CN100474888C - High sensitivity thermal radiation detection with an emission microscope with room temperature optics - Google Patents

Abstract

A light emission microscope includes a low temperature detector array (16) which receives light through a room temperature projection optics system (30) from a device under test (14). A cold aperture including a movable cold stop (32) and a cryogenic filter absorbs unwanted thermal radiation emitted by the optics system. In one embodiment, a high resolution CCD camera can be used with the low temperature detector array (16) and camera with a computer-controlled mirror providing emitted light to both cameras.

Description

Use has the high sensitivity thermal radiation detector of the emission-type microscope of room temperature optical system

The cross reference of related application

The sequence number that the application requires on April 4th, 2002 to submit to is no.60/370,128 common unsettled provisional application, and its content is included in this as a reference.

Technical field

The present invention relates generally to the light emission-type microscope that in semiconducter device testing, uses.

Background technology

Defective in the semiconductor chip presents the LED effect and sends the cold emission that wavelength is 0.4 μ m～1.3 μ m.The wave-length coverage that legacy transmission type microscopic examination is sent from semiconductor chip is the radiation of 0.4 μ m～1.1 μ m, and the defective that has been used for searching semiconductor.

Summary of the invention

The present invention is intended to extended detection range with emission-type microscope to 2.2 mu m ranges.By detecting longer wavelength, emission-type microscope can detect the heat of sending from semiconductor chip.This obviously needs to use the detector until 2.2 mum wavelength sensitivities, and use can be at the lens of this scope work.Yet, such emission-type microscope become also adjust the telescope to one's eyes and other optics in the parasitic thermal radiation sensitivity sent, and these parasitic heat signals greatly reduce the sensitivity of system.Existing infrared microscopy mirror system is attempted to overcome this problem by one of dual mode.At first cool off optics shines detector with minimizing parasitic thermal radiation.Perhaps heating chip makes it send more thermal radiations, and this has overcome the parasitic heat signal that sends in the optical system.In either case, the chip temperature that is detected will be much higher than the temperature of optics.The cooling optical system is a stubborn problem, and this is because it causes cohesion moisture and formation mist on optics, so all parts that are cooled must be placed in the vacuum chamber of sealing.The availability that this seriously restricts the kind of the optics that can use and has limited system.The heating semiconductor chip is also inconvenient, and the analysis of having slowed down, and this is because must make up special permanent plant to heat the cause of every type chip.Heating chip also can change the characteristic of chip or defective, and this should be avoided.

What the present invention was primarily aimed at is microscopical designing technique, this technology reduces the parasitic thermal radiation that shines on the thermal detector element, and allowing a kind of microscope of structure, it can at room temperature detect the thermal radiation of sending from chip, and does not need to cool off most optics.This allows to use in practice the optical arrangement of any complexity, and has improved the quality of the analysis that can carry out greatly.

The invention provides a kind of microscope, comprising:

The low temperature detector array, the output and the above proportional signal of telecommunication of thermal radiation of 1.5 μ m;

A plurality of object lens are used for device the thermal radiation imaging and the projection intermediate image that send by test, and different object lens have different magnification ratios;

Projection lens system is used for that intermediate image is imaged on detector array and lists;

As the cold aperture of the limiting aperture of system, between projection lens system and detector array, described cold aperture is absorbed in the unwanted thermal radiation of sending in this optical system, and this cold aperture is positioned at the place, plane with the outgoing diaphragm conjugation of object lens; The size in described cold aperture is equal to or slightly less than the objective aperture size;

Cold light is learned filtering apparatus, and stopping does not need wavelength imaging on described array;

Be connected to the processor device of low temperature detector array, be used for removing denoising and enhancing signal; And

Be connected to the display unit of this processor device, be used for showing the heat picture of the heat that the device of test sends.

Description of drawings

Fig. 1 is the schematic diagram of emission-type microscope.

Fig. 2 shows has the emissivity microscope of being enclosed the optical tube in the cold house.

Fig. 3 shows the microscopical embodiment of the emissivity with room temperature optical system.

Fig. 4 is the curve of heat flux with respect to wavelength.

Fig. 5 shows the relation of picture contrast relative wavelength.

Fig. 6 is the curve of the transmittance of silicon with respect to wavelength and doping.

Fig. 7 shows the light emission camera of cooling and the combination of high resolution CCD camera.

