Silicon Crystal Structure And Growth

Silicon Crystal Structure and Growth (Plummer - Chapter 3)

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Atomic Order of a Crystal Structure

Figure 4.2

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Amorphous Atomic Structure

Figure 4.3

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Unit Cell in 3-D Structure

Unit cell

Figure 4.4

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Miller Indices of Crystal Planes

Z

Z

Z

Y X

Y X

(100)

Y X

(110)

Figure 4.9

(111)

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Silicon Crystal Structure

Crystals are characterized by a unit cell which repeats in the x, y, z directions.

• Planes and directions are defined using x, y, z coordinates. • [111] direction is defined by a vector of 1 unit in x, y and z. • Planes defined by “Miller indices” – Their normal direction (reciprocals of intercepts of plane with the x, y and z axes). EE-452

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Silicon has the basic diamond crystal structure – two merged FCC cells offset by a/4 in x, y and z. EE-452

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Faced-centered Cubic (FCC) Unit Cell

Figure 4.5

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Silicon Unit Cell: FCC Diamond Structure

Figure 4.6

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Basic FCC Cell

Merged FCC Cells

Omitting atoms outside Cell

Bonding of Atoms

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(Extra line of atoms)

Various types of defects can exist in a crystal (or can be created by processing steps). In general, these cause electrical leakage and are result in poorer devices. EE-452 13

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Point Defects

Vacancy defect

Interstitial defect

Frenkel defect EE-452

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Semiconductor-Grade Silicon

Steps Step

De EE-452

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Czochralski (CZ) crystal growing • Si is purified from SiO2 (sand) by refining, distillation and CVD. • It contains < 1 ppb impurities. Pulled crystals contain O (~1018 cm-3 ) and C (~1016 cm-3 ), plus dopants placed in the melt. EE-452

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CZ Crystal Puller

Crystal puller and rotation mechanism

Crystal seed

Single crystal silicon

Molten polysilicon

Quartz crucible

Heat shield

Carbon heating element

Water jacket Figure 4.10

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• All Si wafers come from “Czochralski” grown crystals. • Polysilicon is melted, then held just below 1417 °C, and a single crystal seed starts the growth. • Pull rate, melt temperature and rotation rate control the growth EE-452

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Silicon Ingot Grown by CZ Method

Photograph courtesy of Kayex Corp., 300 mm Si ingot Photo 4.1

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An alternative process is the “Float Zone” process which can be used for refining or single crystal growth.

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• In the float zone process, dopants and other impurities are rejected by the regrowing silicon crystal. Impurities tend to stay in the liquid and refining can be accomplished, especially with multiple passes. (See the Plummer for models of this EE-452

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Float Zone Crystal Growth Gas inlet (inert) Chuck Polycrystalline rod (silicon)

Molten zone Traveling RF coil

RF

Seed crystal

Chuck

Inert gas out Figure 4.11

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Dopant Concentration Nomenclature

Table 4.2

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Segregation Fraction for FZ Refining

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Basic Process Steps for Wafer Preparation

Crystal Crystal Growth Growth

Wafer Wafer Lapping Lapping and and Edge Edge Grind Grind

Cleaning Cleaning

Shaping Shaping

Etching Etching

Inspection Inspection

Wafer Wafer Slicing Slicing

Polishing Polishing

Packaging Packaging

Figure 4.19

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Ingot Diameter Grind Preparing crystal ingot for grinding

Diameter grind

Flat grind

Figure 4.20

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Internal Diameter Saw

Internal diameter wafer saw

Figure 4.23

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After crystal pulling, the boule is shaped and cut into wafers which are then polished on one side.

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Wafer Notch and Laser Scribe

1234567890

Notch

Scribed identification number

Figure 4.22

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Polished Wafer Edge

Figure 4.24

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Chemical Etch of Wafer Surface to Remove Sawing Damage

Figure 4.25

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Wafer Dimensions & Attributes

Diameter (mm) Table 4.3

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Increase in Number of Chips on Larger Wafer Diameters (Assume large 1.5 x 1.5 cm microprocessors)

88 die 200-mm wafer

232 die 300-mm wafer Figure 4.13

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Developmental Specifications for 300mm Wafer Dimensions and Orientation Parameter

Units

Nominal

Some Typical Tolerances

Diameter

mm

300.00

±0.20

Thickness (center point)

µm

775

±25

Warp (max)

µm

100

Nine-Point Thickness Variation (max)

µm

10

Notch Depth

mm

1.00

+ 0.25, -0.00

Notch Angle

Degree

90

+5, -1

Back Surface Finish

Bright Etched/Polished

Edge Profile Surface Finish

Polished

FQA (Fixed Quality Area – radius permitted on the wafer surface)

mm

147

From H. Huff, R. Foodall, R. Nilson, and S. Griffiths, “Thermal Processing Issues for 300-mm Silicon Wafers: Challenges and Opportunities,” ULSI Science and Technology (New Jersey: The Electrochemical Society, 1997), p. 139.

