8/13/2019 Lecture 18-IR Sensors Demo - IR Motion

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Lecture #18

Basic Intent

This lecture will focus on several recently developed technologies for Infrared Sensing.

Modern thermal infrared detectors

In recent years, the DOD has invested a great deal of R+D funds into detection techniques whichallow long-wave detection fro uncooled platfor s. !n additional focus of this wor" has #eentechniques which are co pati#le with the for ation of dense arrays. One interesting devicewhich has e erged due to this invest ent has #een the $ncooled Detector arrays ade #y

%oneywell.

These detectors are #ased on the si plest ther al design - a resistance ther o eter. &hat isnovel a#out this device is that it co #ines the #est icrofa#rication technology with goodther o eter technology and electronics integration.

Fig. 1: Microbolometer

! drawing of the icro#olo eter is shown in 'ig. (. The #asic idea is to use siliconicrofa#rication techniques )li"e those in the !D* accelero eter to a"e an isolated

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ther al structure with very little heat capacity. !s we saw in the ther o eter lecture, thether al infrared detector is i proved #y ini i/ing the heat capacity.

In the final device, a fla"e of silicon nitride with di ensions of u 0 u 0 . u isfloated a#ove a silicon su#strate. This fla"e is supported #y a pair of legs, and is coated with a

resistive aterial with a good ther al coefficient of resistance. $nderneath the fla"e is atransistor which is used to connect the current- easuring circuit to the device using aconventional row-colu n addressing technique. The device currents are passed out to a

processing circuit on the peri eter of the device #y the 0 and y etal leads.

In this device, uch research went into developing a technique for depositing the nitride on topof a transistor, for releasing the devices with very high yield, and for o#taining a sensitivether o eter in the for of a deposited etal fil . This resistor is ade fro vanadiu o0ide,which offers a T1R of a#out (2 near roo te perature. The resistance change is a result of astructural phase transition in vanadiu o0ide a#ove roo te perature, so this device ust #eheld near roo te perature to allow operation with good sensitivity.

%aving developed this technology, %oneywell has gone on to a"e dense arrays )3 03 , andto continue opti i/ing the perfor ance of the devices. In the last couple of years, a co pleteca era syste has #een de onstrated. This #ase technology has #een offered for licensing, andis presently #eing co erciali/ed #y several anufacturers of infrared i aging syste s.

This device does not out-perfor the 41T i ager, #ut it does ena#le operation at roote perature, and ight #e availa#le at low cost with further develop ent.

Fig. 2: Simplified Model of a !roelectric "ffect

!nother very i portant technology for low-cost uncooled infrared detectors has e erged inrecent years in the for of pyroelectric plastic aterial. 56D' is a pyroelectric aterial that is adecent ther o eter. !nalogous to pie/oelectricity and strain, pyroelectricity is a pheno ena inwhich a change in te perature causes ther al e0pansion, which causes the appearance of charge)through the pie/oelectric effect .

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Infrared detectors have #een availa#le for any years #ased on other speciali/ed pie/oelectricaterials. The #est of the is Deuterated Tri-7lycine 1ulfide )DT7S . This very e0pensiveaterial offers the #est pyroelectric coefficients, and is co only used for IR detection in

la#oratory spectro eters, and in early otion detection syste s.

! variety of other pyroelectric aterials are also availa#le - it is generally true that any aterialwhich is pie/oelectric is also pyroelectric. There are any applications which need good perfor ance )la# spectroscopy, for e0a ple , and these applications generally 8ustify use of the #est aterial availa#le.

On the other hand, there are other applications in which the #est detector perfor ance is notrequired. In these applications, 56D' fil has #eco e the #est choice availa#le - pri arily dueto the tre endously low cost of the device aterial.

! good e0a ple of a low-perfor ance application is an infrared otion detector. 9owadays, it isco on to offer #ac"yard lighting syste s or door opening syste s which detect the presence

of a oving o#8ect with elevated infrared e ission. If you wave your hand a#out, the infraredscene that can #e detected features a variation in the infrared signal to so e pi0el of an i agingsyste . So what is needed is an array of detector ele ents and so e sort of focused optics.&ithout the focused optics, oving your hand a#out does not produce a change in the totalillu ination- and would not produce a varia#le signal.

Re e #er that the pyroelectric detectors do not detect heat - only changes in heat.

So, it has #eco e co on to pac"age a 56D' detector array in a low-cost optical pac"agewhich uses a Teflon lens to focus the light. Teflon lens aterial is also ine0pensive, and istrans issive enough in the IR that is does a decent 8o#.

Typical Teflon lenses used in otion detection syste s are ade with a surface te0ture thatincludes several circular #u ps. These #u ps act as focusing lenses, and will #ring light fro a

particular part of the scene to the detector. !s a war o#8ect oves through the scene, radiationis occasionally focused on the detector, causing a transient in signal which is detected.

Fig. : $ Facet Lens

! good illustration of this concept is shown in 'ig. : )fro the #oo" . !s the ;person< ovesacross the scene, the array of lenses produces an oscillating illu ination on the detector. The

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