Embodiment

Consider now the simplest microscopic system, it has the detector array of micro objective, plain optics tube and cooling, and as shown in Figure 1, wherein the radiation 12 that will be sent by the device 14 in detecting of object lens 10 projects to detector array and lists.In this structure, also will shine detector array from the thermal radiation of the wall of optical tube 20 and list, and will reduce the sensitivity of heat detected from device (DUT) chip that is in detection.

Fig. 2 shows a kind of structure, and wherein identical optical tube 20 is enclosed and comprised in the cold house of detector array.If at room temperature 100 ℃ on the wall of this pipe, the thermal radiation of sending from wall is with insignificant.In order to observe chip, need change object lens continually with the magnification ratio that changes.Therefore, use transparent window, and outside the vacuum chamber of lens before this transparent window, and the size of transparent window is bigger slightly than the maximum lens that need.Yet the lens under the room temperature still can send some parasitic thermal radiation, and this parasitic thermal radiation will impinge upon on the detector and limits sensitivity.Diaphragm (stop) that thermal source is lens or effective diaphragm (aperture diaphragm) (being used to limit the axial cone of the energy that lens collect).In addition, the back side and the mechanical mounting thereof from the microscope lens case sends a certain amount of heat.Because it is unpractiaca increasing luminaire in the cold house, so this structure can not have straight-through lens illumination configuration.Therefore, this system must adopt outside off-axis illumination system, and this will produce relatively poor picture quality.

The huge lens configuration of traditional emission-type microscope (non-heat) use is to observe semiconductor chip under low magnification ratio.Huge lens configuration comprises two big back-to-back camera lens, and even provides high-NA (〉 0.2 under low magnification ratio).On the contrary, the numerical aperture very little (0.05) of conventional microscope lens under low magnification ratio lens.In non-heat emission type microscope, the conversion between huge lens configuration and the microscope lens needs vehicularized sliding surface to shift out with huge lens subassembly shift-in with the Optics in Microscope assembly.This is difficult to realize in the vacuum chamber of sealing.If use big huge lens, the transparency window opening needs to strengthen, and this makes and be in microscopy mode following time when system, enters a large amount of parasitic thermal radiation.Therefore the size of huge lens will be subjected to the restriction of microscope lens size, and this numerical aperture with huge lens is restricted to a little number.Typically, the diameter of microscope lens is 30mm, and the diameter of big huge lens may be 60mm or bigger.Therefore,, will need the transparent window of 60mm diameter, this means when under microscopy mode, using, will allow a large amount of parasitic thermal radiation in order to hold the huge lens of big 60mm.

The present invention has avoided all constraints of the microscopic system of cooling.As shown in Figure 3, use the room temperature optical microscopic system to come the intermediate image of the tested device of projection 14.Projection lens system 30 is imaged on this intermediate image on the low temperature detector array 16 then.Be inserted with the diaphragm 32 of cooling between projecting lens and this array, it is a thin disk with roundlet opening, and this roundlet opening serves as effective diaphragm.The diaphragm of this cooling is made by the sandblast anodised aluminium below room temperature usually.In a preferred embodiment of the invention, the diaphragm of this cooling is positioned at the vacuum chamber 34 of the photomoduel of cooling.If the diaphragm of this cooling is significantly less than room temperature, it drops on absorption the thermal-radiating 90-95% of all incidents on the opaque section.The present invention uses it to absorb unwanted parasitic thermal radiation.If the diaphragm of this cooling is positioned at certain concrete position, it will almost absorb all parasitic thermal radiation of being sent by the wall of optical system, effective diaphragm and object lens case, and the opening of the diaphragm by this cooling and to drop on the radiation that detector array lists just only be the radiation of being sent by the device in detecting.

The position of the diaphragm 32 of this cooling is crucial.The diaphragm of this cooling will be placed on the plane of image that projecting lens forms objective aperture.In this position, the performance of the diaphragm of this cooling is desirable.If the diaphragm that should cool off is away from this position, effect can weaken greatly.Concrete is, and image will seriously swoon reflects (vignetted), and promptly image intensity is away from the center of image.Reason is to absorb a large amount of thermal radiations of the device (especially from device edge) in the self-test on the diaphragm of this cooling.The sensitivity of system is with impaired.

In the common system based on microscope or camera, the limiting aperture of system will be the aperture of object lens 10.Yet the aperture of the diaphragm that cools off among the present invention is the limiting aperture.In the diaphragm of cooling, the size in aperture is crucial.Preferred this aperture should be the limiting aperture of system, and promptly it should have same size or smaller with the aperture of object lens.In this case, the radiation meeting of having only the device in the test to send shines on the detector.The aperture of objective line will be projected on the surface of diaphragm of cooling, and the surface of the diaphragm of this cooling is around the aperture of the diaphragm of cooling, and its all radiation absorb the diaphragm that is cooled.To focus on the position before the diaphragm of cooling from the radiation of microscopical wall 20, the diaphragm of this cooling will absorb the major part of this radiation.

