ALD high-k layer grating couplers for single and double slot on-chip SOI photonics

Solid-State Electronics 74 (2012) 58–63

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ALD high-k layer grating couplers for single and double slot on-chip SOI photonics Maziar M. Naiini ⇑, Christoph Henkel, Gunnar B. Malm, Mikael Östling KTH – Royal Institute of Technology, School of Information and Communication Technology, Integrated Devices and Circuits, P.O. Box 229, SE-164 Kista, Sweden

a r t i c l e

i n f o

Article history: Available online 22 May 2012 Keywords: Integrated photonics Grating couplers High-k thin films Slot waveguides Fabrication and characterization

a b s t r a c t State of the art grating couplers for horizontal single and double slot waveguides are presented; in these devices the input signal is transmitted from a single mode optical fiber to silicon on insulator slot waveguide. In the waveguides, atomic layer deposited (ALD) high-k dielectrics form the low refractive index slot. It is demonstrated that a fully etched design combined with precision of ALD result in highly reproducible devices with theoretical efficiency variations less than 1%. Devices have a peak calculated coupling efficiency of 24% at 1.55 lm. In order to achieve an optimal design, optical properties of high-k films are studied by spectroscopic ellipsometry. Measured refractive indices show variations from reference values, originated from film variation in densities. Chips with a test slot material are fabricated and the optical efficiency of the couplers is characterized. The maximum measured coupling efficiency of the couplers is 18.5%. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Silicon-on-Insulator (SOI) technology is suitable for design and fabrication of a complementary metal oxide semiconductor (CMOS) compatible on-chip silicon photonics platform. High optical confinement is achieved with the large refractive index contrast between the Buried Oxide (BOX) and the silicon layer. Conventional SOI photonic waveguides consist of silicon ridges on top of the BOX layer. In contrast, a horizontal slot waveguide is manufactured by interposing a thin film of low index material between two thicker layers of silicon [1]. Slot waveguides outperform their conventional counterparts in terms of loss [2]. It is shown that the optical power has a confinement factor of 36% [3], and the total stored power in the slots can be further enhanced by increasing the number of slots [4]. High-k dielectrics have a lower index of refraction compared to that of silicon. Therefore, they are suitable candidates for the slot material. Atomic Layer Deposition (ALD) is a precise and sophisticated tool used in the CMOS technology to implement high-k gate dielectrics which enables aggressive scaling of MOSFETs. ALD is also an excellent choice for SOI photonics since the thin films made by this method have low roughness and thickness variations. SOI photonics have been greatly developed in the recent years pursuing its ultimate goal; intrachip interconnects, where high performance active photonic devices are required. SOI Light modulators have been reported using carrier plasma dispersion effect [5,6]. Silicon nanocrystals embedded in slot waveguides have been ⇑ Corresponding author. E-mail addresses: [email protected] (M.M. Naiini), [email protected] (C. Henkel), [email protected] (G.B. Malm), [email protected] (M. Östling). 0038-1101/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.sse.2012.04.012

reported [7]. Electro-optic modulators using an organic material with optical nonlinearity and possible extension to slot waveguides have been introduced [8]. SOI slot waveguides can be studied with a variety of materials enabling fast modulation, light amplification and lasing in silicon photonics. A major challenge for the on-chip photonic circuits is to inject the light from an optical fiber to waveguides. A standard optical fiber core has a circular cross section with the diameter of 10.4 lm, whereas the waveguides have a rectangular cross section with dimensions less than 0.5 lm. A terminal for transmission of optical signal is required. Fig. 1a illustrates this conceptual device as a key component for an Optical Integrated Circuit (OIC). Grating couplers reorient the incoming light from an optical fiber that is almost perpendicular to the waveguides. This reorientation is obtained by diffraction through a periodic structure, or so called gratings. Partially etched grating couplers have been reported for conventional silicon slab waveguides [9–11] and sandwiched silicon waveguides [12,13]. Previously, design and fabrication of grating couplers for single horizontal slot waveguides was reported [14]. With a fully etched design, reproducible couplers were introduced; variation in transmission efficiency is avoided and a one mask fabrication step is maintained. The arrangement of the layers in the slot waveguide is schematically shown in Fig. 1b and c. Transmission efficiency of a grating coupler is highly sensitive to the optical constants (n, k) of the slot material, fiber angle, and waveguide dimensions. Knowing the optical constants of ALD thin films is a key requirement for fabrication of grating couplers with an optimized performance, therefore in this work (n, k) of high-k films are obtained by spectroscopic ellipsometry. Based on the measurement results, grating couplers with optimal efficiencies

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M.M. Naiini et al. / Solid-State Electronics 74 (2012) 58–63 Table 1 ALD precursor and temperatures. Slot material

Precursor

Deposition temp.

