Editing Crystal oscillator (section)

==Crystal structures and materials==

=== Quartz ===
[[File:Crystal Packages.jpg|thumb|right|Common package types for quartz crystal products]]
[[File:Quartz Brésil.jpg|thumb|right|Cluster of natural quartz crystals]]
[[File:Quartz synthese.jpg|thumb|right|A synthetic quartz crystal grown using [[hydrothermal synthesis]], about {{nowrap|19 cm}} long and weighing about {{nowrap|127 g}}]]
[[File:32768 Hz quartz crystal resonator.jpg|thumb|right|Tuning-fork crystal used in a modern quartz watch]]
[[File:Inside QuartzCrystal-SimpleType.jpg|thumb|right|Simple quartz crystal]]
[[File:InsideQuartzCrystal.jpg|right|thumb|Inside construction of an HC-49 package [[quartz crystal]]]]
[[File:Krystalové-výbrusy.jpg|thumb|right|Flexural and thickness-shear crystals]]
[[File:HC13 xtal-int.jpg|thumb|right|Internal construction of an HC-13 package 100kHz [[quartz crystal]]]]
The most common material for oscillator crystals is [[quartz]]. At the beginning of the technology, natural quartz crystals were used but now synthetic crystalline quartz grown by [[hydrothermal synthesis]] is predominant due to higher purity, lower cost and more convenient handling. One of the few remaining uses of natural crystals is for pressure transducers in deep wells. During [[World War II]] and for some time afterwards, natural quartz was considered a [[strategic material]] by the USA. Large crystals were imported from Brazil. Raw "lascas", the source material quartz for hydrothermal synthesis, are imported to USA or mined locally by Coleman Quartz. The average value of as-grown synthetic quartz in 1994 was {{nowrap|60 [[United States dollar|USD]]/kg.}}<ref>Gordon T. Austin, [http://minerals.usgs.gov/minerals/pubs/commodity/silica/810494.pdf Quartz Crystal]. minerals.usgs.gov</ref>

==== Types ====
Two types of quartz crystals exist: left-handed and right-handed. The two differ in their [[optical rotation]] but they are identical in other physical properties. Both left and right-handed crystals can be used for oscillators, if the cut angle is correct. In manufacture, right-handed quartz is generally used.<ref name=terms>[http://www.ndk.com/catalog/AN-SQC_GG_e.pdf Synthetic Quartz Crystal] Terms and Definitions</ref> The SiO<sub>4</sub> tetrahedrons form parallel helices; the direction of twist of the helix determines the left- or right-hand orientation. The helixes are aligned along the c-axis and merged, sharing atoms. The mass of the helixes forms a mesh of small and large channels parallel to the c-axis. The large ones are large enough to allow some mobility of smaller ions and molecules through the crystal.<ref>[http://www.quartzpage.de/gen_struct.html The Quartz Page: Quartz Structure]. Quartzpage.de (2010-10-23). Retrieved on 2012-06-21.</ref>

Quartz exists in several phases. At 573&nbsp;°C at 1 atmosphere (and at higher temperatures and higher pressures) the α-quartz undergoes [[quartz inversion]], transforms reversibly to β-quartz. The reverse process however is not entirely homogeneous and [[crystal twinning]] occurs. Care must be taken during manufacturing and processing to avoid phase transformation. Other phases, e.g. the higher-temperature phases [[tridymite]] and [[cristobalite]], are not significant for oscillators. All quartz oscillator crystals are the α-quartz type.

==== Quality ====
[[Infrared spectrophotometry]] is used as one of the methods for measuring the quality of the grown crystals. The [[wavenumber]]s 3585, 3500, and 3410&nbsp;cm<sup>−1</sup> are commonly used. The measured value is based on the [[absorption band]]s of the [[OH radical]] and the infrared Q value is calculated. The electronic grade crystals, grade C, have Q of 1.8 million or above; the premium grade B crystals have Q of 2.2 million, and special premium grade A crystals have Q of 3.0 million. The Q value is calculated only for the z region; crystals containing other regions can be adversely affected. Another quality indicator is the etch channel density; when the crystal is [[etching|etched]], tubular channels are created along linear defects. For processing involving etching, e.g. the wristwatch tuning fork crystals, low etch channel density is desirable. The etch channel density for swept quartz is about 10–100 and significantly more for unswept quartz. Presence of etch channels and etch pits degrades the resonator's Q and introduces nonlinearities.<ref name="patentstorm1">John R. Vig ''et al.'' ''Method of making miniature high frequency SC-cut quartz crystal resonators'' {{US patent|4554717}}, Issue date: November 26, 1985.</ref>

==== Production ====
{{See also|Crystal growth|}}
Quartz crystals can be grown for specific purposes.

