GB2146865A - Camera with reduced condensation cooled solid-state imager - Google Patents

Abstract

A solid-state imager 410 of a television camera is cooled by a thermo electric cooler 310 having a "cold" surface 314 attached to the imager 410 and a "hot" surface 312 attached to a metal cap 212. The cap is then attached to a thin plastic or ceramic base 210 which also supports a window 320 thereby formed a sealed housing for the imager 412. The housing is filled with dry low thermal conducting gas. Heat from the cooler is supplied via the cap and the thin support to the window. Thus the imager is cooled for good signal to noise ratio and low dark current whilst condensation on the window is reduced. <IMAGE>

Description

SPECIFICATION Camera with reduced condensation cooled solidstate imager This invention relates to a camera including a solid-state e.g. COD imager.

Portable television cameras having solid-state imagers have begun to appear on the marketplace.

These cameras are very advantageous in that they are extremely rugged and have no inherent degradation mechanisms. They do have certain disadvantages when compared with conventional vidicon tubes, among which are the generation of unwanted noise due to mechanisms unique to solidstate imagers. These noise mechanisms include temperature-dependent black-level leakage currents which appear as signal even in the absence of light.

The problem of noise due to temperaturedependent sources may be exacerbated by the power dissipated in the camera during operation, which may cause a temperature rise above the ambient air temperatures. Even for cameras with low temperature rise above ambient, noise may still be a problem because even normal ambient temperatures (such as 20"C) are too high for good imager noise performance.

It is desirable to cool the solid-state imager of a camera for reduced noise and low dark current without creating a problem of condensation.

Summary of the invention A cooling arrangement for cooling a camera imager includes a solid-state imager having a photosensitive surface and a second surface and a cooler having a cold surface coupled to the second surface of the imager for pumping heat from the photosensitive surface by thermal conduction through the body of the imager. A transparent window is mounted between the photosensitive surface and the image and is subject to condensation if cooled. The mount by which the window is affixed includes a relatively low thermal conductivity path between the photosensitive surface of the imager and the window, and a relatively high thermal conductivity path between the window and the hot surface of the cooler whereby the window tends to be heated rather than cooled and condensation is prevented.

Description of the drawing Figure lisa block diagram of a colortelevision camera; Figure 2 is a perspective view of a mount for the imager according to the invention; Figures 3, 4 and 5 are various cut-away views illustrating the mounting of the imager; Figures 6 a-i illustrate in cutaway view sequences of assembly for two manufacturing techniques of the cooled imager; and Figure 7 is a block diagram of the thermal servo bX which the temperature is controlled.

Description of the invention In figure 1, a lens 10 focusses visible light from an image (not shown) past a rotating shutter 12 to a color-splitting prism 14 which divides the light according to its color and applies it to a red (R) imager 16, a green (G) imager 18 and a blue (B) imager 20. imagers 16, 18 and 20 are e.g. solid-state COD imagers of the field-transfer type. A clock generator illustrated as a block 22 provides multiphase clock signals for control of the pull-down and signal transfer of the imagers. Signal preamplifiers 24, 26, and 28 are coupled to the R, G and B imagers respectively for amplifying the signals therefrom and for applying them to processing circuits illustrated as blocks 30, 32 and 34, respectively. The processing includes ordinary camera signal processing which may include shading, dropout (DO) correction, gamma correction, clamping and the like.The process signals are applied to circuits designated as 36-40 for insertion of sync and blanking to produce R, G and B baseband video signals for application to utilization circuits (not shown).

The R, G and B signals are also applied to a matrix 42 which generates Y, I and Q signals in known fashion. The I and Q signals are applied through low-pass filters 44 and 46, respectively, to a modulator illustrated as a block 48 for modulation in known fashion onto a subcarrier received from a subcarrier generator 50. The color signal modulated onto the subcarrier is summed with the Y signal in an adder 52 to form a composite color television signal. The Y, I, and Q signals may be applied directly to a portable videotape recorder without passing through the modulator.

A cooler such as a thermoelectric cooler and its associated circuitry illustrated together as a block 60 is shown thermally coupled to green imager 18. As described hereinafter with reference to Figure 7 coolers (not shown) may be thermally coupled also to red and blue imagers 16 and 20. A cooler may be coupled to the blue imager alone.

Figure 2 illustrates in perspective view the structure of the mount for imager 18. A plastic or ceramic base material 210 is the major assembly support. A thermoelectric cooler (not visible) lies under thermally conductive (metal) cap 212 for pumping heat from the imager towards top portion 214 of cap 212. A thermally conductive braid material such as copper braid 216 is thermally affixed to the top 214 of cap 212 to aid in carrying heat away from the cap to a thermal heat sink (not shown). The imager is formed as an integrated circuit chip which is too delicate for ordinary electrical connections. An array of electrical contact pins designated generally as 220 has connections to the imager and provides convenient electrical connection to a socket. Braid 216 may be oriented so as to reduce interference with the socket connection.