Number of cycles

HfO2

350 °C

300, 600, 1000

Al2O3

Bis (methylcyclopentadienyl) methoxymethylhafnium(HfD04) Hf[C5H4(CH3)]2(OCH2)(CH3) TMA (Trimethylaluminum)

200 °C

AlN

TMA + NH2

350 °C

500, 750, 1000 300, 600, 1000

Samples are characterized by spectroscopic ellipsometry to measure the thickness and optical constants (n, k). Ellipsometry measurements are performed for a spectrum ranging from 140 nm to 2000 nm of wavelength at 70° incident angle using a UVISEL ER from HORIBA. Optical constants of each material were determined, and based on the physical properties of each material an appropriate ellipsometric dispersion model is developed. Every dispersion model consists of parameters that need to be fitted with the measurement results. This requires the ellipsometric model to be evaluated for several thicknesses of deposited material. A set of parameters with the least error for all samples is valid. 2.2. Efficiency calculations Fig. 1. Schematic view of slot waveguide devices: (a) Marking the grating coupler as the input terminal for the interconnects. Wavelength of incoming light is 1550 nm. (b) Schematic cross section of a single slot grating coupler, An ALD layer is sandwiched between two silicon layers with higher refractive index. (c) Schematic cross section of double slot grating coupler, where two ALD layers are sandwiched by three silicon layers.

are designed. In addition, couplers for double slot high-k waveguides are introduced. Further on, test chips are fabricated with silicon dioxide (SiO2) single slot waveguides. Optical characterization of the chips can address the performance of this class of grating couplers. As the simulations in this work show, a change in the slot material has a weak effect on the maximum efficiency. A manual setup is implemented to facilitate wafer scale measurements. Optical characterization results and transmission spectrums of the optical wires will be presented. This article is organized in three main sections: in Section 2 the experimental thin film characterization method is explained, in addition, details of the simulations are briefly explained. In Section 3 measurement results for optical constants of the high-k films are reported and effect of these results on the grating coupler designs are discussed. Section 4 is dedicated to fabrication and characterization of the test chips. In Section 5 conclusions are summarized. 2. Experimental and design details 2.1. ALD layer characterization High-k dielectric layers are deposited on 4-inch silicon wafers by means of an 8-inch ALD BENEQ TFS200 deposition tool. Substrates are prepared with a process that ensures a clean surface without the chemical silicon oxide. By means of ellipsometric measurement spectrums it was confirmed that this substrate has the same characteristics as the database values for a standard silicon substrate. High-k thin films are deposited on the cleaned wafers and this single high-k film structure is characterized. Layers are deposited according to Table 1. Deposition temperatures are harmless for the underlying layers and formation of grains in the amorphous silicon layers is avoided. For each material wafers are prepared with different cycle numbers to achieve various thicknesses.

Coupling efficiency is calculated using a 2D Finite Element Method (FEM) in COMSOL MULTIPHYSICS [14]. The slot effect is expected to be observed for the TM-like mode. This mode is defined by the electric field component parallel to the slot plane being zero. The coupler efficiency is dependent on the BOX layer thickness due to reflections of the leaked light at the bottom of the BOX layer. An efficiency excluding this contribution can be calculated if the BOX layer is assumed to be infinite. The simulation domain is terminated with strongly absorbing sub-domains (perfectly matched layers) to prevent distortion of the wave propagation. The guided mode is studied with a 2D mode analyzer to find the optical mode power and confinement factor. 3. Layer characterization results 3.1. Material dispersion spectrum The thickness of HfO2 layers is measured 18.2 nm, 27.1 nm and 53.9 nm respectively for 300, 600, and 1000 cycles. New amorphous [15] model is used in the measurements. This model is appropriate for the spectrums with photon energies well below the band gap. The spectral dispersion of HfO2 is shown in Fig. 2. Refractive index of thin layers at 1550 nm is measured 1.95. As shown in Fig. 2 the ‘k’ value for HfO2 is zero at 1550 nm. This is due the fact that the photon energy is much smaller than the band gap of HfO2. The thickness of Al2O3 layers is 46.7 nm, 68.1 nm and 98.8 nm for 500, 700 and 1000 ALD cycles. Cauchy absorbent [15] model is utilized for the measurements. This model takes small material absorptions into account. Results are summarized and compared with reference values in Fig. 3. Refractive index of Al2O3 has been reported to be 1.74 at 1550 nm. In the prepared ALD samples the index is measured 1.63 and the ‘k’ value is 0.0009. AlN layers deposited with 300, 750, and 1000 cycles have a thickness of 14.2 nm, 32.2 nm and 44.9 nm. As shown in Fig. 4 parameter fittings show that the refractive index at 1550 nm for aluminum nitride is 1.71 and the ‘k’ is zero. The deposition method has a major effect on the density of the films. The difference between the measured refractive indices and the reference counterparts is directly originated from the density of the films. A smaller index corresponds to a lower film material density.

