We report the use of localized annealing via in situ heaters to induce a semi-permanent change in the refractive index of the cladding in ring resonator filters. When compared to other methods for post-fabrication trimming, this method has the advantage that no additional equipment, other than a supply of electrical power, is necessary to cause the index change. Two cladding materials were used: hydrogen silsesquioxane (HSQ) for samples that were externally annealed, and PECVD oxide for samples that were annealed with in situ heaters. The resonant wavelengths could be adjusted by as much as 3.0 nm and 1.7 nm for the HSQ and PECVD cladded filters, respectively. The trimming of a 5 channel, single ring filter bank, and a single, double ring filter is demonstrated.
© 2016 Optical Society of America
Silicon photonics is a promising technology because of its potential to greatly reduce the size, weight, and power for many electronic and optical systems. The use of silicon fabrication technology is especially well suited to the low-cost integration of large numbers of optical components. One great challenge to fully realizing this potential is the need for extremely precise optical phase control in components such as filters and interferometers. Manufacturing tolerances make it generally impossible to achieve such phase control by fabrication alone. Typically, the effective optical path length in a device is adjusted by varying the optical index using integrated heaters. This solution has several disadvantages, including the amount of power that is consumed and the need for electronics to independently adjust the power to a large number of heaters that may be necessary in a system.
An alternative solution is to permanently modify the index of refraction of the structures to achieve the desired optical path length after fabrication. This post-fabrication trimming method would provide a much simpler set-it-once-and-forget-it mechanism for tuning. Several such methods have been demonstrated such as using visible light or UV exposure to change the index of the cladding [1–3], electronic-beam exposure to cause stress that changes the effective mode index , oxidation of the waveguide using an atomic force microscope probe  or patterning the cladding in an extra fabrication step . Each of these methods has their advantages and disadvantages, but no method offers a quick way to perform trimming without specialized or expensive equipment. The method described here can be done quickly, and only requires a way to measure the optical response of the device, and an electrical power supply to perform the adjustment.
Instead of using optical exposure to change the index of a waveguide cladding, it is possible to use high temperature to permanently change the index of a cladding [7–9]. Integrated heaters are already commonly used for thermal adjustment of the waveguide index, and can achieve temperatures several hundred degrees above the ambient temperature . Such high temperatures can cause a permanent (or semi-permanent) change in the index of a cladding material in addition to a temporary one if the right cladding material is chosen. Individual heaters can be placed at each optical element so that it is easy to individually adjust each optical element. Because the heaters and optical devices are so small, the time scale for heating and cooling the devices is on the order of 10’s of milliseconds. This means that the adjustments and measurements could be done in rapid succession.
2. Temperature dependence of the cladding material refractive index
One essential element for this trimming method is an optical cladding material that has a significant permanent index change when annealed at an appropriate temperature. Earlier demonstrations of this technique have used a standard silica cladding, and index changes of only 0.01% were achieved [7,8]. This is insufficient for most application. The cladding material also must have low optical losses at the wavelength of interest, for the final waveguide to have low propagation loss. This work focuses on the C-band, or wavelengths near 1550 nm. Four materials with known low optical loss were investigated: Hydrogen silsesquioxane (HSQ), polydimethylsiloxane (PDMS), low temperature PECVD oxide (hereafter, referred to as simply PECVD oxide), and polymethylglutarimide (PMGI).
To investigate the permanent index change of these four materials with temperature, we first deposited the materials on silicon wafers and then measured the index before and after baking the wafers. The thickness of the films were approximately 1 µm. The PECVD oxide was deposited at a low temperature of 150° C, which is known to produce oxides of poor quality , making the material susceptible to further thermal modification. The other materials were spin coated. The refractive indices of the films were measured at a wavelength of 1 μm using spectroscopic ellipsometry. This wavelength was the longest wavelength available on the spectroscopic ellipsometer, but it is expected that index changes at 1.55 μm will mirror changes at 1 μm. The results of these measurements are presented in Table 1. The lowest temperature in the test range is the post-apply bake temperature for the materials that were spin-coated. The high temperature in the test range is limited by equipment to 600°C for some materials, and limited by the expected decomposition temperature for others. Shown in the table is the amount of index change and over what temperatures that index change happened. The baking was done in a small furnace-like oven and the temperature was held for 5 minutes. However, it was not possible to add the samples after the oven reached temperature. The additional bake time as the oven ramped-up is not included in the table. This time was about 30 minutes for the highest temperatures.
