Selffocussing phase transmission grating for an integrated optical microspectrometer

Sensors and Actuators A 88 (2001) 1±9

Selffocussing phase transmission grating for an integrated optical microspectrometer Dietmar Sandera,*, JoÈrg MuÈllerb a

b

Eppendorf Instrumente GmbH, Barkhausenweg 1, 22339 Hamburg, Germany Technische UniversitaÈt Hamburg-Harburg, Halbleitertechnologie, 21073 Hamburg, Germany Received 3 March 2000; received in revised form 22 June 2000; accepted 5 July 2000

Abstract An integrated optical microspectrometer in slab waveguides is presented for broadband spectroscopy in the visible wavelength range. Monomode silicon oxinitride (SiON) waveguides on silicon substrates are structured to produce a novel selffocussing waveguide transmission grating with a waveguide to air index transition endface. Regarding re¯ection gratings such transmission gratings have fourtimes larger facettes which is linked to reliable fabrication by standard thin ®lm technology as well as to an exceptional high spectral dispersion. Combined with further functional optic elements, a compact microspectrometer is presented for long-term stable operation. The selffocussing and dispersion properties of the transmission grating are developed theoretically by phase matching and evaluated successively with electromagnetic theory. The spectral and spacial condition of constructive interference is generated in the waveguide plane at a focus length of only 13 mm yielding a miniaturized, integrated optical, chirped `grism' Ð grating prism Ð a specially designed planar imaging optic with structured waveguide endfaces results in a compact and monolithical core. The fabrication sequence based on standard thin ®lm technology consists of a single deposition and etching process which allows an economic manufacturing. The realized microspectrometer exhibits high ef®ciency >60% in the ®rst diffraction order and a spectral range of 350±650 nm with a spectral resolution of 9 nm. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Diffraction grating; Integrated optic; Microspectrometer; Selffocussing transmission grating; Silicon oxinitride slab waveguide

1. Introduction Much emphasis has been drawn in recent years to develop different kinds of diffraction gratings for wavelength division devices. On one hand, integrated optics have an outstanding importance in multichannel communication systems due to the planar design ¯exibility: Passive devices like arrayed waveguide gratings (AWG) are developed to produce at high diffraction orders (>10) optical channel separation of less than 2 nm spectral bandwidth at 1.55 mm [1±3]. Joined to monomode optical ®bers, insertion losses less than 2 dB are aspired as well as negligible birefringence of less than 0.2 nm TE±TM spectral shift and a ¯at-top spectrum. Up to 128 spectral channels in silica waveguides consumes substantial substrate area in the waveguide branch which is translated to costs of several $10 per channel [4±5]. But such devices enables to increase

*

Corresponding author. Tel.: ‡49-40-53997-373; fax: ‡49-40-53997-792. E-mail address: [email protected] (D. Sander).

substantially the information density by a higher number of optical channels in already existing ®ber transmission systems. On the other hand, integrated optics enables economic mass products for sensor applications satisfying demands for industrial process monitoring and control [6]. In particular in the consumer market, low fabrication costs are fundamental conditions to establish thin ®lm technology for sensor devices. Surface relief gratings such as wavelength selective couplers diffract and focus light simultaneously in and out of the waveguide plane but are limited to applications with a narrow spectral bandwidth [7,8]. Broadband imaging devices need high performance imaging of multiple wavelengths which can be accomplished by a transfer of well-known concave re¯ection diffraction pattern to planar waveguides [9]. Due to downscaling of such conventional spectrometers Ð keeping the diffraction properties and hence the grating period constant, Ð miniaturized spectrometers exhibit in general a larger spectral bandwidth of approximately 10 nm. Such re¯ection gratings are usually optimized for ¯at®eld design to obtain broadband focus imaging on a planar photodetector.

