Appendix A Fabrication Technology for Miniaturization

INTRODUCTION visible light or ultraviolet (UV) radiation —expos- ing selected areas of the material below. The ex- posed areas can then be modified in some way. Many of the technologies that have enabled Successive applications of this basic process pro- advances in miniaturization were first developed for microelectronics and allow both lateral and duce a multilevel structure with millions of indi- vidual transistors in a square centimeter as de- thickness control in the creation of structures. scribed in figure A-1. Although the early techniques and tools were directed at silicon, recent years have seen in- Photolithography is the most widely used form creased attention to materials other than silicon: of lithography and is likely to continue to be so for compound semiconductors, superconductors, the near future. UV light, usually with a wave- metals, and insulators. Furthermore, they are be- length less than 400 nanometers, is used to create ing applied to more diverse areas: micromechani- patterns resist layers on flat surfaces. Diffraction cal structures, biosensors, and chemical sensors. of light—the interference of light waves with one another—limits the resolution or minimum re- The same processes that have allowed for de- producible feature size. Diffraction is minimized creases in size also allow parallel production of by reducing the wavelength of the exposing radi- many devices. Thousands or millions of transis- ation. Mercury vapor lamps are routinely used as tors or other devices can refashioned simulta- a source of 436 nanometer (G-line) and 365 nano- neously on one chip the size of a thumbnail, and meter (I-line) light. Shorter wavelength excimer many chips preprocessed at the same time on one laser sources are a new source of UV light with wafer. The key steps in the process of creating operation possible at 248 nanometers and in the these structures are: future 193 nanometers and other wavelengths. By ● lithography, combining shorter wavelengths with improved mask technology (including phase shifted ● pattern transfer, and masks2), better optics, and more sophisticated ● characterization. resist chemistries, it is likely that photolithogra- phy will be usable to 0.25-microns minimum fea- LITHOGRAPHY ture sizes and possibly below 0.2 microns. The longevity of UV lithography is a subject of Integrated circuit fabrication techniques all debate in the semiconductor industry. Many ex- rely on lithography, an ancient technique first perts argue the limits of diffraction and depth of used for artistic endeavors. The basic process focus will prevent use of UV photolithography involves covering an object with a thin layer of below 0.2 microns and that x-ray lithography, material (ancient artisans sometimes used wax) which uses substantially shorter wavelength radi- that can be patterned but will resist subsequent ation, will be the successor to UV lithography. processing and protect the material underneath. X-ray lithography is a candidate for high volume Industrial lithographers use a hydrocarbon poly- production of integrated circuits with line widths mer, appropriately called “resist.” A pattern is below 0.5 microns. The most studied and devel- produced in the resist—usually by exposure to oped form of x-ray use is “proximity printing,”

IH.G. Craighead and M. Skvarla, “Micro and Nanofabrication lkchnology, Applications & Impact,” contractor repofi prepared for the OffIce of lkchnology Assessment, April 1990; and Tlm Studt, “Thin Films Get Thinner as Research Heats Up,’’ R&D Magazine, March 1990, pp.70-80. Zphase shift masks function by Careful& controlling light diffraction, using the constructive and destructive interference to help create the circuit pattern. Phase shift masks hold greatest promise for manufacturing memory and other ICS with regular, repeated patterns.

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36 ● Miniaturization Technologies

Figure A-1 -Fabrication Sequence for a Metal-Oxide-Semiconductor (MOS) Circuit

Silicon nitride Silicon dioxide First polysilicon

1

p-Type silicon

Insulating oxide film Contact “window”