All of the materials except PDMS go through a significant index change when they are baked at an elevated temperature. HSQ undergoes the largest change in refractive index, however, the change is not monotonic. At lower temperatures in the range, the index decreases. This is believed to be due to de-absorption of water, as will be discussed later. At higher temperatures, the HSQ film densifies, raising the index . It should be noted that this non-monotonic characteristic of the index of HSQ upon annealing has been previously reported .
3. Demonstration of ring resonator trimming
3.1 Anealing of entire sample
To demonstrate the technique on ring-resonator-based filters, devices based on a previous filter design were fabricated . This design is suitable for WDM applications with 100 GHz channel spacing. These ring-resonator filters were fabricated on silicon-on-insulator (SOI) wafers with waveguides having 600-nm width and 110-nm height. Other parameters are a waveguide diameter of 13.0 µm, free spectral range of 2 THz, and a 3 dB bandwidth of 50 GHz. This waveguide thickness is thinner than the more commonly used 220 nm. In the original design the thinner waveguide offered the advantage of reduced optical scattering. In this case, the thinner waveguide puts more of the mode in the cladding, thereby allowing greater tuning from an index change in the cladding. Calculations predict that the effective index of the mode changes about 1/5 of the relative index change in the cladding. In a more typical silicon waveguide (with 500 nm x 200 nm cross-section), the fraction decreases to about 1/10. It also should be noted that such thin waveguides effectively only guide the TE mode.
The first device tests were done by using HSQ as the top and side claddings after the silicon waveguide was fabricated. The HSQ thickness is approximately 1 µm, similar to the bulk film measurements. Figure 1 shows the change in the resonant wavelength when filters with the HSQ cladding are baked. These samples were baked in a furnace in the same manner as used in the previous section, and individual adjustment of ring resonators was not possible. As anticipated, the shift in the resonant wavelength follows the same pattern as the shift in the index of the HSQ films described above. The resonant wavelength decreases as the index of the HSQ decreases as the sample is baked at temperatures below 300° C. Above 300° C, the resonant wavelength increases. The total shift in resonant wavelength from the minimum to the maximum is 3 nm, and this would be the expected trimming range possible if an HSQ cladding is used. Also shown in Fig. 1 are the changes in cladding index that would be predicted to cause the measured changes in resonant wavelength. Regardless of the bake temperature there was detectable difference in performance between the HSQ coated samples and control samples prepared with the standard coatings.
3.2 Localized In Situ annealing
For demonstrations with the in situ heaters, PECVD oxide was used as the cladding material. Measurements were made on single ring filters with the same dimensions and performance as the devices in the previous section. PECVD oxide was used because our CMOS facility lacks a particular tool required to achieve a post-coat bake of HSQ without crazing. (In general, HSQ is CMOS compatible). It was important that the PECVD oxide was deposited toward the end of the process, because the processing of the metals necessary for contacting the heaters requires temperatures of 475° C. Toward the end of the fabrication process, the standard oxide overcladding was removed, so that it could be replaced with a 1 µm thick PECVD oxide film. This removal was done through a series of dry and wet etch steps to avoid damaging the silicon waveguides and heaters. Figure 2 shows a SEM image of a filter with two coupled rings after the removal of the first oxide overcladding. Visible are the rings, the input and output waveguides, and the heaters. The heaters are made in the same silicon layer as the waveguide, except that the heaters are n-type doped (to a concentration of 1021 cm−3) to provide a controlled electrical resistance. The heaters are 500 nm wide and are spaced 1500 nm from the edge of the ring. The length of the heaters are about 10 µm, not including the leads. As shown in Fig. 2, the leads between the heaters and the metal contacts are also made of the same doped silicon. These leads are wider further from the ring to lower their electrical resistance. There are two heaters for each ring and the two heaters are wired in parallel. The net resistance (including electrical leads) for the two heaters is about 1.2 KΩ.
