermost electrons. Each pair of Si atoms shares 2 electrons in a covalent bond (equal sharing). In 2 dimensions, the Si crystal might look like: This is how Si forms a crystalline solid. However, it doesn’t really form in a 2d square lattice like this. It forms in a 3d lattice.
The Diamond Crystal Lattice The most important semiconductor materials, like silicon, have a crystal structure that is based on the FCC lattice. FCC lattice the diamond lattice The diamond lattice is formed by having 2 interpenetrating FCC lattices. The atoms of one of the FCC lattices are highlighted in red. If the atoms in this crystal are all carbon, then this is a crystal called diamond. Other materials also have their atoms arranged in this form. These include elemental semiconductors like silicon (Si) and germanium (Ge) as well as compound semi-conductors like GaAs.
Crystal Structures of the Elements Here are the crystal structures taken by the chemical elements. hex = hexagonal hcp = hexagonal close-packed diamond = diamond lattice sc = simple cubic bcc = body-centered cubic fcc = face-centered cubic
Energy Levels of Atoms Electrons in atoms are only allowed to have certain, distinct orbits due to the wavenature of the electrons. This idea leads to the Bohr model of the atom, resembling a miniature Solar System. Here the electrons are simply viewed as particles. Here are the electrons in the various allowed orbits. Note that some orbits are occupied by electrons, but others are empty. Instead of looking at the orbits, we can simply plot the energies of each orbit in an energy level diagram. For E > 0, all energies are possible; this is called the energy continuum. The electrons are free from the atom for E > 0. E > 0 E = 0 E < 0 The actual electron wavelike orbits look like this. E2 For E < 0, the electrons are bound to the atom. Only these energies are allowed for the electrons, no others; these energies are quantized. E1 E0 This is the energy situation for a single atom. What happens if you start to build a larger piece of matter by bringing more atoms close together?
Building Solids Using Atoms Individual atoms have discrete energy levels. As you build a crystal by putting atoms in the lattice and move them closer together, they begin to interact with each other (since they exhibit wave-like behavior). Energy bands & forbidden band gaps E = 0 upper band Discrete atomic energy levels of a single atom forbidden gap lower band of allowed energies atomic spacing As the e- interact, their energy levels broaden to form wide “bands” of allowed energy. These wide energy bands are separated from one another by forbidden energy gaps. These energy bands show the allowed energies in a solid material (crystal or other).
Energy Bands in Solids The behavior of a solid depends on how the available energy bands are filled with electrons. This is called electronoccupancy. Here is a possible energy band diagram (instead of an energy level diagram). The shaded area in a band indicates regions filled by electrons. upper band Empty area indicates unoccupied energies (like empty orbits). forbidden energy range — no available states for e-; energy gap Energy completely filled band (no energies left for e-) middle band maximum band energy Emax The height of the band gives the range of available energies for the electrons. minimum band energy Emin lower band
Materials Properties Determined by Bands For example, electrical behavior is determined by the way in which bands are filled with electrons. Metal—a material that is a good conductor of electricity; copper, gold. It has a full lower band (valence band) and a half-full upper band (conduction band). There are lots of available energies for electrons to move in the conduction band. Insulator—a material which does not conduct electricity; glass, wood, some plastics. Its bands are either completely full or completely empty. There are no empty energies for the electrons to move (no electricity). Semiconductor—a material that may conduct electricity, but it’s a poorer conductor than a metal; silicon, GaAs. It has an upper band that is either mostly empty or mostly full. Some energies remain for it to conduct electrons. The energy range between the valence and conduction bands is called the energy gap or bandgap. There are no allowed energies in the gap. conduction band mostly empty conduction band empty half full bandgap Eg valence band nearly full valence band Energy full full Eg helps to determine many properties of the solid like electrical, optical, etc… full full full Different materials have different Eg (larger or smaller or zero). insulator semiconductor metal