Aluminum

Fabricating this MOS circuit element (a two-level n-channel-negative charge carrier- polysilicon-gate metal-oxide-semiconductor) re- quires six masking steps. The first few process steps involve the selective oxidation of silicon with the aid of a film of silicon nitride, which acts as the oxidation mask. A thin film of silicon dioxide is grown over the entire wafer, and a Iayer of silicon nitride is deposited from a chemical (l). The layer is selectively removed in a conventional photolithographic step in accordance with the pattern on the first mask (2). A p-type dopant (e.g., boron) is implanted using the silicon nitride as a mask, followed by an oxidation step, resulting in a thick layer of silicon dioxide in the unmasked-(3). The silicon nitride is then removed in selective etchant that does not attack either the silicon or silicon dioxide (4). Since silicon is consumed in the oxidation process, the thick oxide layer is partly recessed” into the silicon substrate. The first layer of poly- crystalline silicon is then deposited and patterned in the second masking step (5). A second insulating film of oxide is grown or deposited, followed by the deposition of the second polysilicon layer, which is in turn patterned in the third masking step (6). A short etch in the hydro- fluoric acid at this stage exposes certain regions to an implantation or diffusion of n-type dopant. A thin layer of silicon dioxide is deposited next, and contact “windows” are opened with the fourth mask (7). Finally a layer of aluminum is deposited and patterned in the fifth masking operation (8). The wafer will also receive a protective overcoating of silicon dioxide or silicon nitride (not shown); the fact that openings must be provided in this overcoating at the bonding pads accounts for 6 masking steps. Vertical dimensions are exaggerated for clarity. SOURCE: William G. ‘The Fabrication of Microelectronic Circuits,” (San Francisco, CA: Freeman & Co., 1977), p. 48. Copyright (c) 1977 by Scientific American, Inc.–George Kelvin. AppendixA-Fabrication Technology for Miniaturization .37 where the mask is placed on the substrate materi- laboratories, and in industry for mask-making al and its pattern is reproduced by exposure to and small manufacturing production runs. x-rays, resulting in a substrate pattern the same There are approaches still in research that may size as the mask. Practical problems that must be yield a more versatile lithography tool. A possible addressed include complexity of mask technolo- alternative is to use photolithography for large gies, difficulty in alignment of multiple layers, feature definition and reserve the scanned elec- economical x-ray sources, and resolution limita- tron beam for the critical dimensions. A varietyof tions. X-ray lithography operates similarly to UV approaches for parallel exposure, rather than photolithography, but with much shorter wave- serial scanned exposure, of electron-beams have lengths–around 5 nanometers. Proximity print- been studied for years. There is currently active ing requires a bright source of x-rays. Synchro- research in electron beams reduction projection tron sources are the highest intensity sources of using masks at AT&T Bell Laboratories, and in x-rays but they are very expensive. Most experts multiple source systems and proximity printing at feel it is unlikely that proximity printing with IBM. These approaches would exploit the resolu- x-rays will be practical below 0.2 microns, while tion and alignment possibilities of electron others hold that the technology will be usable to beams with the speed of parallel exposure tech- 0.1 microns. niques. More recent exploratory work in x-rays in- Ion beam lithography is in many ways similar volves “reduction projection lithography.” X-rays to electron beam lithography with beams of are focused through a mask that is kept away charged atoms (ions) taking the place of elec- from the substrate. The principal advantages of trons. Recent advances in ion sources have in- this approach are cheaper x-ray sources (a syn- creased the utility of scanned focused ion beams. chrotron is not required) and improved resolu- Compared to photons (x-rays and light) or elec- tion. In this country, AT&T Bell Laboratories trons, ions chemically react with the substrate, and other laboratories are investigating this tech- allowing a greater variety of modifications. Some nology. One of the greatest challenges to making scattering effects are also reduced compared to projection x-ray lithography useful for manufac- electrons and, as with electrons, diffraction limits turing is the optics used in the process; they are of ion beams are negligible. The fundamental complex, involving the creation of multiple-layer resolution limits for charged particle lithogra- films with precisely controlled thickness. phies are below 1(M) nanometers. Ion beam lithog- raphy suffers the same draw back as electron Another way to get beyond the diffraction limit beam systems; it relies on a serially scanned of radiation-based lithography is to use electrons beam. That limits its applications to products or atoms to expose the