Figure 3 shows the residual resonant wavelength shifts of the ring filters after being heated. The amount of change is dependent on both the amount of heat or electrical power applied, and on the time of the application. For the mid-range powers, the shift dependence on time is roughly linear when time is plotted on a logarithmic scale (data not shown). The highest power applied, 79 mW, is where the heaters begin to be damaged, and higher heating powers are not possible. The amount of temperature change during heating can be estimated by the resonant wavelength shift during heating, and at 79 mW, the ring is heated by about 150°C. It should be noted that this is an average temperature shift. It expected that the areas of the ring close to the heaters are heated more than the other areas of the ring, but the specific temperature gradients were not investigated.
Using this measurement as rough guide, it is possible to trim a series of ring resonator filters. The trimming of 5 channels of a single ring filter bank is shown in Fig. 4. Shown is the transmission through the through port, which allows the measurement of the resonant wavelength of all 5 channels by monitoring a single port. Before trimming, the channel spacing is nearly random. After trimming, the resonant wavelengths are much more closely aligned to the target 0.8 nm spacing.
Many filter designs require multiple small rings to be coupled together. It is therefore important to demonstrate that this technique can be used to independently tune nearby or adjacent rings. The tuning of a filter with two directly couple rings (Fig. 2) is shown in Fig. 5. To show the independent ability to tune each ring one ring was trimmed nearly as much as possible by heating with 79 mW for 2 min. Figure 5(a) shows the drop-port transmission of the filter before and after this trimming of one ring. Notice that the resonant wavelength of the adjusted ring shifts by −1.7 nm, while no observable change is made to the resonant wavelength of the second, adjacent, ring. There is a slight change in the shape of the other resonance due to the difference in coupling between the two resonances as one is shifted. The two rings were then brought into co-resonance by trimming the second ring. Figure 5(b) shows the through and drop port transmission performance after both rings in the filter were trimmed. The low on-resonance transmission of the through port indicates that the two rings are well aligned. The on-chip insertion loss to the drop port when on resonance is between 1 and 2 dB, and the 10 dB width is 0.5 nm. This result is similar to previous results with this filter design with a traditional oxide cladding , and indicates that any additional waveguide loss from this technique is minor.
4. Long-term stability of localized cladding annealing
Low temperature oxides such as those used here are known to absorb moisture . This is believed to be the primary mechanism for the refractive index changes observed in our PECVD oxide. As the oxide is heated, moisture is forced out of the film. It is therefore expected that the changes in index produced by heating will not be stable, unless the additional flow of moisture in and out of the oxide is prevented. Samples stored for 1 day in an ambient atmosphere show relaxation toward the original untrimmed resonant wavelength (Fig. 6). Also shown in Fig. 6 is the comparison to a sample stored for 1 day with a desiccant. There is an improvement in stability with the desiccant, however, there is still some drift toward a longer wavelength. It is therefore clear that moisture is an important contributor to the changes in index that are being observed, but it is not clear whether moisture is the only one.
Similarly, with the HSQ coated samples, the resonant wavelengths will relax back to their original wavelengths after being baked at a temperature of 300° C. When an HSQ coated sample is baked at 475 °C, however, the resonant wavelength is much more stable, although a small shift to longer resonant wavelengths will occur over the course of a month (data not shown). This indicates that the mechanism for the index shift at low temperatures is likely moisture, as in the PECVD case. At temperatures above 300° C, the index change is likely due to the expected densification. After the HSQ film densifies, it appears the film can still absorb moisture, but the effect is not as fast or as strong.
Future efforts will be required to develop to a method to avoid moisture changes after annealing, or to use a different mechanism for the wavelength shift. The densification of an HSQ cladding at temperatures above 300° C offers one possible solution, although a moisture barrier or some other control of moisture is still needed. The problem of stability is not unique to this trimming method. Other post-fabrication methods also may not be stable  and can shift over a time period of months, similar to densified HSQ. The trimming method outlined here, however, can be completely done in situ. It is therefore feasible to periodically readjust the trimming, which would not feasible using the other trimming techniques. This would still offer advantages over adjusting the devices in the usual way using active heaters via the thermo-optic effect. The devices would not need to be heated all the time, reducing power consumption, and the devices can be adjusted one-by-one, perhaps reducing complexity.
