APPARATUS AND METHOD FOR DETECTING AN AMOUNT OF DEPOLARIZATION OF A LINEARLY POLARIZED BEAM

Abstract: The present invention relates to an apparatus and a method for detecting an amount of depolarization of a linearly polarized beam transmitted by a birefringent medium in the direction of the optical axis thereof. The apparatus comprises a first beam splitter (7) for separating an non-axis portion (2) of said linearly polarized beam into the orthogonal components (3, 4), two photodetectors (8, 9) for detecting each components (3, 4), a second beam splitter (12) for separating an off-axis portion (11) of said linearly polarized beam into the orthogonal components (15, 16), wherein said second beam splitter (12) is disposed off-axis of the incident linearly polarized beam (1), a second set of photodetectors for detecting the components (15, 16) separated by said second beam splitter (12), and subtracting device (17) for subtracting the signals received by the second set of photodetectors (13, 14) from the respective signals received by the first tow photodetectors (8, 9).

Description: Description Apparatus and method for detecting an amount of depolarization of a linearly polarized beam The present invention relates to an apparatus and a method for detecting an amount of depolarization of a linearly polarized beam transmitted by a birefringent medium. This apparatus and method, respectively, can conveniently be used for detecting internal stress fields inherent to a semiconductor wafer which normally is isotropic and, thus, does not exhibit any birefringence. As the internal stress fields are induced by dislocations and slip lines, the detection re- suit will be a measure for the amount of dislocations and slip lines and, thus, will be a measure of the quality of the wafer and the resulting semiconductor devices. The linearly polarized beam is preferably transmitted by the birefringent medium in a direction perpendicular to the surface of the bi- refringent medium. Usually, the manufacture of semiconductor devices involves various steps of wafer processing and, in particular, thermal processing steps during which the wafers are mechanically stressed. As a consequence, dislocations, slides and slip lines in the crystal are generated which will for example cause leakage currents and, thus, tremendously deteriorate the device characteristics. Accordingly, it is necessary to assess the amount of dislocations and slip lines and, based on the result, reject those wafers having an amount of dislocations and slip lines which exceeds a previously determined threshold. The degree of dislocations and slip lines can be detected by a method called Scanning Infrared Depolarization. The principle of slip line detection by Scanning Infrared Depolarization is based on the fact that linearly polarized light - K 0 3 rr rr Hi H- PJ a s- ^ Q 3 3 Φ IQ CQ Ω H Φ rt Hi Φ μ- Ω Crj φ rt CD Ω rt ra o a- Φ tr μ- t CQ a Φ tr P> Φ J Φ φ X μ- J tr a tr φ X a 0 rt <; tr TJ 0 H s; φ pi Φ O φ . a H- Hi ti ω μ. J TJ <! μ- P Φ Ω φ ii rt iQ 3 J Φ φ Φ PJ 3 Φ ^ li rr Ω Φ Φ J CQ Φ φ TJ CQ Φ Φ li Φ r a Ω TJ a 0 <! ω h- 1 0 Φ Φ Ω CQ F t tr i ti CQ Φ CQ S a PJ *< * CQ Φ CD rt 0 CD rt Φ H- <! H- Hi Ml TJ o φ μ. μ- P. rt tr J rt O Pi ^ a 3 H CQ Φ I ) Hi H 3 s cπ PJ Pi I- 1 Φ 3 J μ- H Φ Hi fi tr o g rt Φ μ- 0 a i a 4 rr φ Φ TJ T • 3 Φ 3 φ a tr Φ rr Φ <! P. rt μ- P. S 3 O a rt a PJ rr rt tr Ω CQ 0 PJ <J Pi φ a iQ φ ^ CQ S H Φ μ- tr ii μ- φ Φ rt r rt H- Φ rr φ a a H- # J μ- Φ a rt 0 CD CQ Φ a Φ φ Hi ti rt rt Q Φ ^ 4 Ω tr a Φ PL. rr Φ Hi Ω a rr J 0 • rt Φ μ- O CQ Hi Hi • H tr - P. Φ a j PJ Φ <i rr a ω h rt Φ 0 H P. tr Hi a ω Ω J r ii Φ • : φ Ω PJ H PJ φ Φ Φ rr . 