Crystals for [[Crystal oscillator#Crystal cuts|AT-cut]] are the most common in mass production of oscillator materials; the shape and dimensions are optimized for high yield of the required [[Wafer (electronics)|wafers]]. High-purity quartz crystals are grown with especially low content of aluminium, alkali metal and other impurities and minimal defects; the low amount of alkali metals provides increased resistance to ionizing radiation. Crystals for wrist watches, for cutting the tuning fork 32768&nbsp;Hz crystals, are grown with very low etch channel density.

Crystals for [[Surface acoustic wave|SAW]] devices are grown as flat, with large X-size seed with low etch channel density.

Special high-Q crystals, for use in highly stable oscillators, are grown at constant slow speed and have constant low infrared absorption along the entire Z axis. Crystals can be grown as Y-bar, with a [[seed crystal]] in bar shape and elongated along the Y axis, or as Z-plate, grown from a plate seed with Y-axis direction length and X-axis width.<ref name=terms/> The region around the seed crystal contains a large number of crystal defects and should not be used for the wafers.

Crystals grow [[Anisotropy|anisotropically]]; the growth along the Z axis is up to 3 times faster than along the X axis. The growth direction and rate also influences the rate of uptake of impurities.<ref>[http://www.roditi.com/SingleCrystal/Quartz/Hydrothermal_Growth.html Quartz Hydrothermal Growth]. Roditi.com. Retrieved on 2010-02-08.</ref> Y-bar crystals, or Z-plate crystals with long Y axis, have four growth regions usually called +X, −X, Z, and S.<ref>{{cite journal|title=Defects in synthetic quartz and their effects on the vibrational characteristics |journal=Ferroelectrics|date=1982-05-01|bibcode=1982Fer....43...43I |last1=Iwasaki |first1=Fumiko |last2=Kurashige |first2=Masakazu |volume=43 |issue=1 |page=43 |doi=10.1080/00150198208202002 }}</ref> The distribution of impurities during growth is uneven; different growth areas contain different levels of contaminants. The Z regions are the purest, the small occasionally present S regions are less pure, the +X region is yet less pure, and the -X region has the highest level of impurities. The impurities have a negative impact on [[radiation hardness]], susceptibility to [[crystal twinning|twinning]], filter loss, and long and short term stability of the crystals.<ref>[http://www.4timing.com/techquartz.htm Quartz Tech]. 4timing.com. Retrieved on 2010-02-08.</ref> Different-cut seeds in different orientations may provide other kinds of growth regions.<ref>{{cite conference |author1=Shinohara, A. H. |author2=Suzuki, C. K. |book-title=Proceedings of 1996 IEEE International Frequency Control Symposium |pages=72–77 |doi=10.1109/FREQ.1996.559821 |year=1996 |chapter=Study of S- and ξ-bar synthetic quartz by X-ray topography |isbn=0-7803-3309-8}}</ref> The growth speed of the −X direction is slowest due to the effect of adsorption of water molecules on the crystal surface; aluminium impurities suppress growth in two other directions. The content of aluminium is lowest in Z region, higher in +X, yet higher in −X, and highest in S; the size of S regions also grows with increased amount of aluminium present. The content of hydrogen is lowest in Z region, higher in +X region, yet higher in S region, and highest in −X.<ref>{{cite journal|author1=Fumiko Iwasaki |author2=Armando H. Shinohara |author3=Hideo Iwasaki |author4=Carlos K. Suzuki |url=http://www.fem.unicamp.br/~liqcqits/publications/paper_files/JJAP1990v29-6p1139-1142_Iwasaki.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.fem.unicamp.br/~liqcqits/publications/paper_files/JJAP1990v29-6p1139-1142_Iwasaki.pdf |archive-date=2022-10-09 |url-status=live |title=Effect of Impurity Segregation on Crystal Morphology of Y-Bar Synthetic Quartz|journal= Jpn. J. Appl. Phys. |volume=29 |issue=6 |pages=1139–1142 |year=1990|doi=10.1143/JJAP.29.1139|bibcode=1990JaJAP..29.1139I |s2cid=97694219 }}</ref> Aluminium inclusions transform into color centers with gamma-ray irradiation, causing a darkening of the crystal proportional to the dose and level of impurities; the presence of regions with different darkness reveals the different growth regions.