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M.M. Naiini et al. / Solid-State Electronics 74 (2012) 58–63 Table 2 Single slot optical power confinement factors. 50 nm Thick single slot waveguides Slot material

Reference ‘n’

Measured ‘n’

HfO2 Al2O3 AlN

31% 32% 28%

30% 33% 32%

Fig. 2. Refractive index of Hafnium Oxide layers, illustrated for the reference model [18] and the results of New Amorphous model.

Fig. 5. Calculated appropriate grating periodicities for hafnium oxide single horizontal slot waveguides. Dashed line shows the results based on the optical constants provided in [17]. Solid is the result based on the measurements.

Fig. 3. Dispersion spectrum of Al2O3 found by the Cauchy-Absorbent model. Atomic Layer Deposited layers of Al2O3 deposited at 200° show a lower refractive index compared to formerly reported values [19].

Fig. 6. Grating periodicities for Aluminum Oxide single horizontal slot waveguides. Designed couplers have a maximum efficiency at 1550 nm of wavelength.

Fig. 4. Aluminum nitride dispersion curves. Reference relation is based on a Sellmeier model [20]. Measured refractive indices are based on the New Amorphous model in our measurements.

3.2. Single slot grating couplers Grating couplers are designed to have a maximum coupling efficiency of 24%, with the assumption of an infinite buried oxide layer. Optimal coupling efficiency is determined only by the appropriate period of grating structure, since the structures are fully etched. This period which is calculated with first order Bragg diffraction condition, is dependent on the propagation constant of the waveguide [16–17]. Propagation constant is defined as the wave number

of the traveling light in a specific waveguide with a known mode effective refractive index. The propagation constant of the waveguide, on the other hand, is highly affected by the slot material index, therefore; performance of a grating coupler is sensitive to the slot material index of refraction. The confinement factor of optical power is a figure of merit for slot waveguide active devices. This parameter is also altered by the refractive index as summarized in Table 2. Fig. 5 summarizes the calculated grating periods with the optimal efficiency 24% at 1.55 lm for HfO2 single slots. Results are compared for reference index values and the measurement results. For a 50 nm thick slot layer it is demonstrated that a 10% non-uniformity in the slot layer will degrade the coupling efficiency up to 2%. Mapping of the wafers with 50 nm Al2O3 layers show a maximum slot thickness variation of 3 nm. This represents efficiency degradations less than 1% in the devices. Results for Al2O3 slot waveguides are shown in Fig. 6. Designs based on the measurements require a larger grating period for the optimal device performance. This is explained by the formerly reported index of Al2O3, which in Fig. 3 is shown to be larger than the ellipsometry results. Results for AlN slot waveguides are summarized in Fig. 7. A larger deviation is expected, since the refractive index of AlN films is found to vary from the reference value.

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Fig. 7. Grating coupler periodicities for Aluminum Nitride horizontal slot waveguides. The significant deviation is originated from a large difference between the measured optical refractive indices. Fig. 9. Scanning electron microscope image of the grating coupler. Demonstrated test structure is etched to the BOX layer. Table 3 Double slot optical power confinement factors. Two 25 nm thick double slot waveguides Slot material

Reference ‘n’

Measured ‘n’

HfO2 Al2O3 AlN

45% 50% 39%

44% 53% 51%

Fig. 10. Wafer scale photonics probe-station, with tunable fiber tilt angle. The position stages can be fine tuned with piezo micro-motors.

Fig. 8. Efficiency diagrams of double slot grating couplers. Peaks are situated at the measured slot material refractive index, addressing the optimal coupling efficiency.