Simple Energy Bands vs Real Energy Bands Recall that we had a simple picture of the allowed electron energies in a semiconductor. This plot of allowed energies was called an energy band diagram. The realistic band diagram for silicon is more complicated. It looks like this. allowed e– energies conduction band mostly empty conduction band We’re going to concentrate on this region—the top of the valence band (VB), the bandgap Eg, & the bottom of the conduction band (CB). energy bandgap Eg forbidden energies no allowed energies allowed e–energies bandgap Eg valence band nearly full Energy This is where the action is for a device. valence band allowed e– energies full semiconductor band diagram different directions in the 3d Si crystal
Other Semiconductors Recall the Periodic Table of the elements and how electrical behavior is distributed around the table. The semiconductors are distributed in & around this region. VI V IV III C B O N Al Si P S II Ge Se Ga As Zn Te In Sn Sb Cd Hg Pb
Semiconductor Property Table Here’s a partial list of the different kinds of semiconductor materials that can be made using the chemical elements in the Periodic Table. 5.43 elemental semiconductors (group IV) Si / GaAs ~ 4% mismatch (too much) 5.65 III-V binary semiconductors compound semiconductors GaAs/AlAs ~ 0.2% mismatch (matched) 5.66 III-V ternary semiconductors wide bandgap II-VI binary semiconductors semimetal II-VI ternary semiconductors narrow bandgap If you want to use 2 or more materials together (to take advantage of their differing properties), the lattice constants need to be very close (lattice-matched) to avoid crystalline defects (which degrade performance).
Controlling the Si Electrical Resistivity name Sym Z Silicon is a member of the Group IV column of elements. In the periodic table, this means that it (along with all the other elements in the column) has 4 valence electrons in its outermost electron shell. These are shared to form chemical bonds between Si and neighboring atoms. I II III IV V VI VII VIII Legend n-type dopants (Group V) p-type dopants (Group III) Lanthanides Actinides
Doping of the Substrate To control the electrical behavior of Si, impurity atoms are deliberately added to the crystal. Doping is the method by which these impurities are put into the substrate. Recall the Si crystal structure. In 2-dimensions, it looks like the picture. Silicon, from group IV in the periodic table, bonds with its 4 nearest-neighbors to share a pair of electrons with each neighbor. If an atom from group V (5 electrons in the outer orbit) was put in the lattice in place of a Si atom, four of its electrons would bond with the neighbors, leaving the 5th electron unbonded & free. If a lot of these atoms were added to the crystal, there would be a lot of extra unbonded electrons. This would form n-type Si. If an atom from group III (3 electrons in the outer orbit) was put in the lattice in place of a Si atom, all of its 3 electrons would bond with the neighbors. But this would leave one Si electron unpaired, giving a missing electron or hole. This hole behaves like a positively-charged particle and is free to move throughout the crystal. If a lot of these atoms were added to the crystal, there would be a lot of holes. This would form p-type Si. extra electron missing electron (or hole) Atoms that do this in Si come from group III. The most common one is B. Atoms that do this in Si come from group V. They include As, P, Sb.
Si Resistivity vs Dopant Concentration As the dopant concentration is varied in a Si crystal, the electrical resistivity varies inversely. The greater the impurity concentration, the smaller the resistivity becomes. For comparison, the Si atomic density is ~ 5 1022 at/cm3.
Comparison between Si Energy Band & Other Semiconductors First a closer look at the Si band diagram. Finally compare the GaAs diagram to Si. Next compare the Ge diagram to Si. Ge bands GaAs bands Si bands Note that Ge is also an indirect gap semicon-ductor. It has a smaller Eg than Si. The fact that these 2 points don’t line up vertically means that Si is an indirectgap semiconductor. GaAs has a larger Eg than Si. It is also a direct gap material. 5 4 3 This means that Si is a very inefficient producer of light. 2 highest point of valence band 1 lowest point of conduction band GaAs is an efficient light source with LED applications 0 1 For this reason, there aren’t any Si light-emitting diode (LED) devices in consumer electronics. 2 3 LEDs are all made from other materials which are efficient producers of light. Also, note that the curvature of the bands is different. This changes how fast electrons & holes can move through Ge compared to Si. Also, the bands are curved differently, making GaAs a faster conductor than Si. But they aren’t lattice-matched
Energy Bandgap versus Lattice Constant A better way to look for compatible materials is to plot bandgap versus lattice constant. ZnS Smaller Bandgap Materials Larger Bandgap Materials HgTe
Still another way to view the nature of the gaps—including direct/indirect gap info for alloys.