0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 0 ) 0 0 4 9 9 - 4

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Table 1 Characteristic data of the integrated optical microspectrometer Optical waveguide SiO2 substrate SiON film SiO2 cladding Optical microspectrometer Spectral range Blaze wavelength Maximum efficiency Diffraction order Spectral resolution FWHM Structuring resolution Width of entrance slit Height of entrance slit Numerical aperture

nsubstrate ˆ 1.46, dsubstrate ˆ 3 mm nfilmˆ1.48, dfilm ˆ 1 mm ncladding ˆ 1.46, dcladding ˆ 1 mm 350±650 nm 400 nm 50% mˆÿ1 10 nm > 0.5 mm 150 mm 1.5 mm 0.2 Fig. 1. Diffraction of light at a periodically structured waveguide endface.

The LIGA-spectrometer as such an economic selffocussing re¯ection grating is fabricated by hot embossing of polymer waveguides (PMMA) of LIGA-made masters [10,11]. An exceptional high structuring resolution of <0.25 mm is needed across the entire waveguide thickness of 100 mm in order to achieve diffraction ef®ciencies of approximately 10%. Caused by waveguide inhomogenities during embossing, signi®cant stray light generates cross talk resp. reduces detection accuracy in particular in spectral ranges of low light intensity. Due to additional optical losses in PMMA in the UV range, the useful spectral range is indicated between 380±780 nm. Our approach is to develop an integrated optical microspectrometer for the visible and near UV wavelength range (Table 1). Highly transparent SiON waveguides are utilized for the planar optical system which are deposited by LPCVD batch processing [12,13]. Due to the selffocussing phase transmission grating and the imaging optic in re¯ection, the complete spectrometer is integrated in a monolithical and compact `waveguide core'. In comparison to re¯ection gratings, transmission gratings with an appropriate refractive index transition of n1 ÿ n2 ˆ 0:5 have a 4±5 times larger grating facette [14]. Thus, designed for a blaze wavelength of lblaze ˆ 500 nm in the ®rst diffraction order, such gratings have facette widths of approximately 1 mm. Drawing attention to this grating design, structuring can be carried out reliably by standard thin ®lm technology of UV-lithography and plasma etching. 2. Periodic phase transmission gratings in SiON slab waveguides A periodic non-focussing phase transmission grating is evaluated theoretically in order to obtain both by classical phase matched diffraction as well as by solving Maxwell's equation basic diffraction properties. Successively, such patterns are transferred to a selffocussing approach described in Section 3.

The diffraction grating pattern in waveguides is related to the lateral periodic refractive index transition by constructive interference condition of two adjacent rays, Fig. 1 [15,16]. dn2 sin bm ÿ dn1 sin a ˆ ml

(1)

By simplifying formula 1 for perpendicular light incidence, the facette a is given by a ˆ l=2n1 for re¯ection gratings resp. a ˆ l=…n1 ÿ n2 † for transmission gratings [17]. Since a low refractive index results in a higher facette width a, a predestined material for n1 is SiO2, SiON with low nitride concentration or silica. Regarding waveguide technology, long term stability, in particular UV transparency of SiON as well as batch processing feasibility promotes SiO2/ SiON as a standard thin ®lm waveguide material. The spectral dispersion D resp. the angular separation is expressed as follows: Dˆ

dbm m ‡ d sin a…dn1 =dl†ÿd sin bm …dn2 =dl† ˆÿ dl dn2 cos bm

(2)

By imaging a wavelength intervall Dl onto the detector length Dl, the focal length f can be reduced by a higher dispersion D according to D ˆ Dl=Dl ˆ …Df †ÿ1 . Designed for a high angle of incidence (>308), such transmission gratings are called grisms which is an abbreviation for grating and prism. Due to Table 1 with n1 ˆ 1:5 and n2 ˆ 1 for a SiON waveguide to air index transition, the calculated facette a amounts to 1 mm. Facette b is also 1 mm in order to obtain according to (2) a high spectral dispersion of D ˆ 1 mm/rad and to assure structuring feasibility [14]. To focus on the latter, electromagnetic theory is carried out on the periodic transmission grating pattern by differential methods of Neviere [18]. The desired structuring resolution of the fabrication process is computed by simulating the impact of rounded grating corners on the diffraction ef®ciency. Assuming symmetric rounding of concave and convex corner, the spectral ef®ciency is depicted in Fig. 2 for TE polarized incident light. As a result, a maximum of 60% is