The dominant type of [[crystallographic defect|defect]] of concern in quartz crystals is the substitution of an [[aluminium|Al(III)]] for a [[silicon|Si(IV)]] atom in the [[crystal lattice]]. The aluminium ion has an associated interstitial charge compensator present nearby, which can be a [[hydrogen|H<sup>+</sup>]] ion (attached to the nearby oxygen and forming a [[hydroxyl group]], called Al−OH defect), [[lithium|Li<sup>+</sup>]] ion, [[sodium|Na<sup>+</sup>]] ion, [[potassium|K<sup>+</sup>]] ion (less common), or an [[electron hole]] trapped in a nearby oxygen atom orbital. The composition of the growth solution, whether it is based on lithium or sodium alkali compounds, determines the charge compensating ions for the aluminium defects. The ion impurities are of concern as they are not firmly bound and can migrate through the crystal, altering the local lattice elasticity and the resonant frequency of the crystal. Other common impurities of concern are e.g. iron(III) (interstitial), fluorine, boron(III), phosphorus(V) (substitution), titanium(IV) (substitution, universally present in magmatic quartz, less common in hydrothermal quartz), and germanium(IV) (substitution). Sodium and iron ions can cause [[inclusion (mineral)|inclusions]] of [[acnite]] and [[elemeusite]] crystals. Inclusions of water may be present in fast-grown crystals; interstitial water molecules are abundant near the crystal seed. Another defect of importance is the hydrogen containing growth defect, when instead of a Si−O−Si structure, a pair of Si−OH HO−Si groups is formed; essentially a hydrolyzed bond. Fast-grown crystals contain more hydrogen defects than slow-grown ones. These growth defects source as supply of hydrogen ions for radiation-induced processes and forming Al-OH defects. Germanium impurities tend to trap electrons created during irradiation; the alkali metal cations then migrate towards the negatively charged center and form a stabilizing complex. Matrix defects can also be present; oxygen vacancies, silicon vacancies (usually compensated by 4 hydrogens or 3 hydrogens and a hole), peroxy groups, etc. Some of the defects produce localized levels in the forbidden band, serving as charge traps; Al(III) and B(III) typically serve as hole traps while electron vacancies, titanium, germanium, and phosphorus atoms serve as electron traps. The trapped charge carriers can be released by heating; their recombination is the cause of [[thermoluminescence]].

The mobility of interstitial ions depends strongly on temperature. Hydrogen ions are mobile down to 10 K, but alkali metal ions become mobile only at temperatures around and above 200 K.
The hydroxyl defects can be measured by near-infrared spectroscopy. The trapped holes can be measured by [[electron spin resonance]]. The Al−Na<sup>+</sup> defects show as an acoustic loss peak due to their stress-induced motion; the Al−Li<sup>+</sup> defects do not form a potential well so are not detectable this way.<ref>{{cite journal|author=Harish Bahadur |url=http://www.crystalresearch.com/crt/ab41/631_a.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.crystalresearch.com/crt/ab41/631_a.pdf |archive-date=2022-10-09 |url-status=live |title=Radiation induced modification of impurity-related point defects in crystalline quartz – a review|journal=Crystal Research and Technology |volume=41|issue=7|pages=631–635|year=2006|doi=10.1002/crat.200510641|bibcode=2006CryRT..41..631B |s2cid=95333080 }}</ref> Some of the radiation-induced defects during their thermal annealing produce [[thermoluminescence]]; defects related to aluminium, titanium, and germanium can be distinguished.<ref>Harish Bahadur [http://ursi-test.intec.ugent.be/files/URSIGA08/papers/BDPS2p6.pdf Investigations on irradiation and structural characteristics of high quality cultured quartz crystals used in satellite communication] {{webarchive|url=https://web.archive.org/web/20110716081852/http://ursi-test.intec.ugent.be/files/URSIGA08/papers/BDPS2p6.pdf |date=2011-07-16 }}</ref>