In a system, the required aperture of huge lens and microscope lens differs widely.The aperture of microscope lens is corresponding to the f16 aperture.Yet the aperture of huge lens is corresponding to the f2 aperture.The present invention uses swiveling wheel that a plurality of apertures are placed in cold camera chamber.

Must be noted that intermediate image and projection lens system are the key elements of the correct work of the present invention.For example, the diaphragm that only increases cooling in the system of Fig. 1 can not be worked satisfactorily.Formed image will seriously be swooned to be reflected, and this is because the signal that sends from chip center absorbs the diaphragm that is cooled.The signal that arrives array edges weakens greatly.The radiation of sending from warm wall will be partially absorbed rather than all absorptions.Therefore, do not have projecting lens, the performance of system is degenerated greatly.

In case the diaphragm of cooling is installed between projecting lens and image sensing array, the performance limitations of most of warm optical system has just disappeared.Now warm huge lens configuration becomes practical, and the large-numerical aperture of big camera lens can't influence its performance.Similarly, it is practical that straight-through lens illumination scheme becomes now, and this is that the cooling chamber of luminaire being put into sealing is unpractiaca because use cold light to learn system.

We find that it also is desirable using the short wavelength of thermal spectrum.That heat picture and hot-probing use traditionally is long wavelength 3-7 μ m.Reason is that thermal radiation increases exponentially with the long wavelength, sees Fig. 4.Therefore, easier use long wavelength comes constructing system.Yet the spatial resolution of system is subjected to the restriction of known limit R=wavelength/N.A, and wherein N.A is the numerical aperture of optical system.

Therefore use the long wavelength to reduce the resolution of system significantly, and, need to reduce microscopical resolution more and use shorter wavelength along with the reducing fast of characteristic size on the chip.In addition, wavelength is short more, and heat contrast (variation of the heat flux of every degree C) also improves, and sees Fig. 5.The Another reason of using the short wavelength is because need the heat of observation from backside of silicon chips.Present chip has the multiple layer metal superficial layer until chip before chip, outside thermal radiation is captured by these metal levels.If observe chip from the back, with regard to the energy head it off, this is because silicon is transparent to the wavelength more than the 1.1 μ m.But the long wavelength, silicon begins to absorb the thermal radiation of sending, and as shown in Figure 6, and the parasitic heat emission that silicon sends begins to increase.Therefore, weakened the long wavelength by the thermal signal that defective produces, and increase from the parasitic heat signal of silicon.Therefore, in fact it is desirable to use short as far as possible wavelength.

It is desirable to use the filter of block long wavelengths.Improved sensitivity greatly in the aforesaid innovation that reduces in noise and the input.This allows us to use than common shorter wavelength.As shown in Figure 7, we use to from radiosensitive MERCAD (HgCdTe) array 40 of 0.8 μ m-2.6 μ m as transducer, and use cold filter 42 to stop that unwanted wavelength acts on this array.Our experiment shows that low heat flux situation (for example, at the chip defect that is higher than under the room temperature several years situation) preferably uses the filter that stops the radiation more than the 2.2 μ m to detect.Yet when chip was warmmer, by stopping the thermal radiation more than the 1.8 μ m, heat flux was many more, and hot contrast and temperature resolution that we obtain are good more.Yet the filter of room temperature sends heat, so filter also must be cooled, and is placed in the sealing camera chamber 44 of cooling.When being powered, can also send chip from the cold recombination radiation of 0.4 μ m to 1.4 μ m.We find must stop short wavelength (＜1.5 μ m) in order correctly thermal defect and pop-up point to be distinguished.Therefore we use three types filter: be used for the 1.6-2.2 μ m bandpass optical filter of low discharge thermal defect, be used for the 1.6-1.8 μ m bandpass optical filter of high flow capacity thermal defect, and the 1.5 μ m optical low-pass filters that are used for the pop-up defective.These filters are placed on the rotation circular wheel.In the first embodiment of the present invention, these cold filters are installed on the same wheel with different apertures, but we plan to use separately aperture wheel and filter wheel in ensuing embodiment.