D. Sander, J. MuÈller / Sensors and Actuators A 88 (2001) 1±9

Fig. 2. Simulated spectral efficiency in the ÿ1 diffraction order (transmission grating: n1 ˆ 1:5, n2 ˆ 2, a ˆ b ˆ 1 mm, TE-polarisation).

achieved at 400 nm indicating an excellent transmission performance even at rounding of up to 400 nm (sinusodial pattern). In order to avoid diffractive overlap of different waveguide modes k (with n1 ˆ neffk ), the SiON slab waveguides are designed for monomode propagation in the visible wavelength range. Based on the matrix method for waveguide simulation [19], Fig. 3 illustrates the spectral effective refractive index for TE and TM propagation. The dispersion of the composite SiO2 and Si3N4 is considered with the Bruggman formula [20]. As a result, low waveguidance for the listed index pro®le assures negligible birefringence of less than 0.5 nm TE±TM spectral shift. According to the high transparency of quartz glass, such waveguides exhibit low optical losses in the UV and visible wavelength range. Due to additional losses of the silicon substrate by leaky wave (l > 800 nm, simulation according to [21]) as well as mode cut-off losses, monomode SiON waveguides are optimized for a wavelength range of 350± 650 nm (Fig. 4). LPCVD waveguide processing is optimized for low light scattering resulting in optical losses of less than 0.05 dB/cm.

Fig. 3. Birefringence of the SiON-slab waveguides Ð effective refractive index (ncladding ˆ nsubstrate ˆ 1:46, nfilm ˆ 1:48, dsubstrate ˆ 3 mm, dfilm ˆ 1 mm, dcladding ˆ 1 mm).

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Fig. 4. Optical losses of the SiON slab waveguides (nfilm ˆ 1:5).

3. Non-periodic selffocussing phase transmission grating The design of the selffocussing phase transmission grating is based according to the periodic design on constructive interference condition at the refractive index transition. In order to obtain selffocussing properties in the waveguide plane, two different wavelengths have to interfere for parallel incident light in two different foci (Fig. 5). Hence, the position with the unknown parameters yi, zi of the grating facette i is given by two equations, each for the wavelength l1 and l2, Eq. (3) [21] r q  …yl1 ÿ yi †2 ‡…zl1 ÿ zi †2 n2 ÿ y2l1 ‡ z2l1 n2 ‡ yi n1 ˆ iml1 ; r q  …yl2 ÿ yi †2 ‡…zl2 ÿ zi †2 n2 ÿ

y2l2 ‡ z2l2 n2 ‡ yi n1 ˆ iml2

(3) De®ning the refractive index transition n1, n2 and the diffraction order m ˆ ÿ1, the equation system is solved by iteration. In order to avoid cumulative approximation errors, subsequent facette calculations are related to the coordinate origin (y ˆ z ˆ 0) increasing on the other hand, the diffraction order by m. Polarisation effects as well as the facette shape are not considered with such classical design approach. Taking into account divergent light from an entrance slit by additional terms in (3), the equation system can be solved similarly for other designs. Thus, a complete imaging system can be achieved which avoids any further optical elements for collimation. Inserting different parameters in (3) regarding mainly foci and the spectral dispersion, the grating endface evolvent can be varied within a concave and convex curve with respectively. The transition between such shapes is characterized by an almost planar grating evolvent. Due to the selffocussing properties, as generating convergent diffracted light at the endface, the grating results in a non-periodic pattern. In view to the design, spacial and spectral diffraction properties have to be considered simultaneously. To obtain

