BACK-SIDE IL­LU­MI­NA­TION SEN­SORS

Re­cent break­throughs in semi­con­duc­tor processing and wafer-scale pack­ag­ing have meant that a num­ber of 2017’s cam­era re­leases boast back-side il­lu­mi­na­tion sen­sors (or BSI for short). We un­cover how they work and what they ac­tu­ally mean for your im­agery

New Zealand D-Photo - - EXPLAINED -

The cam­era’s sen­sor is of­ten talked about as though it is the heart of the de­vice: a lit­tle solid­state de­vice from which all the imag­ing magic is pumped. But a closer look at its work­ings re­veals that the more ac­cu­rate bod­ily com­par­i­son is ac­tu­ally the eye­ball. The sen­sor is the site where light is gath­ered and trans­lated into a sig­nal that can be pro­cessed as an im­age — the mech­a­nism through which the world is re­vealed. Just like eye­balls, not all sen­sors are cre­ated equal. One of the most preva­lent trends in dig­i­tal imag­ing has been dubbed the ‘megapixel race’, in which brands have com­peted to see who can stuff the most pix­els — tiny pho­ton sen­sors — onto a sin­gle sen­sor. Boast­ing about the megapixel count of a par­tic­u­lar model makes for a con­ve­nient mar­ket­ing tool, but, in truth, more pix­els does not au­to­mat­i­cally mean a bet­ter im­age. It’s the sen­sor’s de­sign that re­ally counts. To that end, there is a fair amount of lingo to brush up on. You have your dif­fer­ent cat­e­gories of sen­sor, like CCD, CMOS, and LiveMOS. There are dif­fer­ent sen­sor sizes, such as full frame, APS-C, and Four Thirds. And an­other im­por­tant vari­ance is the way a sen­sor is con­structed, which is the purview of this ar­ti­cle. A sen­sor com­prises many in­di­vid­ual pic­ture el­e­ments (pix­els), each with its own lens, lightre­ceiv­ing pho­to­di­ode, and con­nec­tive wiring. The con­ven­tional front-il­lu­mi­nated sen­sor de­sign ar­ranges th­ese pixel el­e­ments with the lens on top (fac­ing the light), the wiring in the mid­dle, and the pho­to­di­ode at the back. This means that the pho­to­sen­si­tive re­gion of the chip is ac­tu­ally fur­thest away from the light — a de­sign that mim­ics the hu­man eye — and the wiring stacked on top blocks a cer­tain amount of light from reach­ing the sil­i­con pho­to­di­ode. One of the most im­por­tant de­vel­op­ments in sen­sor tech­nol­ogy has been the rise of the back-side il­lu­mi­na­tion (BSI) sen­sor, which in­verts the de­sign to al­low much more light to reach its des­ti­na­tion. Tra­di­tional sen­sor de­sign takes a base of light-sen­si­tive sil­i­cone and lay­ers the me­chan­ics on top; the BSI de­sign be­gins the same way, but adds a sec­ond sil­i­con layer on top, and the chip is flipped. This re­sults in a layer of light-re­ceiv­ing sil­i­con on top of the wiring, as well as a layer at the chip’s base. Think of the dif­fer­ence be­tween a pizza and cal­zone. The mer­its of this de­sign are straight­for­ward. While a con­ven­tional chip will cap­ture be­tween 30 to 80 per cent of the light ap­proach­ing the sen­sor, a BSI de­sign will grab close to 100 per cent. The down­side has gen­er­ally been the ex­tra pro­duc­tion costs. As the layer of sil­i­con at the top of the chip has to be pre­cisely thinned from an orig­i­nal thick­ness of 1mm down to around 5μm in or­der to let light through, the pro­duc­tion process can be in­ten­sive and ex­pen­sive. This is why, un­til re­cently, BSI sen­sors were largely re­served for spe­cial­ist equip­ment like sur­gi­cal mi­cro­scopes, se­cu­rity cam­eras, and as­tron­omy sen­sors, where ex­treme low-light sen­si­tiv­ity is a must. How­ever, tech­nol­ogy, as it tends to, has marched on, and pro­cesses have been re­fined to the point where BSI sen­sors have also be­come a more com­mon fea­ture of con­sumer cam­eras. One of the most im­pres­sive demon­stra­tions of the power that a BSI sen­sor can lend a DSLR comes in Nikon’s new high­end D850. Chief among the model’s many con­sid­er­able fea­tures is the com­pany’s first BSI full-frame CMOS sen­sor. With the BSI de­sign al­low­ing light to hit the senor’s whop­ping 45.7-mil­lion pix­els far more ef­fi­ciently than a tra­di­tional sen­sor de­sign, the cam­era can go to a na­tive ISO of 25,600 with­out the as­so­ci­ated im­age noise that would usu­ally re­sult from such a pixel-dense chip. At the other end of the sen­si­tiv­ity spec­trum, way down at ISO 64, the BSI sen­sor is still able to de­liver an im­pres­sively wide dy­namic range. Com­bine the pow­er­ful sen­sor with more ef­fi­cient BSI wiring, the re­moval of the low-pass fil­ter, and the Ex­peed 5 im­age-processing en­gine, and you now have a de­vice that can cap­ture im­mac­u­late de­tail, freeze ac­tion, shoot at high-speeds, and nav­i­gate low-light sit­u­a­tions all in one tidy pack­age. And it all started with the bright idea of flipping a lit­tle chip on its head.

NIKON D850 CAM­ERA Sen­sor............................................................. Full frame Res­o­lu­tion......................................................... 45.7MP ISO............................ 64–25,600 (32–102,400 exp.) Shut­ter.............................................................. 1/8000s Burst shoot­ing............. 9fps with MB-D18 bat­tery grip Aut­o­fo­cus.......................... 153-point phase-de­tec­tion (99 cross-type) Lens mount................................................................... F Di­men­sions...................................... 146 x 124 x 78.5mm Weight.................................................................. 1005g

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