resist. Diffraction does not that do not require high throughput—e.g., masks limit the resolution of electron-beam lithography and small batches of electronics. Research efforts because the quantum mechanical wavelengths of are underway to develop parallel projection and high energy electrons are exceedingly small. Elec- multiple source ion beam lithographic equipment trons scatter quickly in solids, limiting practical that might overcome the limitations of serially resolution to dimensions greater than 10 nanome- scanned ion beam lithography. ters—significantly greater than current demands Scanned probe modification of surfaces, using of any practical technology. Electron-beam lith- recently developed scanning tunneling microsco- ography has demonstrated resolution as small as py (STM) methods, is being done by researchers 2 nanometers (0.02 microns) in a few materials. in many research laboratories. Since STMs can Electron beam technology, however, is limited in manipulate individual atoms, such approaches usefulness because an electron beam must be have theoretical resolution limits of single atoms. scanned across the entire wafer. Electron-beam This approach is still in early phases of research lithography tools are in use in universities, in and is focusing on basic physics measurements 38• Miniaturization Technologies and better understanding of the behavior of sur- ● sputtering, faces. The lack of stability of STM tips and slow . laser ablation deposition, speed, however, make it far from a practical tech- nology for manufacturing in the foreseeable fu- ● chemical vapor deposition; and ture. ● molecular beam epitaxy such as organo- metallic chemical vapor deposition. PATTERN TRANSFER Spin-on deposition is the simplest deposition technique. It involves spinning the sample while a liquid is poured onto it; the spinning action dis- The various lithography systems produce relief tributes the liquid evenly. Subsequent heating patterns in resists, and a subsequent pattern bakes the liquid into a solid thin film. transfer step is necessary to fabricate the struc- ture. There are a number of options available and Thermal evaporators use a heated filament or each involves some engineering tradeoffs. Once an electron beam to vaporize material. The mate- the resist pattern is in place, and selected areas of rial passes through a vacuum to impinge on the the underlayer are exposed, there are three possi- sample, building up a film. Evaporators emit ma- ble next steps: 1) add to, 2) remove, or 3) modify terial from a point source, resulting in “shadow- the exposed areas. ing” and sometimes causing problems with very small structures. Adding to the exposed area involves either dep- osition or growth of a material on the exposed Sputter deposition systems erode atoms from a areas. Growth is different from deposition be- broad target that then travel to the sample. This cause it involves consumption of the surface (usu- results in better line width control and uniformi- ally combining with some introduced chemical) ty, especially with high aspect ratio (height/width) to create the new substance. The most common structures. Refinements of these processes, e.g., example is a silicon surface being consumed in in situ cleaning with an ion gun, or using electron the process of forming silicon dioxide. It involves cyclotron resonance to confine and densify a heating the silicon wafer in the presence of oxy- plasma, result in better adhesion, higher quality gen, causing the growth of silicon dioxide from films, and more versatility. Using lasers or par- the exposed silicon. A number of materials are ticle beams in conjunction with deposition pre- grown in this way, since the consumption of the sents the possibility of defining patterns on the surface results in excellent adhesion, better elec- sample in a single step. trical and mechanical properties, and improve- Laser ablation deposition uses intense laser ments in other materials properties. This specific radiation to erode a target and deposit the mate- reaction has been exhaustively investigated and rial onto a substrate. This technique is particular- perfected and now can be used to grow oxide ly useful in dealing with compounds of different films whose thickness, and even lateral dimen- elements —e.g., yttrium-barium-copper oxide su- sions, can be measured in atomic layers (a few perconductor films. angstroms). Chemical vapor deposition (CVD) deposits Deposition creates new layers of material on thin films by passing reactive gases over the sam- the exposed area. These techniques usually in- ple. The substrate is heated to accelerate deposi- volve evaporating or sputtering a piece of mater- tion. CVD is used extensively in the semiconduc- ial-the target-so its atoms or molecules fly off tor industry and has played an important role in and land on the sample. Deposition techniques past transistor miniaturization by making it pos- include: sible to deposit very thin films of silicon. Most CVD is performed in vacuum, but new tech- ● spin-on, niques allow operation without vacuums. Radio ● thermal evaporators, frequency (RF) or photon radiation can be used

