Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyLaser Scanning Thermomechanical Imaging ofMicroelectronic DevicesS. Grauby, A. Salhi, J-M. Rampnoux, W. Claeys, S. DilhaireCentre de Physique Moléculaire Optique et Hertzienne,Université Bordeaux 1, 351, cours de la Libération,33405 Talence cedex, France,s.grauby@cpmoh.u-bordeaux1.fr, tel :33 (0)5 4000 2786, fax : 33 (0)5 4000 6970Abstract- We present a scanning imaging system usinggalvanometric mirrors dedicated to the thermomechanicalimaging of microelectronic devices. Using a classical He-Ne laseras source, it constitutes a scanning thermoreflectance imagingset-up leading to images of reflectivity relative variationsproportional to the device surface temperature variations. Usingan heterodyne interferometric probe, it becomes a scanninginterferometric set-up leading to surface displacement images.Both thermoreflectance and interferometric images arepresented for two samples.I. INTRODUCTIONAs integration density of microelectronic circuits goesincreasing, there is a need for methods able to measure localtemperature variation or surface displacement images atsubmicronic scales with short time acquisition.As a consequence, well-known temperature measurementmethods such as infrared imaging [1-3], liquid crystalsmeasurements or temperature measurements usingmicrometric thermocouples deposited on the surface of thedevice[4] are not adapted to this kind of samples as they offera bad spatial resolution (5 to 10 µm minimum) regarding thedevice dimensions. Moreover, a thermocouple implies acontact with the sample that can damage it or disrupt itsfunctioning. When studying structures as thin as a fewhundreds nanometers, only a Scanning Thermal Microscope(SThM) [5-7] can theoretically reach temperature variationsmeasurements at this scale but this contact method is limitedto low frequency measurements and the nanometer resolutionis not clearly demonstrated on microelectronic devicetemperature mappings [7].Among the optical methods for submicronicthermomechanical mapping, thermoreflectance[8-15] andinterferometry [13,16-19] are useful non contact and noninvasive methods which present a good spatial resolution aslimited by diffraction to the order of magnitude of theilluminating wavelength. Two main approaches ofthermoreflectance and interferometry are currently used:point measurement and imaging techniques. The first ones[8-10,16] use a focused laser source and a photodiode asdetector coupled with a lock-in amplifier. The sensitivitiesare very good but they are time consuming as, usually, thedevice under test (DUT) is moved using a micrometrictranslation stage and a measurement is made for eachposition. In the imaging techniques[11-15, 17-19], the lightsource is not focused, a whole surface of the DUT isilluminated and a CCD camera is used as detector.Unfortunately, the saving of time is obtained to the detrimentof the sensitivity.The originality of the set-up presented in this paper is tocombine both approaches using a fast scanner made of twogalvanometric mirrors, which enable to rapidly sweep afocused laser light on the device under test. Hence, thedetector is made of a photodiode coupled to a lock-inamplifier to reach a good sensitivity.After the description of the experimental set-up, we willpresent an optical image to evaluate its spatial resolutionlimitation. Next, using two different sources, reflectivityvariation and surface displacement images obtained on twosubmicrometric dissipative structures are presented. Finally,we discuss the set-up performances in terms of sensitivityand acquisition time, comparing them with pointmeasurement and CCD imaging technique performances.II. SET-UP AND DEVICE UNDER TESTThe experimental set-up is presented in figure 1. A lasersource is sent on the scanner after passing through apolarizing beam splitter (PBS). The half wave plate (λ/2) isused to adjust the transmitted light intensity. Then, using aninverted microscope, the laser is focused on the sample andreflected towards the PBS. The lens (L 2 ) is used to focus thereflected beam on the galvanometric mirrors. After crossingtwice the quarter wave plate (λ/4), its polarisation haschanged and the laser is deviated by the PBS and focused bythe L 1 lens on the photodetector coupled to a lock-inamplifier. A webcam associated to the L 3 lens enables tolocalize the laser position on the sample.The scanner is made of two galvanometric mirrors: thefirst one, named X, sweeps a line when moved whereas thesecond one, named Y, is vertically moved to create theimage. The scanner driver system [20] creates the threesynchronisation signals necessary for the image: the pixelclock signal, the line signal associated to the X mirror andthe frame signal associated to the Y mirror. Three parameterscan be chosen as independent inputs: the image size, thepixel sampling frequency f p (T p =1/f p is the time spent oneach pixel) and the image resolution (in fact the number ofpixels). Each combination of these three parameters defines aconfiguration. With this system, the pixel clock frequencyrange varies from 2kHz to 500kHz, the image resolutionfrom 60×60 to 500×500 pixels and the image size from20×20µm 2 to 3×3mm 2 .©EDA Publishing/THERMINIC 2008 183ISBN: 978-2-35500-008-9