Swept crystals are crystals that have undergone a solid-state [[electrodiffusion]] purification process. Sweeping involves heating the crystal above 500&nbsp;°C in a hydrogen-free atmosphere, with a voltage gradient of at least 1 kV/cm, for several hours (usually over 12). The migration of impurities and the gradual replacement of alkali metal ions with hydrogen (when swept in air) or electron holes (when swept in vacuum) causes a weak electric current through the crystal; decay of this current to a constant value signals the end of the process. The crystal is then left to cool, while the electric field is maintained. The impurities are concentrated at the cathode region of the crystal, which is cut off afterwards and discarded.<ref>Arthur Ballato ''et al.'' Method of sweeping quartz {{US patent|4311938}}, Issue date: January 19, 1982/</ref> Swept crystals have increased resistance to radiation, as the dose effects are dependent on the level of alkali metal impurities; they are suitable for use in devices exposed to ionizing radiation, e.g. for nuclear and space technology.<ref name="ieee-uffc1">[http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=vigrad Frequency Control|Teaching Resources] {{webarchive|url=https://web.archive.org/web/20100706000255/http://www.ieee-uffc.org/frequency_control/teaching.asp?vig=vigrad |date=2010-07-06 }}. Ieee-uffc.org. Retrieved on 2010-02-08.</ref> Sweeping under vacuum at higher temperatures and higher field strengths yields yet more radiation-hard crystals.<ref name="freepatentsonline.com">James Claude King ''Vacuum electrolysis of quartz'' {{US patent|3932777}}, Issue date: Jan 13, 1976.</ref> The level and character of impurities can be measured by infrared spectroscopy.<ref>[https://web.archive.org/web/20110716212100/https://authors.aps.org/eprint/files/1997/Apr/aps1997apr04_003.1/main.pdf Infrared study of defects in alpha quartz caused by sweeping effects]. authors.aps.org (April 1997). Retrieved on 2012-06-21.</ref> Quartz can be swept in both α and β phase; sweeping in β phase is faster, but the [[phase transition]] may induce twinning. Twinning can be mitigated by subjecting the crystal to compression stress in the X direction, or an AC or DC electric field along the X axis while the crystal cools through the phase transformation temperature region.<ref name="freepatentsonline.com"/>

Sweeping can also be used to introduce one kind of an impurity into the crystal. Lithium, sodium, and hydrogen swept crystals are used for, e.g., studying quartz behavior.

Very small crystals for high fundamental-mode frequencies can be manufactured by photolithography.<ref name="patentstorm1"/>

Crystals can be adjusted to exact frequencies by [[laser trimming]]. A technique used in the world of [[amateur radio]] for slight decrease of the crystal frequency may be achieved by exposing crystals with silver electrodes to vapors of [[iodine]], which causes a slight mass increase on the surface by forming a thin layer of [[silver iodide]]; such crystals however had problematic long-term stability. Another method commonly used is electrochemical increase or decrease of silver electrode thickness by submerging a resonator in [[lapis lazuli]] dissolved in water, citric acid in water, or water with salt, and using the resonator as one electrode, and a small silver electrode as the other.

By choosing the direction of current one can either increase or decrease the mass of the electrodes.
Details were published in "Radio" magazine (3/1978) by UB5LEV.

Raising frequency by scratching off parts of the electrodes is not advised as this may damage the crystal and lower its [[Q factor]]. Capacitor [[trimmer (electronics)|trimmer]]s can be also used for frequency adjustment of the oscillator circuit.

=== Other materials ===
Some other [[piezoelectric material]]s than quartz can be employed. These include single crystals of [[lithium tantalate]], [[lithium niobate]], [[lithium borate]], [[berlinite]], [[gallium arsenide]], [[lithium tetraborate]], [[aluminium phosphate]], [[bismuth germanium oxide]], polycrystalline [[zirconium titanate]] ceramics, high-alumina ceramics, [[silicon]]-[[zinc oxide]] composite, or [[dipotassium tartrate]].<ref>Arthur Ballato ''Method of making a crystal oscillator desensitized to accelerationfields'' {{US patent|4871986}}, Issue date: October 3, 1989.</ref><ref>[http://www.txc.com.tw/download/tech_paper/2002-NTUIAM-1-English.pdf Recent Development of Bulk and Surface Acoustic Wave Technology for Frequency Control Applications], December 23, 2002 Institute of Applied Mechanics National Taiwan University, C. S. Lam, TXC Corporation.</ref> Some materials may be more suitable for specific applications. An oscillator crystal can be also manufactured by depositing the resonator material on the silicon chip surface.<ref>Fumio Nakajima ''Quartz crystal oscillator angular velocity detector circuits'' {{US patent|5420548}}, Issue date: May 30, 1995.</ref> Crystals of [[gallium phosphate]], [[langasite]], [[langanite]] and [[langatate]] are about 10 times more pullable than the corresponding quartz crystals, and are used in some VCXO oscillators.<ref>Bernd Neubig, [https://web.archive.org/web/20060305152252/http://www.vhfcomm.co.uk/pdf/Pressworks%20-%20VCXO.pdf VCXOs with wide pull-in range using alternatives to quartz]. VHF Communications, 2/2003, pp. 66–70.</ref>

{{Anchor|Stability and aging}}