Appendix A--Fabrication Technologyfor Miniaturization .39 to enhance the process and is known as plasma optical transitions in some materials, producing enhanced CVD (PECVD). lasers, detectors, and other optical elements. Materials modification processes are used Some deposition techniques are so precise that mainly to vary the electrical conductivity in the material can be built up literally atom by atom appropriate areas. In the past, dopants (atoms with the crystal structure of the new material that can either contribute or subtract electrons exactly matching that of the underlying layer. In from silicon atoms) were diffused into the sub- molecular beam epitaxy (MBE), the sample is strate thermally; now they are implanted as high placed in an ultra high vacuum in the path of energy ions (see figure A-2). Implantation offers streams of atoms from heated cells that contain the advantage of being able to place any ion at any targets of various types. These atomic streams depth in the sample, independent of the thermo- impinge on the surface, creating layers whose dynamics of diffusion and problems with solid structure is controlled by the crystal structure of volubility and precipitation. Ion implantation, the surface, the thermodynamics of the constitu- originally developed for high energy physics, is ents and the sample temperature. Organo-metal- now an indispensable part of semiconductor lic chemical vapor deposition (OMVPE, or some- manufacturing. Ion beams produce crystal dam- times MOCVD) relies on the flow of gases age in addition to the chemical or electronic effect (hydrides like arsine and phospine or organome- of the dopants. Since crystal damage reduces tallics like tri methyl gallium and tri methyl alu- electrical conductivity, this effect can be ex- minum) past samples placed in the stream. ploited to electrically isolate devices from one Again, the sample surface and thermodynamics another. Ion beams can implant enough material of the processes determine the compounds de- to actually form new materials--e.g., oxides and posited. Both MBE and OMVPE provide thick- nitrides-some of which show improved wear ness control within one atomic layer (a few ang- and strength characteristics. stroms) and are especially useful in creating compound semiconductors and exploiting quan- Subtractive processes —the removal of materi- tum effects and band-gap engineering. This al-are also vital to the field. Some of the steps structural control enables researchers to exploit are still done the same way they were centuries