Except forming heat picture, also need to form the high-resolution reflected light image (illumination image) of chip.Needs to the high-resolution reflected image that uses the CCD camera help with reference to the heat picture with characteristic on the chip surface.This computer double exposure by heat picture and reflected light image is realized.The resolution of reflected light image needs maximization.This realizes by using short as far as possible wavelength to form reflected light image.The wavelength sensitive of 40 couples 0.8 μ m-2.5 of HgCdTe array μ m.Therefore, we use the wideband light source of similar standard tungsten filament to shine chip, and use filter to remove the long wavelength.Another optimizes the illumination that reflected light image method for quality is to use straight-through lens, but not in infrared thermography the side lighting scheme of standard.The illumination of straight-through lens is standard in laboratory microscope, but not be used in thermal imaging system and the infrared system.Because from the heat emission of luminaire parts, straight-through lens illumination is unpractiaca on the camera of cooling.Our diaphragm by making cooling has been eliminated the heat emission from the luminaire sidewall in camera.Speculum 48 also will reflect the parasitic heat of being launched, and for it is minimized, we do not use the speculum of local silver-plating, and use existing common flint glass speculum.For the parasitic heat by the illuminator mirror reflection is minimized, the reflectivity of this speculum in 1.8 μ m to 2.1 mu m ranges must be minimized.Common flint glass has 7% reflectivity, but we are are just researching and developing special coating, and it will reduce the reflectivity in this spectrum.

The HgCdTe detector array is made by RockwellScient ific, and has 256 * 256 form, and this array is at the commercial PICNIC array that is known as.We also use the low signal detection. electronics by Rockwell Scientific and our common research and development.Signal from this electronic installation is fed into the frame grabber that is present in the personal computer 50.We handle the image that is obtained by frame grabber in several modes.At first, image is read out repeatedly also by average, so that electronic noise is minimized.Under the situation of low discharge, read output signal 8 times is also average with it.Under high traffic condition, repeatedly do not read.Secondly, the different pixels in the detector array has different gains and deviation.We use the processing that is known as flat field correction (flat fielding) in image processing techniques that these heterogeneities are proofreaied and correct.It is included in will gain after the manufacturing system and straggling parameter is stored in the computer, and the necessary mathematical operation of utilization.

The processed image that on monitor 52, shows all acquisitions.In a preferred embodiment of this system, computer display serves as display monitor, and processed images shows in window.All images also back up on hard disk, and system keeps the Disk Backup of 8 nearest images.

We have created the thermal flux map of being sent by chip now.Yet we at first make this figure under the situation of chip outage, and energising repeats this figure again to chip then.Obtained the difference of two figure then, this is because the extra radiation of being sent by chip that chip energising is caused.This is enough for the hot stain on the chip (hot spot).Can use a computer this thermal flux map double exposure on the reflected light image of chip or on the CAD figure.This image by double exposure helps the hot stain in location on chip.Under more and more higher objective lens magnification, repeat this operation, up to the image that obtains high power to help the location defective.The user use the most at last scanning electron microscopy in addition higher magnification ratio (because electron microscope has higher magnification ratio) observe this position down, be hopeful to obtain high-definition picture about the defective that causes hot stain.Use for some, what must obtain is the hygrogram rather than the thermal flux map of chip.Reason is that chip comprises different materials, and the heat emissivity coefficient of these materials differs widely.The heat flux that object sends is the function of the heat emissivity coefficient of temperature and this object.For example, the heat emissivity coefficient of aluminium is very low, be 0.05, and aluminium oxide has 0.80 high-heating radiation coefficient.For the identical temperature that raises, the heat flux that aluminium oxide sends will be 19 times of aluminium.In order to obtain hygrogram, must make more images by the heat emissivity coefficient of each pixel on the computing chip and handle.This thermal chuck (hot chuck) by the control that uses a computer comes the heat flux under two different temperatures on each pixel of measured chip to finish, the heat emissivity coefficient of this each pixel of COMPUTER CALCULATION thus, and this information is used to change thermal flux map into hygrogram.