3.3. Double slot grating couplers The grating couplers for double slot waveguides are also designed. If the slot layer is an active region in the device then an optimized optical power concentration at the slot region makes these structures more favorable. Calculation results for power confinement are listed in Table 3. The power confinement in the waveguides is enhanced from 30% to 50% by doubling the number of slots. For a double 25 nm slot waveguide of HfO2, Al2O3, and AlN optimal periods of 980 nm, 1016 nm, 1005 nm are found. Compared to 50 nm single slot waveguides optimal periods are larger, this is originated from the higher propagation velocity of light in a double slot waveguide [17]. The efficiency diagrams of the designed couplers are shown in Fig. 8. The couplers have 3 dB bandwidth of 75 nm. 4. Fabrication and characterization

icon substrate at 1100 °C. This layer is the bottom cladding layer for the waveguide, also for an optimized thickness the light leakage to the substrate can be minimized. The first amorphous silicon layer is deposited using PECVD at 250 °C. A low temperature deposition is suitable since it can prevent formation of small silicon grains in the amorphous material. These grains increase the surface roughness of the film introducing more loss to the waveguides. The two sandwiching Si layer have a thickness of 220 nm. The slot layer in the chip is a 25 nm silicon dioxide film that is deposited using PECVD. These layers have more thickness variations compared to the case of ALD, affecting the yield in the test wafer. Variation in the layer thickness will manifest as a peak wavelength shift in the transmission spectrum. In addition to the second amorphous silicon layer using PECVD, a 200 nm silicon dioxide hardmask is deposited to protect the features while etching. Since the waveguide cladding is also silicon dioxide, this hardmask does not need to be removed. A SPR 700 photoresist is spin coated and the wafer is exposed using an i-line mask stepper. The designed mask has a minimum feature size of 485 nm, which also corresponds to the smallest grating period. This is very close to the resolution limitations of the lithography equipment, which makes process critical. An experimentally optimized exposure dose of 165 mJ/cm2 is used. The hard mask is etched using the photoresist and the resist is striped to avoid silicon fencing while etching of the silicon layers. Etching is done using an Applied Materials P5000 cluster tool, in three steps. A top view of the fully etched device is depicted in Fig. 9. The devices are finally capped by a 1 lm SiO2 layer to form the upper cladding layer and protect the devices from optical fiber scratches.

4.1. Grating coupler fabrication process Test chips are fabricated on 4 inch silicon wafers using the inhouse clean room facility. After wafer cleaning, a silicon oxide layer with thickness of 1260 nm is grown with wet oxidation of the sil-

4.1.1. Optical measurement results Inspired by the conventional electrical probe-stations a wafer scale optical measurement setup is developed. Fig. 10 demonstrates the characterization of an SOI photonic wafer, where the

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M.M. Naiini et al. / Solid-State Electronics 74 (2012) 58–63 Table 4 Summary of coupling methods. Method name

Achieved efficiency

Manufacturing details and requirements

Fully etched gratings Partially etched gratings Nanotapers

18.5%

Single mask process

20% [12]

Double mask process. Highly uniform etching process required to maintain the yield.

Up to 96% [22–24]

High resolution lithography, down to 80 nm for a hyperbolic shape taper. Wafer dicing followed by façade polishing.

Fig. 11. Schematics of the test chip and optical probes. The input fiber is connected to the laser and the polarizer. Output fiber is connected to the detector.

Fig. 12. Efficiency spectrum of the grating coupler with the period of 980 nm and a fiber tilt angle of 17°. For input and output grating couplers peak efficiency is measured 18.5%.