Figure 3 illustrates the structure 200 in more detail and partially cut away. In Figure 3c, braid 216 can be seen to lie flat against cap top 214 for good thermal contact. A thermoelectric cooler designated generally as 310 includes an upper thermal plate or bus 312 which bears against the inside of cap top 214 and a lowerthermal plate or bus 314 which bears against the surface to be cooled.

Semiconductor material 316 coupled between plates 312 and 314 pumps heat from plate 314 towards plate 312 when electrically energized with direct current, in known fashion. A transparent window 320 is mounted to allow light to reach the photosensitive portion of the imager(not visible in Figure 3).

Figure 4 is a cross-sectional view corresponding to Figure 3b, in which the relationship of the imager chip to the cooler and window can be seen. In Figure 4, a thinned imager chip 410 bonded to a thin glass plate 412 is thermally bonded to cooled thermal plate 314 and electrically connected by a number of bonding wires (two of which are shown as 414, 416) to pins 220. Window 320 is affixed to body 210 by means of adhesive gasket material illustrated as 420 so as to cover window opening 450 to form a sealed cavity in which the imager is mounted. It will be noted that there is no mechanical contact between glass plate 412 and body 210, and the only connection between thinned imager 410 and body 210 is by means of bonding wires 414. Since the bonding wires are extremely small in diameter and relatively long in relation to their diameter, they have significant thermal resistance.As a result, the thermal resistance between imager410 and window 320 or body 210 is quite high, whereas the thermal resistance between imager 410 and cooled thermal bus 314 is very low. Window 320 is offset relative to the center of imager 410 so as to allow light to fall onto the "A" register, which is the register which has been selected for light sensing. Also visible in Figure 4 is a pair of electrical conductor wires 430 which passes through a hole in cap 212 to carry electrical energizing current to thermoelectric cooler 316. The hole through which wires 430 pass is sealed with epoxy illustrated as 432.

Dry gas is used to fill the cavity formed between the cap 212 and the window 320 in which cavity the imager resides. The lack of moisture in the gas helps prevent condensation forming on the imager 410 or glass plate 314 when they are cooled. It is desirable to use a gas having low thermal conductivity so as to increase the thermal resistance between imager 410 and window 320. Gases having low thermal and electrical conductivity are well known, e.g.

halocarbon gas and the fluorocarbon known commercially as Freon is suitable.

The plastic or ceramic base 210 is thin relative to its length and breadth and provides sufficiently low thermal resistance between the metal cap 212 and the window 320 to conduct heat from the cap to the window.

Thus the dry gas of low thermal conductivity (together with thin bonding wires 414, 416) provides a relatively low thermal conductivity path between the cooled photosensitive surface of the imager whilst the metal cap and the relatively short path from the cap to the window through base 210 provides a relatively high thermal conductivity path from the hot surface of the cooler 310 to the window 320.

Figure 5 illustrates the general structure in cutaway fashion as an aid to understanding the structure of the invention.

Figures 6a to 6e illustrate one inventive assembly procedure by which the structure illustrated in Figure 4 may be manufactured. In Figure 6a, thinned imager 410 bonded to glass plate 412 is set onto a resilient gasket 610 sitting on a ledge of mount 210.

The imager is thinned to reduce thermal resistance between the rear surface of the imager and the photosensitive surface facing glass plate 412.

Thinning also improves blue sensitivity. Figure 6b illustrates the bonding of wires between appropriate points of imager 410 and corresponding contacts on mount 210. Figure 6c illustrates the bonding of a cap 212 with thermoelectric cooler 310 to the structure of Fig. 6b in such a fashion that thermal bus 314 presses against imager 410 which therefore presses glass plate 412 so as to somewhat compress gasket 610. Figure 6d illustrates the result of removing gasket 610 by pulling it through window opening 450. Figure 6e illustrates the structure with window 320 affixed in position over opening 450 to form the sealed cavity.