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Ω rt CD O tr rr Φ O TJ iQ Φ tr rr a J μ- 0 3 Hi H CQ Ω PJ Hi PJ H Φ μ- P 3 Φ Mi ^ ^ tr o H Φ Φ PJ 3 Ω rt μ- φ μ- O rt φ CQ IQ Pi a H J rt μ- O φ TJ i a ^ PJ Ω I— ■ O φ rt ii ISI Hi 3 μ- ii φ μ- J H Mi H Φ ii CQ tr Φ Φ CQ rr CQ a CQ rt Φ J μ- TJ O a S H Φ PJ μ- rt (-> p. o rr a O Ω TJ P. Φ a rt Φ O a 0J P. CQ μ- rr * t CQ rt tr H- Φ Φ rr rr <T\ H- H 0 c rt Pi Ω μ- a CQ ii μ- rt > 0 CQ tr 0 TJ rt O Ω tr . TJ H- Ω tr φ 0 P. Φ Φ Hl Hi tr PJ Ω tr a tQ φ rr O φ ω TJ rr rt Φ tr a • a O Ml ii a Ω Φ PJ ~> ii O Ω p. l- I- 1 - 0 rt 0 • : a rt H φ O O 1 o φ a rr μ- rt ii H- I Φ H - P CQ ϋ 3 μ- μ, $ 1 PJ 0 1 tr PJ a K PJ PJ 1 a 1 Hi i o φ l Φ 1 φ 1 1 1 ii ii lines but it is also caused by scattering of light. Due to the imperfectness of the wafer surface such as surface roughness or impurities, the polarized light is scattered. In particular, patterns from the semiconductor device such as trenches and other structures will cause light scattering. As a consequence, the polarized light will change its polarization direction or will even become unpolarized. The momentary parallel and perpendicular components of this scattered light will then be detected by both detectors. If the amount of scattered light exceeds the true signal by magnitudes, as it is in particular the case when the wafers are already patterned, the amplifiers start to work nonlinear or are driven into saturation. In both cases the amplifiers be- come blind for the weak signal representing the true depolarization effect. The difficulties arising during a scanning infrared depolarization measurement can partially be avoided when the easure- ment is performed before structures such as trenches which will largely cause light scattering are patterned. However, since during the trench formation also heat processing steps are performed, the amount of dislocations and slip lines will still increase during and after the trench formation. Accord- ingly, a measurement before the trench formation will cause false measurement results. Moreover, such a measurement will avoid scattering due to trenches. However, scattering due to impurities or surface roughness cannot be suppressed. It is therefore an object of the present invention to provide an improved apparatus and an improved method, respectively, for detecting an amount of depolarization of a linearly po- larized beam. w ω t DO μ> H in σ in o σi O in μ- a Ω μ- P. φ a rt μ> μ- a Φ PJ H > J o J ϋ μ- rsi φ P. tr φ J 3 detecting the orthogonal components separated by said second beam splitter by means of a second set of photodetectors; and subtracting the signals received by said second set of photodetectors from the respective signals received by said first set of photodetectors. Preferably, the linearly polarized beam is transmitted in a direction perpendicular to the surface of the birefringent medium and wherein the first portion of said transmitted beam is an on-axis portion and the second portion of said transmitted beam is an off-axis portion. In addition, the present invention provides an apparatus for determining an internal stress field in a semiconductor de- vice comprising a laser device emitting a linearly polarized infrared laser beam and an apparatus for detecting an amount of depolarization of said linearly polarized infrared laser beam after said laser beam has been transmitted by said semiconductor wafer, wherein said apparatus for detecting an amount of depolarization of said linearly polarized laser beam is as defined above. Furthermore, the present invention provides a method for determining an internal stress field in a semiconductor wafer comprising the step of irradiating a linearly polarized infrared laser beam into said normally isotropic medium and the method of detecting an amount of depolarization of said linearly polarized laser beam after said infrared laser beam has been transmitted by said semiconductor wafer wherein said method of detecting an amount of depolarization of said linearly polarized laser beam is as defined above. The present invention is based upon the understanding that since the effect of light scattering cannot be suppressed nor it can be avoided that the scattered light will enter the photo detectors, this portion of light has to be subtracted L ) to to μ» μ 1 in o in o in o in a μ> 3 J rt H P. 0 μ- rt PJ CQ CQ H rt rr μ- in O μ- H •A ^ύ P. μt μ- Ml Φ lO μ- X tr a PJ μ- TJ a tr a TJ TJ a 0 tr N • P. 