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Fig. 5. Schematic illustration of the selffocussing diffraction of the transmission grating.

suf®cient blaze effect with a perpendicular arranged facette pattern to incident and diffracted light, both foci have to be positioned paraxial with a center ray symmetry (Fig. 5). Furthermore, if the spectral distance l1 ÿ l2 is less than 200 nm, signi®cant defocussing is avoided for wavelengths in between. As a result from choosing yl1 ˆ 10 mm, zl1 ˆ 2:5 mm, yl2 ˆ 10 mm, zl2 ˆ 5 mm, l1 ˆ 400 nm and l2 ˆ 550 nm, the spectral dispersion D is D ˆ …l2 ÿ l1 †=…zl2 ÿ zl1 † ˆ 60 nm/mm with a focus length of approximately 13 mm. Inserting these parameters into Eq. (3) yields an almost linear grating chirp, thus, a planar grating endface is achieved. Both facette widths a and b are approximately equal yielding an outcoupling endface with 458 inclination to the incident light. The range of the facettes is between a ˆ b ˆ 0:5 mm (y ˆ z ˆ 0) to a ˆ b ˆ 1:5 mm which is linked to the overall grating width Hz of 7 mm. 4. Simulated selffocussing imaging The grating calculation in Section 3 which is based on classical constructive interference expresses the selffocussing imaging insuf®ciently. Neither the facette width and the light intensity respectively, nor the radiation characteristic of the facettes are considered thoroughly. In addition, abberations of random errors by fabrication technology have to be taken into account as well as imaging properties of wavelengths far off l1 and l2. To overcome these problems, the far®eld intensity Ifar®eld is calculated by superposition of the radiation of each facette i. The far®eld is simulated by a Fourier transformation of a slit in (4) which has an amplitude proportional to the facette b [22]. Each facette is expressed by an additional different phase factor Fi yielding the complete far®eld in (5) sin C ej…kn2 r‡fi † ; with r   C p zi C… y; z† ˆ n2 bi sin arctan r l 2 N X  Ifarfield … y; z† ˆ Eifar iˆ1

Fig. 6 illustrates the far®eld intensity across a grid in yl1 and zl1 for the wavelength l1. An image with a spacial FWHM of 2 mm (in z-direction) resp. 0.3 nm is calculated as well as a negligible deviation to the original focus yl1 and zl1 of 0.12 nm. As pointed out in Fig. 5, the foci of different wavelengths are located on a curve. Utilizing common planar detectors in both foci, a broadband (350±650 nm) spectrum exhibits wavelength ranges of different foci. This insuf®cient property has to be taken into account for de®ning the foci for l1 and l2. Thus, the calculation of the grating performance is a `reverse' design tool to give a feedback impact to the grating calculation in Section 3. Fig. 7 shows the broadband imaging in a plane of the foci of l1 and l2. As a result, a FWHM of less than 9 nm within the visible wavelength range is determined as well as an exponential intensity decay of more than 3 decades from the normalized maximum.The impact of random position errors of each facette is presented in Fig. 8. The errors originate from the rounding of the calculation (3) of the facette positions yi, zi which is caused by Ð in this case exaggerated Ð address discretisation of 100 nm accuracy of the photolithography mask. Since additional stitching errors resulting in side lobes are not taken into account, the `white noise' is indicated as spectral broadband stray light. An accuracy of 20 nm of highly

 … y; z† ˆ E0 Eifar

(4) (5) Fig. 6. Farfield intensity image in the focus yl1, zl1 (l1 ˆ 400 nm).