Figure 6f-6i illustrates an alternative inventive assembly procedure which avoids contamination which may occur when removing resilient gasket 610. Imager 410 and glass plate 412 are placed in mount 210. A glue or adhesive may be used in the interface region. The wire bonds are made as illustrated in Figure 6f. During the next step, a plunger 620 is placed against glass plate 412 and pushed to separate plate 412 and imager 410 from mount 210. The plunger may be heated or the separation may be accomplished in an oven if the adhesive is loosened by heat. Depending on the characteristics of the adhesive, it may be necessary to place a backstop (not shown) to prevent the movement of the imager and glass plate from pulling the wire bonds loose.Figure 6h illustrates the bonding of the cooled surface of the imagerto the cold surface 314 of cooler 310 by pressing plunger 620 against glass plate 412 while 9n adhesive (not shown) between imager410 and plate 314 cures. During this operation, cover 212 is pressed against mount 210. The operations of Figures 69 and 6h may be combined with cover 212 and attached cooler 310 acting as a backstop for imager 410 as plunger 620 presses glass plate 412 and imager 410 away from mount 210. Figure 6i illustrates the last step of mounting window 320 by an adhesive 650 to mount 210 to form the sealed cavity.

The dry gas which is used to fill the cavity may be introduced into the cavity after mounting of the window 320 by evacuation and refilling through a small hole in the cap, or the window may be affixed while the assembly is immersed in a gas atmosphere.

Figure 7 illustrates in block-diagram form the control and drive circuits of a thermal servo. In figure 7, connections in a portion of the output "C" register of a frame-transfer imager and the output amplifier region are illustrated in a region bounded by a surface 710. The various gates of a CCD imager are labeled OG, H1, H2, and H3. These gates are driven by the polyphase clock signals from generator 22 of Figure 1 for causing signal representative charges in "wells" in the p-region to travel towards an output diode region, illustrated as the junction between a region N+ and the p-region.

The gate connections are not illustrated in detailed fashion because they are well known and are not central to the invention. At the right end of the C register is an output amplifier designated generaliy as 712 and at the left of the C register is an output amplifier designated generally as 714. The imagers are made with two output amplifiers so that signals may be clocked in either direction by adjustment of the phasing of the drive clocks to reverse the image right-to-left for special effects or for compensating for the effects of a mirror in the light path.

Ordinarily, only one output amplifier is used and the other one is not bonded for connection to the outside world. In the arrangement of Figure 7, the signal output is taken from output amplifier 712 to which the various clock signals are applied for transferring signal from diode 711 to an output terminal 718 for further processing. Several connections of output amplifier714 are not bonded during the bonding operation, as indicated by an "X". During the bonding operation, the gates of a dual-gate field-effect transistor (FET) 720 of output amplifier 714 are coupled together and to the drain.

This forms a junction diode between an output terminal 722 and the N+ region 724. The junction diode formed from FET 720 is thermally coupled to imager 410 and tends to remain at either the same temperature as the remainder of the chip or at a fixed offset temperature relative thereto.

Consequently, otherwise-unused diode-connected Fet 720 may be used to sense the temperature of the imager chip to aid in the operation of a temperaturecontrol thermal feedback loop or servo. Naturally, if output amplifier 714 is used for signal handling, FET 719 of amplifier 712 would be bonded as a junction diode for temperature sensing.

Since a junction diode produces no significant voltage in the absence of a current passing therethrough, production of a temperaturedependent sense voltage requires that a current be passed through temperature-sensing diode 720.

However, it has been discovered that a current flow through diode-connected FET 720 perturbs the generation of light-representative charge in the photosensitive regions of the imager. It has further been discovered, however, that injection of current into the diode does not perturb transfer of charge from the photosensitive regions to the output register. Terminal 722 is therefore connected to a controllable high impedance current source 730 coupled to a source B+ of supply for gating current from B+ to diode-connected FET720 during the pull-down interval of the imager. Source 730 is controlled by sync signal from sync signal generator 54 so as to conduct for an interval of approximately 800 microseconds during vertical blanking, which corresponds generally with the pull-down interval.

Sufficient current must flow to produce an adequate sense voltage, but not so much as to significantly raise the temperature of the diode above that of the surrounding chip. It has been found that a current of approximately 100uA is adequate in this regard.

When the specified current flows in the diode, a voltage which depends upon temperature appears between terminal 722 and ground, and is compared with a reference voltage by a comparator or amplifier 732. In Figure 7, comparator 732 compares the sense voltage with ground.

During the integrating interval following each pull-down interval, the sense voltage drops to zero because current source 730 is turned off.

Consequently, the error voltage produced by comparator 732 becomes zero. If the error voltage were used to directly control the thermoelectric coolers illustrated as 740-744, the coolers would be energized with a low duty cycle (800/16,600= 0.48), and higher peak cooler drive currents would be required to maintain proper operating temperature.