Figure A-2–ion Implantation

Accelerated boron ions Polysilicon gate /

p-Type - silicon Thick oxide

Ion implantation is employed to place a precisely controlled amount of dopant (in this ease boron ions) below the gate oxide of a MOS transistor. By choosing a suitable acceleration voltage the ions can be made to just penetrate the gate oxide but not the thicker oxide (left). After the boron ions are implanted polycrystalline silicon is deposited and patterned to form the gate regions of the transistor. A thin layer of the oxide is then removed and the source and drain regions of the transistor are formed by the diffusion- of n an n-type impurity (right). SOURCE: William G. “The Fabrication of (San CA: Freeman Co., 1977), p. 50. Copyright (c) 1977 by -George V. Kelvin. 40 ● Miniaturization Technologies ago. The object, with its lithographically defined tigating new materials, since it can be exploited to areas exposed and the remainder protected by uncover ways to etch new and exotic materials. the “resist,” is dipped into a chemical bath and a reaction or dissolution is allowed to take place. Unlike ancient lithographs, modern semicon- Unfortunately, liquid chemicals usually have no ductors must undergo many processing steps. sense of direction and will dissolve at an equal The mask must remain intact until the substrate rate on all sides. As device geometries shrink, it etch is complete, without changing feature size or becomes important to control directionality. shape or surface characteristics (morphology). Most modern etching is done in vacuum without Multiple operations are often required because of contact with liquids. the processing limitations of materials. For exam- ple, to grow a thin layer of silicon dioxide at Ion milling uses the sputtering process to bom- 1,000°C, a resist is used to pattern a silicon ni- bard the substrate with accelerated ions. The tride layer which is then capable of surviving the sputtering process relies on a physical mecha- high temperature during the oxide growth. Multi- nism whereby energy is transferred to a surface ple layers are often processed in succession to by impinging ions. This energy is enough that better allow a straightedge to be carried through some atoms break free of the surface. This mech- a thick stack of material. anism erodes all materials, with small differences in rate between any two. Selectivity is poor, and any masking material (resist) will disappear as CHARACTERIZATION fast as the underlayer, limiting the (relief) depth. Characterization has become an indispensable Reactive ion etching (RIE) can be highly selec- tool in the art and science of fabrication. The tive because it is a chemical reaction between ions designer needs information about device per- (radicals) formed in a plasma and atoms on the formance, the experimenter needs to know com- sample surface, and can be highly selective. RIE position and concentration, and the process engi- has excellent lateral control because the electric neer must have immediate confirmation of the fields produced by the plasma cause ions to dif- electrical, mechanical, optical, or chemical prop- fuse directionally. Straight parallel trenches a mi- erties of a thin film. In fact characterization, or cron or more deep can be etched without affect- failure analysis, is in some cases as difficult and ing a rather fragile resist mask; minimum feature extensive as the original experiment. The most sizes of a few nanometers are possible. RIE is common questions addressed by characteriza- highly selective in its etching: oxygen etches hy- tion concern the shape and size of the structure. drocarbons (resists), fluorine compounds are Optical and electron microscopes provide this useful in working with silicon systems, and many information over size ranges from hundreds of materials are susceptible to chlorine etch, espe- microns to fractions of a nanometer (near-atomic cially the compound semiconductors like gallium dimensions). arsenide. A great deal of work is directed at dis- covering the etching characteristics of other com- Instruments also exist to measure a range of pounds. materials properties, including the thickness, re- fractive index, and electrical resistivity. While The combination of physical and chemical these techniques involve the measurement of thin etching is achieved in a process called chemically film properties over large distances, microchar- assisted ion beam etching (CAIBE). The com- acterization can determine composition or struc- bined effects of ion sputtering and chemical reac- ture on a size scale comparable to the mean free tion with reactive ions results in a faster etch rate path of an electron and some techniques have an than that due to either mechanism alone. CAIBE ultimate resolution on the order of a few atomic is of particular interest to research groups inves- diameters. Appendix A--Fabrication Technology for Miniaturization .41

There are a wide range of techniques usually IMMA – Ion microprobe mass analysis: identified by acronyms, and some of the more high resolution secondary ion useful are listed here: mass spectroscopy (SIMS). EELS – Electron energy loss spectrosco- py: identifies composition by SCANNED BEAM TECHNIQUES measuring energy loss of elec- trons during electron interaction. SEM – Scanning electron microscope: ED – Electron diffraction: measures creates image of a surface by crystal structure by diffraction of measuring the reflection from a electrons. beam of electrons. LEED – Low energy electron diffraction: TEM – Transmission electron micro- measures surface structure by scope: determines crystal struc- diffraction of electrons. ture by measuring transmission RHEED – Reflection high energy electron of electrons through a thin sam- diffraction: measures surface ple; has high resolution. structure; particularly useful for AES – Auger electron microscopy: iden- epitaxial film growth. tifies constituents in the surface layer by measuring energy of elec- SCANNED PROBE TECHNIQUES trons emitted due to “Auger tran- sitions.” STM – Scanning tunneling microscopy: RBS – Rutherford backscattering spec- creates image of surface using troscopy: identifies constituents electron tunneling between the by measuring the spectrum of surface and a sharp tip near the scattered ions off a sample. surface; very high resolution. SIMS – Secondary ion mass spectrosco- AFM – Atomic force microscopy: creates py: analyzes composition of mate- image of surface using repulsion rial removed by ion bombard- of electron charge distributions ment. between surface and a tip; near- atomic resolution is possible. XRF – X-ray fluorescence: identifies composition from photon emis- SxM – Other developing scanned probe sion due to x-rays. microcopies: create images of surface characteristics for mag- EDS (EDX)— Energy dispersive x-ray spectros- netic force, electrochemical mea- copy: identifies composition from surement, etc., using the principle dispersion of x-rays. of the STM.