A new post-fabrication trimming method using a thermal or annealing process has been demonstrated. This method uses in situ heaters to cause a shift of up to −1.7 nm in the resonant wavelength of a ring resonator filter when low temperature PECVD oxide is used as a cladding. Tests indicate that this shift is at least partially due to the removal of moisture that is present in these oxide films. A larger possible wavelength trimming range was shown using hydrogen silsesquioxane (HSQ) as a cladding material. With HSQ, the resonant wavelength shifted over a range of 3 nm when annealed above 300° C. This shift is likely due to densification of the HSQ, making the change more permanent than a shift relying on changes of moisture.
The authors gratefully acknowledge technical discussions with Ted Lyszczarz, Ted Fedynyshyn, Corey Stull, Rabindra Das, Reuel Swint, Suraj Bramhavar, and Cheryl Sorace-Agaskar. We also acknowledge assistance with measurements from Sara Mouser and Fred O’Donnell, and assistance with processing from Sandra Deneault, Peter Baldo, Karen Magoon, and Molany Neak. This work was sponsored by the Assistant Secretary of Defense for Research and Engineering (ASDR&E) under Air Force Contract #FA8721-05-C-0002. The opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.
References and links
1. A. Canciamilla, F. Morichetti, S. Grillanda, P. Velha, M. Sorel, V. Singh, A. Agarwal, L. C. Kimerling, and A. Melloni, “Photo-induced trimming of chalcogenide-assisted silicon waveguides,” Opt. Express 20(14), 15807–15817 (2012). [CrossRef] [PubMed]
2. S. Grillanda, V. Raghunathan, V. Singh, F. Morichetti, J. Michel, L. Kimerling, A. Melloni, and A. Agarwal, “Post-fabrication trimming of athermal silicon waveguides,” Opt. Lett. 38(24), 5450–5453 (2013). [CrossRef] [PubMed]
3. D. K. Sparacin, C. Y. Hong, L. C. Kimerling, J. Michel, J. P. Lock, and K. K. Gleason, “Trimming of microring resonators by photooxidation of a plasma-polymerized organosilane cladding material,” Opt. Lett. 30(17), 2251–2253 (2005). [CrossRef] [PubMed]
5. Y. Shen, I. Divliansky, D. Basov, and S. Mookherjea, “Perfect set-and-forget alignment of silicon photonic resonators and interferometers,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2011, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPC3. [CrossRef]
7. M. Abe, Y. Inoue, K. Moriwaki, M. Okuno, and Y. Ohmori, “Optical path length trimming technique using thin film heaters for silica-based waveguides on Si,” Electron. Lett. 32(19), 1818–1820 (1996). [CrossRef]
8. T. Mizuno, M. Kohtoku, M. Oguma, Y. Hida, and Y. Inoue, “Birefringence and path length adjustment of silica-based waveguide using permanent heater trimming,” Electron. Lett. 40(6), 371–372 (2004). [CrossRef]
9. S. Spector, J. M. Knecht, and P. W. Juodawlkis, “Permanent Trimming of Silicon Ring Resonator Filters by Thermal Modification,” in Conference on Lasers and Electro-Optics: Laser Science to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2015), paper JTh2A.46. [CrossRef]
10. M. Watts, W. Zortman, D. Trotter, G. Nielson, D. Luck, and R. Young, “Adiabatic Resonant Microrings (ARMs) with Directly Integrated Thermal Microphotonics,” in Conference on Lasers and Electro-Optics/International Quantum Electronics Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper CPDB10. [CrossRef]
11. M. S. Haque, H. A. Naseem, and W. D. Brown, “Stress in high rate deposited silicon dioxide films for MCM applications,” in Proceedings of IEEE Symposium on Reliability Physics (IEEE, 1996), pp. 274–280. [CrossRef]
12. D. Többen, P. Weiganda, M. J. Shapiro, and S. A. Cohen, “Influence of the cure process on the properties of hydrogen silsesquioxane spin-on-glass,” in Proceedings of the Materials Research Society Symposium, (Materials Research Society, 1996), pp. 195–200. [CrossRef]
13. S. Spector, A. Khilo, M. Peng, F. Kaertner, and T. Lyszczarz, “Thermally tuned dual 20-channel ring resonator filter bank in SOI (Silicon-on-Insulator),” in Conference on Lasers and Electro-Optics: Laser Applications to Photonic Applications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper CWM2. [CrossRef]