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O rr PJ rt a N t μ- 0 Hi PJ < Φ a φ tr Hi 01 rt μ- μ- t Ω O 01 Φ 0 ti rt LQ rt J Φ μ- CQ a H J rt μ- 0 Hi P ii ii Φ a 03 ^ H r 0 Hi Ω P. Hi rt Φ 4 rt CQ 3 TJ ii μ- 01 LQ a 1 tr Φ Φ μt Ω LQ μ- μ- a J Hi μ- P. P. Φ μ- TJ CQ tr PJ P a rt μ- tr PJ rt TJ TJ a rt 03 Pi LQ 0 Ω P. rt 1 Ω Φ O CD P PJ μ- o a PJ rt O LQ PJ X ϋ ii ϋ o P. φ φ tr a ii Φ rt rt J " < a P ii ti LQ rt LQ rt Φ a LQ X μ- PJ Φ Φ TJ H Φ a rt rr Φ rt Φ tr X φ J μ- Ω Ω μ- LQ a O μ- μ- ϋ rt PJ J μ- CQ Ω CQ CD PJ CQ <i 03 Φ 0 rt Φ li Φ μ- 01 01 Ω PJ 3 tr rt Φ J Pi Φ a 0 a tr μ- CQ rt Φ φ H μ- μ- Ω J Hi Φ Ω Φ ϋ CD Φ Φ a Φ μ- LQ a J Φ CQ a <! μ- a a r ii Ω rt rt 01 rt P. Φ O H a J PJ φ • <; rt < φ a rt rr μ- Φ φ μ- μ- rt •<; O P. ii CD μ- CQ CD CQ Ω Φ PJ 0 TJ 3 PJ Φ ti LQ μ- μ- Ω CQ <! rt a 3 4 ii H 1 μ- μ- - tr a P 01 J Ω Hi CQ PJ μ- 3 H rt a a P P Hi φ LQ Φ Q μ- tn μ Φ ϋ LQ μ rt P a rt ω rt ii rt TJ rt μ- P. LQ LQ μ. O rt LQ φ μ- PJ P Φ li CQ TJ 0 φ rt ϋ PJ tr μ- Ω Φ PJ rt ii 0 rt rt tr μ- tr H tr Φ μ- Ω PJ a Ω a 3 Ω 3 Φ P TJ tr li Ω < rt rt ϋ μ- a tr H Φ Q rr O rt rt a Φ P Ml a ϋ ϋ Φ 01 0 Φ 01 tr Hi PJ μ- a Ω Hi Φ 01 φ μ- μ- Ω Φ 1 tr Φ L a CQ Φ PJ J Ϊ > μ- TJ tr a Q P rt CQ a 01 01 J 0 PJ PJ li ii rt tr PJ CQ PJ P. 3 LQ φ Ω Φ rt tr tr 0 CQ Φ φ 01 O μ- tr ii Ω O 0 0 tr μ- 1 Φ Hi rt a ii Q Φ PJ • Φ Ω CQ J 3 rt Hi 1 a O a a 1 CQ ii CQ Φ Φ 3 a Mi φ Φ P. Φ a Hi tr μ- 01 Φ 3 1 1 Hi Φ rt o μ- P. 3 1 1 Mi 1 such as trenches. Thus, this measurement will be more exact and substantial than the usually performed measurements before patterning the device structures. Moreover, the effect of impurities and surface roughness can now advantageously be suppressed. As a consequence, quality evaluation can be performed at much higher precision. In order to ensure the laser beam to be transmitted by the semiconductor wafer, its wavelength has to be appropriately chosen. In particular, an infrared laser beam emitted by a semiconductor laser such as a GaAs laser is normally used. Usually, an experimental setup is chosen in which the laser device is fixed beneath the semiconductor wafer while the de- tectors as well as the beam splitters are fixed above the semiconductor wafer. The semiconductor wafer is rotated and laterally shifted so that the entire semiconductor wafer is scanned. When determining the off-axis angle of the second beam splitter, it has to be born in mind that the scattering angle of light is not isotropic but rather lobar. Accordingly, in order to detect the portion of the scattered light which is also included in the on-axis portion, it is necessary that the displacement angle of the second beam splitter with respect to the optical axis to the semiconductor wafer be very low. However, a minimum angle is determined by the geometric arrangement of the apparatus . The present inventors found out that an angle of 3° to 5° will provide optimum results. The accuracy of the measurement as described above can be remarkably improved by using a lock-in amplifier. In this case the laser beam irradiated into the semiconductor wafer is chopped with a predetermined frequency. The lock-in amplifier will then only take into account those signals from the two sets of detectors which signals have this predetermined frequency. The lock-in amplifier subtracts the signal supplied by the set of off-axis detectors from the respective signals supplied by the set of on-axis detectors. The present invention is in particular useful when wafers having highly scattering surfaces or various wafer topographies have to be examined. Although the present invention has been explained with respect to the examination of semiconductor wafers, it is ap- parent to those skilled in the art that it can be applied