Such high peak currents are disadvantageous, especially for portable cameras which must be powered from batteries. In order to maintain low peak cooler drive current, a sample-and-hold (S & ) circuit illustrated as 746 is coupled to the output of comparator 732 and is gated during the pull-down interval to store the error voltage throughout the next following integrating interval for operating power amplifier 748 for driving thermoelectric coolers 740-744 with a constant current.A gate pulse for S & 746 may be generated by a circuit consisting of a cascade of a 400-microsecond astable multivibrator 750 coupled to an output of sync generator 54 for being triggered into its astable condition at the beginning of the pull-down interval and for producing a pulse 400uS after the beginning of the pull-down interval, which pulse triggers a SuS astable multivibrator 752 for triggering S & 746.

Thermoelectric cooler 744 as illustrated is located within a box 760 which represents a thermal coupling between cooler 744 and imager 410. Thus, the thermal servo loop controls the temperature of imager 410, which is coupled to receive the green components of light from the image being televised.

Since the thermal resistances of the red and blue imagers to their surroundings are approximately equal to the thermal resistances associated with the green imager, application of equal cooling drive will result in similar temperatures. Consequently, thermoelectric coolers 740 and 742 associated with the red- and blue-responsive imagers respectively are electrically coupled in series with cooler 744.

The series electrical connection is advantageous because of the relatively low electrical resistance of the thermoelectric coolers. For coolers having higher impedance, a parallel or series-parallel electrical connection may be more advantageous, as will be obvious to those skilled in the art.

Other embodiments of the invention will be obvious to those skilled in the art. For example, the cooled imager may be green-or luminance responsive and an uncooled single imager having a color checkerboard or stripe filter may be used to respond to red and blue in known fashion. Also, a single checkerboard-filter-imager responsive to both red and blue may be open-circuit cooled by a thermal servo controlling the green chip temperature. The resilient gasket may be made of paraffin or other substance which may be removed by melting, ratherthan by pulling, or the gasket may be of a soluble substance which is removed by a noncorrosive solvent. The temperature-sensing element may be a diode formed as such within the imager chip, or it may be an element other than a diode, as for example a resistor, which has a more linear temperature-resistance characteristic than a diode. Further, the temperature-sensing element may be external to the imager chip, and clamped to the chip to sense the temperature; however such an external sensor is disadvantageous in that it requires further assembly and also because it tends to be less sensitive to changes in the chip temperature.

Furthermore a cooler may be coupled to the blue imager alone, orto especially the blue imager but also to the other imagers, for improving the signal to noise ratio of the blue imager, in particular in low light level conditions. Athermal servo as described herein is then preferably associated with the blue imager and its cooler.

Claims (14)

1. An imaging system comprising: a solid-state imager having a first major, photosensitive, surface and a second major surface; cooling means having a cold surface coupled to said second surface for removing heat from the said second surface for cooling said photosensitive surface by thermal conduction through the body of said imager and a hot surface to which the heat is moved; a transparent window means supporting said transparent window in front of said photosensitive surface for allowing lightfrom a source to reach said photosensitive surface, said window being subject to condensation and a consequent reduction of transparency if cooled below the dew point of the surrounding atmosphere; a relatively low thermal conductivity path between said photosensitive surface of said imager and said window; and a relatively high thermal conduction path between said window and the hot surface of said cooling means whereby said window tends to be heated rather than cooled thereby preventing condensation on said window.

2. A system according to claim 1, wherein the imager is spaced from the window and supporting means.

3. A system according to claim 1 of 2 wherein as closed housing comprising said supporting means and window encloses the imager.

4. A system according to claim 3 wherein said housing contains a dry gas having low thermal conductivity.

5. A system according to claim 4, wherein the said gas is a halocarbon gas.

6. A system according to claim 5, wherein the said gas is a fluorocarbon gas.

7. A system according to claim 3, 4, 5 or 6 wherein the housing comprises a cap of material having relatively high thermal conduction sealingly connected to the supporting means with the window sealingly connected to the supporting means, and in that the said hot surface is highly thermally conductively coupled to the said cap.

8. A system according to claim 3, 4, 5, 6 or 7 wherein the imager is supported in the housing solely by the cooling means.

9. A system according to any preceding claim wherein there are electrical connections between the imager and electrical conductors associated with the supporting means.

10. A system according to any preceding claim wherein means for sensing the temperature of the imaging means and means for controlling the cooling means to cause the sensed temperature to tend towards a predetermined temperature.

11. A system according to claim 10 wherein the sensing means comprises a temperature sensitive element within the solid state imager.

12. A system according to any preceding claim wherein the cooling means is a thermoelectric cooler.

13. A system according to any preceding claim wherein it is a television camera and the solid state imager is responsive to visible light.

14. An imaging system substantially as hereinbefore described with reference to Figures 1, 2, 3a to c, 4 and 5 of the accompanying drawings.