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Fig. 7. Calculated intensity in the detector plane of y1, z1 and y2, z2. Fig. 9. Schematic illustration of the interated optical microspectrometer.

resolved pattern addressing (Photronics MZD) results in an acceptable stray light level of 2  10ÿ3 [21]. 5. Optical system Since the selffocussing transmission grating is designed for parallel incident light, the divergent light which originates from the entrance slit of the spectrometer has to be collimated. In order to avoid imaging abberations of lenses, a parabolic mirror with a focus length of f ˆ 10 mm collimates the light with an off-axis alignment for total internal re¯ection. Considering technological feasibility of such a re¯ective optic, the imaging is not effected by the refractive index of the waveguide which can alter by temperature rise as well as within different deposition runs. Since the endface structuring is already accomplished by the grating, a single etching process structures the complete imaging system. Hence, different functional waveguide endfaces such as the diffraction grating, the collimating mirror and the incoupling

buttface of the entrance slit form the planar optical spectrometer core. The planar design of the integrated optical microspectrometer is illustrated in Fig. 9. The entrance slit coincidences with the focus of the parabolic mirror whose curve is given by z ˆ y2 =4f. The optical system of mirror and the selffocussing grating images the entrance slit (width of slit: s) with a scale of approximately 1:1 onto the detector plane. As a consequence, the spectral bandwidth is limited to a minimum of Ds. The dielectric mirror (neff ˆ 1:48) is designed for an angle of light incidence >478 to obtain total internal re¯ectance for lossless propagation. In order to reduce substrate area, three further dielectric planar mirrors fold the optical path `circular'. Taken into account a maximized ray divergence at the entrance slit of 88 for optimum light throughput, the tilt of the ray for each mirror is 788. Thus, the lateral aperture of 0.2 is equal to the waveguide guidance, resulting in a spherical entrance aperture. Such compact rectangular design minimizes the substrate consumption to 11  11 mm2. 6. Fabrication technology

Fig. 8. Calculated intensity with a grating pattern of 100 nm address discretisation (y1, z1 and y2, z2).

The ef®ciency of the diffraction grating is linked to the structuring resolution of the waveguides. The smaller the edge radii the more light is transmitted into the desired order (Fig. 2). Hence, the essential fabrication development procedure comprising of deposition and structuring process is pointed out as the pattern transfer process into the SiON waveguides. In order to maximize the structuring resolution for a waveguide of 3 mm thickness, a plasma etching sequence is developed which uses selectivity enhancing steps by an additional metal mask. The photoresist which exhibits low resistance to SiON plasma etching processes (such as a CHF3-plasma) is transferred into a CHF3-etch-resistant metal mask of tungsten. Tungsten is structured with a fast etching SF6-plasma keep-

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ing with this selectivity enhancing procedure the patterned photoresist undegraded. Thus, a thin resist mask of only 300 nm is reliable to enable furthermore highly resolved grating transfer from an original photolithographic mask Ð pattern resolution: 100 nm. Hence, the etching process optimization is the tungsten mask transfer into the SiON waveguides by an anisotropicly etching CHF3-plasma. The mechanism of SiON etching in different carbon± halogen±¯uorine containing plasmas was investigated in numerous papers which focus mainly on silicon masks [23±26]. The surface mechanisms in the reactive ion etching mode (RIE, Leybold Z 401) are summarized as follows. Isotropic polymerisation of a te¯on-like ®lm counteracts etching and sputtering by the impact of ions. Using pure CHF3, mainly CF3‡ ions interact with the SiON surface while released hydrogen scavanges F radicals by forming HF. Selective etching of different materials originates from the composition of Si and SiON, the latter due to the release of volatile COxFy by oxygen [27]. Since an activation energy of several hundred eV is necessary, vertical directionality of the ion impact yields highly anisotropic etching behavior. On the other hand, the mask materials Si or W in ¯uorine plasmas form volatile SiF4 and WF6 resp. at high etchings rates e.g. in CF4-plasmas. But exposed to a CHF3-plasma, the mask is only etched by sputtering and residual but negligible ¯uorine radicals. By adding further gases like H2 and O2, the etching mechanism promotes either enhanced polymerisation or supresses ®lm formation with reduced selectivity. Basic etching process optimization is carried out sequentially on the two parameters: pressure and applied power. The pressure adjustment just below the polymerisation (®lm formation without any etching) yields nearly maximum SiON etching rate at almost maximum selectivity. The latter can be reduced by adding 10% H2 which also reduces the SiON etching rate by 10%. SiON etching rate and selectivity is depicted in Fig. 10 as a function of applied power. A maximum selectivity of 25 is found within a power range of 100±300 W. At higher supplied power, increased sputtering