The HgCdTe that we use now is limited to 256 * 256 forms.Can use larger sized array, but cost is an astronomical figure.Modern chips needs the highest possible resolution.We obtain this effect by the technical scheme of using two cameras.Except HgCdTe, we also use the ccd array 46 of the astronomical grade of being made by Kodak, and its resolution is 1317 * 1035.The high-definition picture that is obtained by CCD camera 46 also can be used as illuminated image.These two optical channels that have a camera transfer capability by use dispose to be realized.Projection lens system comprises two back-to-back f2 lens, and golden speculum is arranged therebetween.HgCdTe camera and based on the shared first projecting lens element of the camera of CCD, but the second independent lens element had.In order to be transformed into another, need mobile mirror 48 to make it be transformed into another from a position from a camera.This computer-controlled motor by pinpoint accuracy is finished.The spectral response of CCD 46 is from 0.4 μ m～1.1 μ m.Because the recombination luminescence defective is less than 0.8 μ m, so its spectral response has compensated the response of HgCdTe array.So ccd sensor not only can be used to form the emission image but also can be used to form illuminated image.This system has the software calibration feature, and it is mapped to the CCD photographing space in the photographing space of HgCdTe array.This allows our the illuminated image and the emission image double exposure that uses HgCdTe to take with the shooting on the CCD camera.In a word, the configuration of double camera has embodied the advantage of HgCdTe array and astronomical grade CCD best.

Though described the present invention with reference to specific embodiment, these descriptions are to explanation of the present invention.Under the situation of the spirit and scope of the present invention that do not break away from claims and limited, can make various variations.

Claims (19)

1. microscope comprises:

The low temperature detector array, the output and the above proportional signal of telecommunication of thermal radiation of 1.5 μ m;

A plurality of object lens are used for device the thermal radiation imaging and the projection intermediate image that send by test, and different object lens have different magnification ratios;

Projection lens system is used for that intermediate image is imaged on detector array and lists;

As the cold aperture of the limiting aperture of system, between projection lens system and detector array, described cold aperture is absorbed in the unwanted thermal radiation of sending in this optical system, and this cold aperture is positioned at the place, plane with the outgoing diaphragm conjugation of object lens; The size in described cold aperture is equal to or slightly less than the objective aperture size;

Cold light is learned filtering apparatus, and stopping does not need wavelength imaging on described array;

Be connected to the processor device of low temperature detector array, be used for removing denoising and enhancing signal; And

Be connected to the display unit of this processor device, be used for showing the heat picture of the heat that the device of test sends.

2. according to the microscope of claim 1, wherein cold light is learned the thermal radiation that filtering apparatus stops that 2.2 μ m are above.

3. according to the microscope of claim 1, this microscope has by the cold light of the radiation between 1.5 μ m～2.2 μ m learns filtering apparatus.

4. according to the microscope of claim 1, wherein cold light is learned the thermal radiation that filtering apparatus stops that 1.8 μ m are above.

5. according to the microscope of claim 1, this microscope has by the cold light of the radiation between 1.5 μ m～1.8 μ m learns filtering apparatus.

6. according to the microscope of claim 1, this microscope has the movable fixture that allows a plurality of cold light filtering apparatus.

7. according to the microscope of claim 6, wherein this detector array is to the non-theramal radiation sensitivity from 0.8 μ m to 2.6 μ m.

8. according to the microscope of claim 7, have the straight-through lens illumination device that allows the high-resolution reflected light image, this high-resolution reflected light image can double exposure on the heat picture that obtains by identical object lens.

9. according to the microscope of claim 1, has the movable fixture that allows a plurality of cold apertures, for each object lens is optimized described cold aperture.

10. according to the microscope of claim 9, wherein a plurality of object lens of claim 1 comprise and are used to obtain the global view of device and have huge lens greater than 0.2 high-NA, and the microscope lens of high power that is provided for observing the subregion of device.

11. according to the microscope of claim 10, wherein the leviathan mirror comprises two back-to-back lens that have greater than 0.2 high-NA.

12. according to the microscope of claim 11, this microscope has the thermal-radiating cold light that stops greater than 2.2 μ m and learns filtering apparatus.

13. according to the microscope of claim 12, wherein cold light is learned filtering apparatus by the radiation between 1.5 μ m～2.2 μ m.

14. according to the microscope of claim 11, this microscope has the thermal-radiating removable cold light that stops greater than 1.8 μ m and learns filtering apparatus.

15. according to the microscope of claim 12, this microscope has by the cold light of the radiation between 1.5 μ m～1.8 μ m learns filtering apparatus.

16. according to the microscope of claim 10, this microscope has the movable fixture that allows a plurality of cold light filtering apparatus.

17. according to the microscope of claim 16, wherein detector array is for the non-theramal radiation sensitivity from 0.8 μ m to 2.6 μ m.

18. according to the microscope of claim 17, have the straight-through lens illumination device that allows the high-resolution reflected light image, this high-resolution reflected light image can double exposure on the heat picture that obtains by identical object lens.

19. according to the microscope of claim 10, this microscope has the movable fixture that allows a plurality of cold apertures, for each object lens is optimized described cold aperture.

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