to any kind of birefringent materials, in particular normally isotropic media, having rough and highly scattering surfaces, Incident laser beam transmitted on-axis laser beam on-axis vertical component on-axis parallel component semiconductor wafer slip line first beam splitter first photodetector for the vertical component first photodetector for the parallel component trenches off-axis portion of the laser beam second beam splitter second photodetector for the parallel component second photodetector for the vertical component off-axis parallel component off-axis vertical component subtracting device high gain amplifier infrared laser device

Claims: Claims 1. An apparatus for detecting an amount of depolarization of a linearly polarized beam (1) transmitted by a birefringent medium (5) , comprising: a first beam splitter (7) for separating a first portion (2) of said transmitted beam into orthogonal components (3, 4) ; a first set of at least two photodetectors (8, 9) for detecting a respective one of the orthogonal components (3, 4) separated by said first beam splitter (7) ; a second beam splitter (12) for separating a second portion (11) of said transmitted beam into orthogonal components (15, 16) , wherein said second beam splitter (12) is disposed off- axis of the incident linearly polarized beam (1) ; a second set of at least two photodetectors (13, 14) for detecting a respective one of the orthogonal components (15, 16) separated by said second beam splitter (12) ; and a subtracting device (17) for subtracting the signals received by said second set of photodetectors (13, 14) from the respective signals received by said first set of photodetectors (8, 9) . 2. The apparatus according to claim 1, wherein the linearly polarized beam (1) is transmitted in a direction perpendicu- lar to the surface of the birefringent medium (5) and wherein the first portion (2) of said transmitted beam is an on-axis portion and the second portion (11) of said transmitted beam is an off-axis portion. 3. The apparatus for detecting an amount of depolarization of a linearly polarized beam according to claim 1 or 2, wherein said second beam splitter (12) is disposed at an angle of 3° to 5° off-axis. 4. The apparatus for detecting an amount of depolarization of a linearly polarized beam according to any of claims 1 to 3, to l CO to μ> μ> in o in o in o in 01 Pi PJ Φ μ- rt Pi Φ Ω Mi rt μ- 0 μ ii CD CQ rt , — . 03 μ> Φ Id rt 0 H Mi ( ■ — J tr Ml 0 ii r 0 0 3 P. Φ rt rt tr Φ Φ Ω rt ii 0 Φ ii 01 CD TJ Φ * ~. Ω 00 rt - μ- <! KΩ φ • — ' • 01 μ- LQ a PJ CQ ii Φ Ω φ μ- < φ Pi tr ■ - 8. The method for detecting an amount of depolarization of a linearly polarized beam according to claim 7 wherein the linearly polarized beam (1) is transmitted in a direction perpendicular to the surface of the birefringent medium (5) and wherein the first portion (2) of said transmitted beam is an on-axis portion and the second portion (11) of said transmitted beam is an off-axis portion. 9. A method for detecting an amount of depolarization of a linearly polarized beam according to claim 7 or 8 , further comprising the step of amplifying the signal which results from the step of subtracting. 10. A method for determining an internal stress field in a semiconductor wafer (5) comprising the step of irradiating a linearly polarized infrared laser beam (1) into said semiconductor wafer (5) and the method of detecting an amount of depolarization of said laser beam after said linearly polarized laser beam (1) has been transmitted by said semiconductor wa- fer (5) wherein said method of detecting an amount of depolarization of said linearly polarized laser beam is as defined in any of claims 7 to 9.

Inventors: KUERNER WOLFGANG, KUERNER, WOLFGANG
Applicants: INFINEON TECHNOLOGIES SC300, KUERNER WOLFGANG, INFINEON TECHNOLOGIES SC300 GMBH & CO. KG, KUERNER, WOLFGANG
Application year: 2001
Main classification: G01N21
IPC codes: G01J4, G01L1, G01N21