Fig. 11. (a) SEM photo of an anisotropically etched SiON-structure; (b) SEM photos of the waveguide transmission grating.

effects reduce the selectivity, thus optimal parameters are indicated at 200 W and 5 Pa. The resulting etching pro®le of vertically structured SiON waveguides is illustrated in Fig. 11a. The residual mask visualized by the material contrast of the SEM allows further etching to depths of 10 mm with the same sub-micron accuracy. A pattern resolution of 300 nm of the complete process sequence is shown in Fig. 11b, underlining the capability of standard thin ®lm technology. The parameters of the etching process are listed in Table 2. Since the photoresist is removed in a O2-plasma, the complete etching sequence is carried out in situ in the order of the processes 1, 2, 3 and 1. All etching gases are widely established which guarantees the implementation into other fabrication plants. The realized spectrometer is shown in Fig. 12 with marked optical elements. 7. Performance of the integrated optical microspectrometer

Fig. 10. Etching rate and selectivity vs. supplied RF-power.

The performance of the optical microspectrometer is characterized by the selffocussing transmission grating as well as the optical system. As a result of the optical ray forming, the light propagation within the waveguide core is presented in Fig. 13, highlighting four times total internal

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Table 2 Plasma etching sequence of SiON waveguides structuring Parameter

Process 1: tungsten

Process 2: resist

Process 3: siliciumoxinitride

Gas supply Pressure Power Bias voltage Etching rate Anisotropy Mask/selectivity

13 sccm SF6 12 Pa 50 W 20 V 50 nm/min 0.9 Resist/2

24 sccm O2 20 Pa 100 W 800 V 200 nm/min

50 sccm CHF3 5 Pa 100 W 1000 V 40 nm/min >0.95 Tungsten/27

Fig. 14. Diffraction efficiency (ÿ1 order) of the selffocussing transmission grating. Fig. 12. Photo of the realized integrated optical microspectrometer.

re¯ection. Additional stray light due to the re¯ection of four surfaces has not been detected. The performance of the microspectrometer is related to different characterizing issues: The light throughput results from the diffraction ef®ciency, the width of slit, the aperture of the waveguide as well as additional losses by the waveguide and the imaging optic. Measuring accuracy of intensity is limited mainly by inherently generated noise, namely by the electronic detection unit and the shot noise of the light itself. The performance of the ¯at®eld design, abberations due to optical imaging and the width of slit determine the

spectral bandwidth. Furthermore, the spectral range combines all items since overlapping stray light and measuring accuracy limits applications in the wavelength range of low light intensity. Characteristic grating data of the diffraction ef®ciency are shown in Fig. 14. Vertical interference due to substrate re¯ection at the endface causes a sinusodial overlap onto the grating ef®ciency which exhibits a maximum of approximately 50% at a blaze wavelength of 350 nm. As a further result of the transmission grating performance, the spectral separation is illustrated in Fig. 15. In accordance with the

Fig. 13. Photo of the optical beam propagation by a HeNe laser.

Fig. 15. Spectral dispersion of the selffocussing transmission grating with parabolic fitting.

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Fig. 16. Spectrum of a Hg-discharge lamp (FWHM: 9 nm).