Table 4.4

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Wafer Polishing

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Double-Sided Wafer Polish

Upper polishing pad

Wafer Slurry Lower polishing pad

Figure 4.26

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Improving Silicon Wafer Requirements 1995 (0.35 µm) Wafer diameter (mm) Site flatnessA (µm) Site size (mm x mm) MicroroughnessB of front surface (RMS)C (nm) Oxygen content (ppm)D Bulk microdefectsE (defects/cm2) Particles per unit area (#/cm2) EpilayerF thickness (±% uniformity) (µm)

Year (Critical Dimension) 1998 2000 (0.25 µm) (0.18 µm)

2004 (0.13 µm)

200

200

300

300

0.23 (22 x 22)

0.17 (26 x 32)

0.12 26 x 32

0.08 26 x 36

0.2

0.15

0.1

0.1

≤24 ±2

≤23 ±2

≤23 ±1.5

≤22 ±1.5

≤5000

≤1000

≤500

≤100

0.17

0.13

0.075

0.055

3.0 (±5%)

2.0 (±3%)

1.4 (±2%)

1.0 (±2%)

Adapted from K. M. Kim, “Bigger and Better CZ Silicon Crystals,” Solid State Technology (November 1996), p. 71.

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Quality Measures • • • • • • •

Physical dimensions Flatness Microroughness Oxygen content Crystal defects Particles Bulk resistivity EE-452

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“Backside Gettering” to Purify Silicon Polished Surface

Backside Implant: Ar (50 keV, 1015 /cm2) The argon amorphizes the back side of the silicon. The wafer is heated to 550oC, which regrows the silicon. However, the argon can not be absorbed by the silicon crystal so it precipitates into micro-bubbles and prevents some damage from annealing. The wafer is held at 550oC for several hours, and all mobile metal contaminants are attracted to and then captured by the argon stabilized damage. Once captured, they never leave these sites. EE-444

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Chapter Review (Wafer Fabrication)

• Raw materials (SiO2) are refined to produce electronic grade silicon with a purity unmatched by any other available material on earth. • CZ crystal growth produces structurally perfect Si single crystals which are cut into wafers and polished. • Starting wafers contain only dopants, and trace amounts of contaminants O and C in measurable quantities. • Dopants can be incorporated during crystal growth • Point, line, and volume (1D, 2D, and 3D) defects can be present in crystals, particularly after high temperature processing. • Point defects are "fundamental" and their concentration depends on temperature (exponentially), on doping level and on other processes like ion implantation which can create non-equilibrium transient concentrations of these defects. EE-444

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Measurement of Wafer Characteristics Darkfield and Brightfield Detection Brightfield imaging

Darkfield imaging Viewing optics

Viewing optics Two-way mirror Light source

Lens

Light reflected by surface irregularities Lens

Figure 7.15

t gh i L

e c r sou

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Schematic of Optical System Phase and and intensity intensity Phase detection detection Photo detector array

Data generation, generation,processing, processing, Data display are are networked networked with with display factory management management software software factory Split mirror

Video camera Lens CRT Light source

Viewing optics

Objective lens assembly Three-axis piezo substage

Wafer positioning stage

Vibration isolation pad Figure 7.16

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Principle of Confocal Microscopy Detector Pinhole

Beam splitter

Laser

Pinhole Objective lens Wafer is driven up and down along Z-axis Center of focus

+Z 0 -Z

Figure 7.17

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Particle Detection by Light Scattering

Photo detector

Detection of scattered light

Reflected light

Incident light Beam scanning

Wafer motion

Particle

Scattered light

Figure 7.18

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Measurement of Wafer Characteristics

Hot Point Probe

The hot point probe is a simple and reliable means to determine whether a wafer is N or P type is the Hot Point Probe. The basic operation of this probe is illustrated in the next slide. Two probes make ohmic contact with the wafer surface. One is heated 25-100°C hotter than the other. A voltmeter placed across the probes will measure a potential difference whose polarity indicates whether the material is N or P type. EE-452

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Basic principle of the hot probe, illustrated for an N-type sample, for determining N- or P-type behavior in semiconductors. EE-452