Si O Silicon Dioxide Silicon dioxide, SiO2, is a dielectric material that is used as an insulator in microelectronic devices. It is also employed as a sacrificial material in some versions of MEMS surface micromachining. In crystalline form, SiO2 forms into a hexagonal lattice as shown. When in macroscopic form, this is called quartz. The SiO2 lattice doesn’t match the Si lattice either in crystal form or in lattice constant. When SiO2 is grown thermally on a Si crystal, the resulting oxide film is amorphous, called a continuous random network (CRN) as shown below. single-crystal SiO2 lattice (shown as a 2d lattice) amorphous SiO2 film on Si (CRN)
TEM Cross-section of Device Structure Shown in the picture is a cross-sectional electron microscope photograph of all three types of atomic structures (single crystalline, polycrystalline, & amorphous) polysilicon polycrystalline layer amorphous layer crystalline layer silicon dioxide (100) silicon substrate Motorola APRDL
a Other Insulating Materials Wide bandgap epitaxial semiconductors as advanced insulators: The lattice parameter measures how far apart the atoms are in the crystal structure. If a film is deposited epitaxially, then its atoms line up with the atoms in the substrate. Materials with a large Eg (wide bandgap) can be used as insulators with Si. wide bandgap semiconductors ZnS is an epitaxial wide bandgap material that is lattice-matched to Si. There are others.
Other Insulating Materials… (continued) Lattice Matching If you deposit a wide bandgap film that isn’t lattice-matched to Si, then the resulting insulating layer is not crystalline. It will have lots of crystal defects. If you deposit a lattice-matched wide bandgap film on the Si, its atoms will line up with the underlying substrate. This will be a much better insulating layer (depending on the size of Eg). This insulating film will not be the best insulator. 2nd layer of crystalline Si on top of the epi-insulator wide bandgap semiconductor film without lattice matching wide bandgap epitaxial semiconductor lattice-matched to Si epitaxial insulator With an epitaxial insulator, now it may be possible to deposit a new layer of crystalline Si on top of the insulator. This new Si layer can be used to fabricate an entirely new layer of devices on top of the 1st layer of devices (in the Si substrate). This process would enable multiple layers of devices to be stacked, greatly increasing the device density & complexity.
Metals in Device Fabrication There are a number of desirable properties of metals in microelectronic and MEMS device applications: • High conductivity • Ease of deposition (such as a reliable CVD process) • Stability on insulating films, no diffusion • Chemical compatibility with other materials (in adjacent layers) • Good thermal stability to survive subsequent processing • No (or little) native oxide formation • Good mechanical stability, stress tolerance • Corrosion resistance • Ease of plasma etching (reliable plasma etch process) • Good stable grain structure • Good adhesion to underlying films
Metal Reliability in Devices—Electromigration While metals are required in devices for their high conductivity, they can present reliability and lifetime issues. One of these is electromigration, in which a conductor will degrade over a period of operation until it fails. When deposited under a variety of conditions, metal films form a grain structure as shown. The grain structure can be controlled to a certain extent by the deposition control parameters. metal film with grain structure substrate As the current flows, certain criticial grains will begin to lose material. The material travels in the direction of the “electron wind” and is redeposited somewhere downstream. Different grains will have their own sizes, shapes, and resistivities. Over time as that location continues to lose material, eventually the metal wire feature develops a complete break, opening the circuit, and causing a device failure. During operation, electrical current flows through the metal. wire at start premature wire failure electromigration begins
Electromigration Example This result used either Al:Cu or Cu interconnects with current densities up to ~ 107 A / cm2. current flow time from start of current flow G. Schneider, M.A. Meyer, G. Denbeaux, et al., J. Vac. Sci. Tech. B 20, 3089 (2002)
Other Material Effects In addition to the standard material properties, other effects play a part in the behavior of materials. These effects have been of lesser importance in the fabrication of devices until now. However as device requirements become more precise (as seen with epitaxial insulators) and as device sizes are reduced, these effects become more significant. One such effect is the behavior of crystalline surfaces, of increasing importance in multilevel device architectures. This effect arises due to the rearrangement of atoms at the crystal surface. It is caused by the difference between atoms in the crystal interior in comparison to the surface atoms. The effect is called a surface reconstruction.