transmission of light is measured. Schematic view of the test structure is illustrated in Fig. 11. Generated light travels through a polarizer and is delivered to the surface of the coupler. Transmitted light is then coupled back to the output fiber and measured in the detector. Fiber tilt angle can be manipulated using rotating stages. Attenuation of the light signal is due to the loss in the waveguide and the loss per each coupler. In the measurements it is assumed that the grating couplers are reciprocal structures therefore both input and output couplers have the same loss. In order to isolate the grating loss waveguides with different lengths are characterized. Waveguide loss is caused by the surface roughness of each deposited layer at the slot boundaries, the line edge roughness of waveguide sidewalls caused by lithography and etching, and mainly the material loss of amorphous silicon. Performance of the grating coupler is demonstrated in Fig. 12. As discussed above the optimized grating period size is proportional to the slot size. If a very small period size is dictated by the thickness of the slot, optimized coupling can be achieved by increasing the angle. For the fabricated slot waveguide with slot thickness of 25 nm and grating period size of 980 nm (grating teeth width of 490 nm), 17° fiber tilt angle is appropriate to achieve the peak efficiency at 1540 nm. After subtraction of the waveguide loss, a peak coupling efficiency of 18.5% for each coupler is measured. A lower coupling is caused by the line edge roughness of the grating teeth and non idealities in fiber alignment. It is worthwhile to compare the results achieved through this method with other methods reported. Table 4 summarizes the efficiencies, manufacturing requirements, and advantages of each method. Formerly introduced grating couplers for horizontal slot waveguides [12] require a double mask process which is more complex. Also etching non-uniformity is a drawback to this method. Nanotaper mode convertors have been manufactured with very high efficiencies; this method on the other hand does not enable wafer scale measurements and requires sample preparation. Transmission spectrums show resonance cavities with size corresponding to the length of the waveguide. Reflections at each end

Fig. 13. Compared to the results in Fig. 12 the high sensitivity of measurements to the grating period size is demonstrated. With period size of 1010 nm and the fiber tilt angle of 17° a 20 nm red-shift in the spectrum is measured.

of the waveguide are originated from the strong perturbation of the waveguides by fully etching. These reflections can be eliminated if the grating openings are filled with a higher refractive index material compared to that of SiO2. Another solution to remove the resonance cavity was proposed by applying antireflective photonic crystal interfaces [21]. Implementation of this approach requires a half size grating tooth at the interface of the waveguide. Considering the resolution limits of an i-line stepper, this feature size can be achieved by double patterning technique. Grating couplers with a grating period of 1010 nm are also characterized. A comparative study of the two couplers can verify the results. In addition, the sensitivity to the grating period can be demonstrated. Bragg’s diffraction condition dictates that among all geometrical parameters if only the grating period size is increased the transmission spectrum will have a red shift of

Dk0 ¼ DK  ½neff  nc sin hc 

ð1Þ

where DK is the change in the grating period, neff is the effective refractive index of the coupler, nc is cladding layer index and sin hc is the fiber tilt angle. The spectrum in Fig. 12 has a peak at 1540 nm and from (1) it is expected that the spectrum is moved to the right side. A comparison between the measurements in Figs. 12 and 13 show a 20 nm shift of the peak in the spectrum, confirming the discussed physical criterion. 5. Conclusions Reproducible photonic devices are designed and fabricated for light transmission to CMOS compatible SOI photonics waveguides. In these waveguides ALD high-k thin films are used as low index slot material. The novel fully etched design eliminates efficiency degradations. The designed grating couplers have a simulated coupling efficiency of 24% at 1.55 lm, assuming an infinite BOX layer. Coupling characteristics are highly sensitive to the thickness of the high-k film, and it is shown that the precision of the ALD method

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minimizes the coupling variations to less than 1%. In order to achieve optimal results optical characteristics of the ALD materials are studied by spectroscopic ellipsometry. Results indicate deviations from reference values, which originate from the density of deposited material. Especially, a larger variation is observed for AlN films. Compared to plasma-assisted process, thermal ALD layers of AlN have a relatively low refractive index, because of a lower density of material. Grating couplers for double slot waveguides require a relatively larger grating period than single slot waveguides. This is originating from a changed optical confinement factor. By increasing the number of slots a larger fraction of the light is guided in the slot region; therefore the effective refractive index of the waveguide will be lower. Due to this increased wavelength, and from the Bragg condition of diffraction, it is implied that a larger period is needed to couple the light, if the guided wave has a larger wavelength. Interconnect test chips are fabricated and characterized to evaluate the transmission performance of these couplers. For a 25 nm thick slot grating coupler a maximum coupling efficiency of 18.5% is measured. Roughness and alignment non-idealities are the origin of a lower experimental efficiency. Strong perturbation of the waveguide introduces some reflections in the waveguide that can be reduced by increasing the cladding layer refractive index. Fabricated test chips have demonstrated high-yield grating couplers that can be utilized for SOI on-chip photonics. Acknowledgment The financial support by the ERC advanced grant OSIRIS is greatly acknowledged. References [1] Preston K, Lipson M. Opt Express 2009;17:1527. [2] Xu Q, Almeida VR, Panepucci RR, Lipson M. Opt Lett 2004;29:1626.

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