Fig. 18. Measured extinction of absorbing liquids Patentblau (636 nm) (graph a), Erythrosin (535 nm) (graph b) and Nitrophenol (400 nm) (graph c).

theoretical dispersion of 60 nm/mm, experiments yield 63 nm/mm. Since the dispersion is a non-linear, a parabolic ®t generates consistently a spectral accuracy of 1.4 nm across the wavelength range of 200 nm. Considering the integrated microspectrometer, the spectral bandwidth is determined by a Hg-discharge lamp (Fig. 16). Neglecting the native spectral bandwidth of the emission lines, the FWHM of 6 pixels (150 mm) or 9 nm resp. is mainly due to the imaged entrance slit. Blocking of better than 20 dB is measured for a single emission line in the spectral distance of 30 nm. A white light reference spectrum (micro tungsten halogen lamp (Welch Allyn: 5 V, 5 W)) is illustrated in Fig. 17 with inserted long pass ®lters. The integration time of the Hamamastu photodiode array S3903 256 of 100 ms for pixel saturation arises from a light intensity of 0.16 nW in the maximum. The maximum blocking of log(I ref =I† ˆ 1:2ÿ1:5 absorption units (AU) decreases at smaller wavelengths due to the reduced intensity but an almost constant stray light level. Although stray light might be subtracted as dark current, inherently generated noise reduces the measuring accuracy DAU. Thus, the useful spectral range is indicated between 350±650 nm.

Three different liquids with variable absorption are inserted into the optical path in order to specify the spectral range for photometric applications. Hence, the data for such applications consists of a parameter matrix of wavelength, absorbance and accuracy. As illustrated in Fig. 18, an extinction saturation of 0.6±1.5 is detected at higher absorption for the liquids Patentblau, Erythrosin and Nitrophenol. Measuring accuracy Ð at the referred extinction Ð amounts to DAU ˆ 0:002 at AU ˆ 1:6 at l ˆ 636 nm for Patentblau and DAU ˆ 0:002 at AU ˆ 1:2 at l ˆ 400 nm for Nitrophenol.

Fig. 17. Spectral extinction of different longpass filters.

8. Conclusion An integrated optical microspectrometer is presented in particular for photometric applications in the visible wavelength range. The feasibility of manufacturing with standard thin ®lm technology arise from a special designed nonperiodic selffocussing transmission grating. Functional waveguide endfaces for ray forming result in a `monolithical' spectrometer core. The fabrication sequence is carried out by a single deposition and structuring process which guarantees a high reproducibility at low fabrication costs and a high throughput. Characteristic data of the performance demonstrate various applications for spectra analysis within a range of 350±650 nm with 9 nm spectral resolution. Regarding light throughput of standard white light sources, a S/N ration better than 1000 yields satisfactory accuracy of the complete system. As an outlook, a sophisticated deposition process will be very promising to reduce further considerably the stray light. In regard to the spacial distribution of the stray light by the detected spectra, computational correction of the spectra can be carried out to compensate saturation effects at low light levels. In addition, the design capability of the transmission grating enables to expand the wavelength range into the near infrared as well as the UV. In particular as WDM devices at 1.3 and 1.55 mm, such gratings could also generate attractive applications for optical communication systems.

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Biographies JoÈrg MuÈller received the Dipl.-Ing, Dr.-Ing. and Habilitation degrees in subjects like MIS varactors, high-frequency mixers and IMPATT, p-i-n and fast photodiodes, while studying electrical engineering at the Technical University Braunschweig Germany. Since 1983 and after several years at Siemens AG, where he was head of microwave diode development and fabrication, he has been Professor of Electrical Engineering at the Technical University Hamburg-Harburg as Head of the Department of Semiconductor Technology. He is engaged in thin film technology, SOI processes, integrated optics, high-temperature semiconductors and development of microsystems and processes. Dietmar Sander received the Dipl.-Ing. in electrical engineering and Dr.Ing. degree in subjects like micro moulding, integrated optics, thin film technology from the Technical University of Hamburg-Harburg. At the Department of Semiconductor Technology he has been engaged for his PhD thesis in particular in microsystem technology. Since 1998 he is employed at the company Eppendorf Netheler Hinz GmbH for research and development of microfluidic systems and devices.