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Hot Point Probe

Consider an N-type sample. The majority carriers are electrons. At the hot probe, the thermal energy of the electrons is higher than at the cold probe so the electrons will tend to diffuse away from the hot probe, driven by the temperature gradient. If a wire were connected between the hot and cold probes, this would result in a measurable current, whose direction would correspond to the electrons moving right to left. (The current by definition would be in the opposite direction.) If we place a high-impedance voltmeter between the probes, no current flows, but a potential difference is measured, as illustrated. As the electrons diffuse away from the hot probe, they leave behind the positively charged, immobile donor atoms that provided the electrons. The negatively charged mobile electrons tend to build up near the cold probe. This results in the hot probe becoming positive with respect to the cold probe. By a similar set of arguments, if the material were P type, positively charged holes would be the majority carriers and the polarity of the induced voltage would be reversed. The direction of the current between the two probes would also be reversed in P-type material, if they were shorted with a wire. Thus a measurement of either the short-circuit current or the open circuit voltage tells us the type of the material. EE-452

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“Four-point probe” measurement method. The outer two probes force a current through the sample; the inner two probes measure the voltage drop. EE-452

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Measurement of Sheet Resistance • The most common method of measuring the wafer resistivity is with the four-point probe. We measure the sample resistance by measuring the current that flows for a given applied voltage. This could be done with just two probes. However, in that case, contact resistances associated with the probes and current spreading problems around the probes are important and are not easily accounted for in the analysis. Using four probes allows us to force the current through the two outer probes, where there will still be contact resistance and current spreading problems, but we measure the voltage drop with the two inner probes using a high-impedance voltmeter. Problems with probe contacts are thus eliminated in the voltage measurement since no current flows through these contacts. EE-452

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Four Point Probe Constant current source

V ρ s= x 2π s (ohms-cm) I

R

I V

Voltmeter

Wafer Figure 7.3

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“Van der Pauw” Sheet Resistivity

(similar to 4-point probe, but uses shapes on wafer) I

(b) V (a)

Contact Conductive material

(c)

(d) Figure 7.4

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Hall Effect Measurements The Hall effect was discovered more than 100 years ago when Hall observed a transverse voltage across a conductor subjected to a magnetic field. The technique is more powerful than the sheet resistance method described above because it can determine the material type, carrier concentration and carrier mobility separately. The basic method is illustrated in the next slide. The left part of the figure defines the reference directions and the various currents, fields and voltages; the right part of the figure illustrates a top view of a practical geometry that is often used in semiconductor applications. EE-452

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Conceptual representation of Hall effect measurement. The right sketch is a top view of a more practical implementation.

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Fourier Transform Infrared Spectroscopy (FTIR)

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FTIR (Oxygen and Carbon Detection)

The CZ crystal growth process introduces oxygen and carbon into the silicon. These elements are not inert in the crystal. It is important is to be able to measure them and to control them. The method is Fourier Transform Infrared Spectroscopy. FTIR measures the absorption of infrared energy by the molecules in a sample. Many molecules have vibrational modes that absorb specific wavelengths when they are excited. By sweeping the wavelength of the incident energy and detecting which wavelengths are absorbed, a characteristic signature of the molecules present is obtained. Oxygen in CZ crystals is located in interstitial sites in the silicon lattice, bonded to two silicon atoms. Low concentrations of carbon are substitutional in silicon since carbon is located in the same column of the periodic table as silicon and easily replaces a silicon atom. Oxygen exhibits a vibrational mode that absorbs energy at 1106 cm-1 (wavenumber), that is at a wavelength of about 9 microns; carbon absorbs energy at 607 cm-1 .There are other wavelengths of IR light that are absorbed by the silicon atoms themselves. By measuring the absorption of a particular wafer at 1106 or 607 cm-1 , and comparing this absorption with an oxygen or carbon free reference, the FTIR technique can be made quantitative. An IR beam is split by a partially reflecting mirror and then follows two separate paths to the sample and the detector. For pure silicon, if the movable mirror is translated back and forth at constant speed, the detected signal will be sinusoidal as the two beams go in and out of phase. The Fourier transform of this signal will simply be a delta function proportional to the incident intensity. If the frequency of the source is swept, the Fourier transform of the resulting signal will produce an intensity spectrum. If we now insert the sample, the resulting intensity spectrum will change because of absorption of specific wavelengths by the sample. The benefit of using the Fourier transform method as opposed to simply directly measuring the intensity spectrum is simply that the signal to noise ratio is improved and as a result, the detection limit is reduced. With modern instruments, the detection limit for interstitial oxygen in silicon is about 2x1015 /cm3. Carbon can be detected down to about 5x1015 /cm3. Oxygen precipitated into small SiO2 clusters can be detected by FTIR because in the SiO2 form, the oxygen does not absorb at 1106 cm-1 . As the precipitation occurs, the IR absorption at this wavenumber decreases.