Atoms in the Interior of a Solid Consider a simple square crystal. Each individual atom is bound to its neighbors by the chemical bond. We can visualize these as forces (provided by springs) which pull on an atom in different directions. These forces hold each atom in its place in the crystal F F F F Each atom feels these forces from its neighbors & is held in place by them.
What Happens at a Surface? From an atom’s point of view, being at a surface is quite different from being in the interior of the solid. Take the same crystal and make a new surface by breaking it in half. Now, the top layer of atoms (the new surface layer) has no upward forces holding it in place. So the top layer moves downward, closer to the lower neighbor layer. break here
Reconstruction of the Crystal Structure at a Surface Making this new surface has completely altered the square crystal structure at the surface layer. This is due to the unbalanced forces that happen when the surface was formed. Now that the top layer has moved closer to the lower plane, the surface atoms will also rearrange themselves in the surface plane. This further distorts the original crystal structure. So the surface crystal structure is dramatically altered from the structure that the interior atoms use. This distortion of the crystal structure at a surface is called a surfacereconstruction. Recall: the crystal structure is a major factor in defining the energy bands & bandgaps for a material. The energy bands and bandgaps Eg determine the properties of the crystal.
Examples of Surface Reconstruction—Gold The crystal structure of bulk gold is face-centered cubic (FCC) (check the periodic table with crystal structures from last time). If you were sitting on one of the cube faces (called the 100 faces), the gold atoms would be arranged like this. Wikipedia Instead, the gold surface changes. The surface atoms form long columns that are several atoms wide with deep trenches between the columns. This is a reconstructed gold surface.
Examples of Surface Reconstruction—Silicon Silicon forms in a diamond crystal structure (see last discussion). From the 111 direction, the silicon atoms appear to form into triangles (or parallelograms). Northwestern U This is the Si 111 reconstruction, called the 7 × 7 reconstruction. Note the distortions from the bulk structure.
Examples of Surface Reconstruction—Silicon Looking from the 100 direction in the Si diamond lattice, the atoms are arranged in a square pattern. The reconstructed Si 100 surface looks like the STM image. U Wisc U Mich U Ariz This is called the 2 × 1 reconstruction due to the pairs of Si atoms that have formed (called dimers). Yet another view showing the reconstructed surface with a step edge and defects. Also, the white spheres are buckyballs (C60). Another view of the 2 × 1 surface showing the dimers, an atomic step edge and some buckling at the edge.
Examples of Materials Use in Devices Surface topography due to conventional LOCOS Original IBM Cu Damascene process 5-level metal process, 120 nm, STMicrotech
MEMS Materials used in Surface Micromachining In surface micromachining, MEMS devices are fabricated using both a structural material and a sacrificial material to act as a scaffolding on which to place the device structures. There are several forms of this technique. Each uses different structural / sacrificial materials choices with different capabilities. Some have demonstrated commercial products (TI DLP) The precise selection of materials, both structural & sacrificial, depends upon the application. It also depends upon the entirety of properties of the candidate material including mechanical properties (stress, Young’s modulus, hardness), thermal budgets, available material processing capabilities, etc…
Typical MEMS Devices by Surface Micromachining electrostatic micromotor (MEMS Exchange) cantilevers (Ga Tech) structural material: poly Si sacrificial material: SiO2 chain microgear (Sandia)
Sandia SUMMiT V Process—Surface Micromachining The Sandia SUMMiT V Process uses a specific set of fabrication processes to make MEMS devices by surface micromachining. The structural material is poly silicon deposited by LPCVD. The sacrificial material is SiO2, also deposited by LPCVD. Other parts of the process sequence include plasma etches (RIE) for small parts of the devices and a wet etch for certain parts (hubs). The final release step is a wet etch using HF acid. optical switch (Sandia) ratcheted microgear (Sandia)