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Schematic of “TEM” Transmission Electron Microscope Electron gun Anode

Liquid N2 Dewar

Condenser lens X-ray detector Objective aperture Aperture

Sample stage

}

Lenses

Displayed sample image

CRT CCD video camera

Fluorescent screen

Detector

Energy-loss spectrometer

Wavelength of 1 MeV Electron ~ 1Angstrom EE-452

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Electron Microscopy (TEM) of SiO 2 on Si

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Oxygen Contamination in Silicon

Oxygen is the most important impurity found in silicon. It is incorporated in silicon during the CZ growth process as a result of dissolution of the quartz crucible in which the molten silicon is contained. The oxygen is typically at a level of about 1018 /cm3. It has recently become possible to use a magnetic field during CZ growth to control thermal convection currents in the melt. This slows down the transport of oxygen from the crucible walls to the growing silicon interface and reduces the oxygen concentration in the resulting crystal. Oxygen in silicon is always present at concentrations of ~10-20 ppm (5x1017 - 1018 /cm3) in CZ silicon. The oxygen can affect processes used in wafer fabrication such as impurity diffusion. Oxygen has three principal effects in the silicon crystal. (1) In an as-grown crystal, the oxygen is believed to be incorporated primarily as dispersed single atoms designated OI occupying interstitial positions in the silicon lattice, but covalently bonded to two silicon atoms. The oxygen atoms thus replace one of the normal Si-Si covalent bonds with a Si-O-Si structure. The oxygen atom is neutral in this configuration and can be detected with the FTIR method. Such interstitial oxygen atoms improve the yield strength of silicon by as much as 25%, making silicon wafers more robust in a manufacturing facility. (2) The formation of oxygen donors. A small amount of the oxygen in the crystal forms SiO4 complexes which act as donors. They can be detected by changes in the silicon resistivity corresponding to the free electrons donated by the oxygen complexes. As many as 1016 /cm3 donors can be formed, which is sufficient to significantly increase the resistivity of lightly doped P-type wafers. During the CZ growth process, the crystal cools slowly through ~500oC temperature and oxygen donors form. The SiO4 complexes are unstable at temperatures above 500°C and so usually wafer manufacturers anneal the grown crystal or the wafers themselves after sawing and polishing, to remove the oxygen complexes. These donors can reform, however, during normal IC manufacturing, if a thermal step around 400-500°C is used. Such steps are not uncommon, particularly at the end of a process flow. (3) The tendency of the oxygen to precipitate under normal device processing conditions, forming SiO2 regions inside the wafer. The precipitation arises because the oxygen was incorporated at the melt temperature and is therefore supersaturated in the silicon at process temperatures. EE-452

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Carbon Contamination in Silicon

Carbon is normally present in CZ grown silicon crystals at concentrations on the order of 1016/cm3.The carbon comes from the graphite components in the crystal pulling machine. The melt contains silicon and modest concentrations of oxygen. This results in the formation of SiO that evaporates from the melt surface. Generally, the ambient in the crystal puller is Ar flowing at reduced pressure, and the SiO can be transported in the gas phase to the graphite crucible and other support fixtures. SiO reacts with graphite (carbon) to produce CO that again transports through the gas phase back to the melt. From the melt, the carbon is incorporated into the growing crystal. Four Effects of Carbon on Silicon (1) Carbon is mostly substitutional in the silicon lattice. Since it is a column IV element, it does not act as a donor or acceptor in silicon. Carbon is known to affect the precipitation kinetics of oxygen in silicon. This is likely because there is a volume expansion when oxygen precipitates and a volume contraction when carbon precipitates because of the relative sizes of O and C. There is thus a tendency for precipitates that are complexes of C and O to form at minimum stresses in the crystal. Since precipitated SiO2 is crucial in intrinsic gettering, this can have an effect on gettering efficiency. (2) Carbon is also known to interact with point defects in silicon. Silicon interstitials tend to displace carbon atoms from lattice sites, presumably because this can help to compensate the volume contraction present when there is carbon in the crystal. (3) Thermal donors (Oxygen Effects) normally form around 450°C. There is also evidence that if C is present at ~1 ppm, these donors may also form at higher temperatures (650-1000°C). (4) Higher concentrations of C to Si (levels of a few percent) can change the bandgap of the silicon and may allow the fabrication of new types of semiconductor devices in the future. EE-452

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Chapter Review (Wafer Metrology) • • • • • • • •

Microscopic examination for particulates. Hot Point Probe (wafer doping) Four Point Probe (wafer resistivity) Hall Effect (carrier mobility) FBIR (oxygen and carbon detection) TEM (atomic resolution of defects / surface) Effects of Oxygen on IC fabrication Effects of Carbon on IC fabrication EE-452

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