2 State of the Art
2.4 State of the art of Chip Scale Atomic Clocks
Considerable effort has been devoted in recent times to the fabrication of low-power, chip-scale packaged atomic clocks. The effort to miniaturize and to reduce the power consumption of atomic clocks is due to the fact that a low-dimension, low-power device may be used in important applications such as wireless base stations and telecom networks and in portable equipment for navigation and positioning.
In 2004, Knappe et al. [75] presented the first miniature atomic clock. This clock was based on the Coherent Population Trapping (CPT) technology: this means that the presence of the large resonance cavity is not needed in the atomic clock, and this allows significant reduction of the dimensions. The disadvantage is that the stability of such clocks is usually five times less than the DR-based atomic clocks. The physics package of the miniature atomic clock presented in 2004 by Knappe et al. was based on microelectromechanical system (MEMS) fabrication techniques; in particular, the reference cell (filled with Cs) was fabricated using anodic bonding [76] of borosilicate glass and Si. The heater used consisted of a planar structure made from a film of lacked a local oscillator and the miniaturized control electronics to stabilize the cell and laser temperatures and lock the laser wavelength. The physics package of this clock is showed in Figure 2.6.
18
Figure 2. 6: Physics package of the miniature atomic clock presented by Knappe in 2004: (1), Schematic assembly, (2), Photodiode assembly, (3), cell assembly, (4) Optics assembly, (5) laser assembly, (6), the full atomic clock physics package presented. The black lines all indicate 1 mm.
Image taken from [75].
In 2007, the Symmetricom team presented the last version of their Chip Scale Atomic Clock (CSAC) [77] using again CPT as the interrogation scheme. The total dissipated power of this clock was 125 mW, the frequency stability was 1.6 × 10-10 at 1 s of integration and the total volume of the physics package was 15 cm3. The reference cell of this clock was fabricated using a similar technique than the previous described system: a silicon body 2 mm square and 2 mm thick was anodically-bonded with a transparent Pyrex window. The fabricated cell and the VCSEL laser were placed onto a polyimide (k = 0.2 W·m-1·K-1) support structure which provided minimal thermal conductivity, limiting conduction loss. On top of the polyimide support, a Pt heater and temperature sensor was patterned (upper suspension). On the lower suspension of the polyimide heater system, bond-pads for flip-chip attachment of the VCSEL were patterned. An illustration of such heater is shown in Figure 2. 7; this figure also shows an illustration of the physics packaging as well as a photo of the final product.
19
Figure 2. 7:(a) polyimide heater, lower suspension, with the bond pads for the flip-chip attachment of the VCSEL. (b), polyimide heater, upper suspension, including the Pt resistive heater and temperature sensor: (c) illustration of the physics package of the clock. (d) photo of the final
product. Images taken from [78].
A further improvement of the miniature atomic clock was presented by De Natale et al.
in 2008 [79]. They were able to fabricate a system with a total volume of 1 cm3 with the electronics integrated, a power consumption of 30 mW and an Allan deviation better than 1 × 10-11. They configured the system in a way so that the VCSEL and the reference cell could share a common heater: this allowed an important reduction of the size and of the power consumption of the system. In this system, the heaters were again MEMS-based, consisting of a Pt resistor patterned on top of an insulating material, so they use the same concept illustrated in Figure 2. 7. The reference cell of this system was made out of single Si crystal, etched to form the cavity which contains metallic Cs and anodically-bonded with a glass window. Figure 2. 8 shows a photo of the physics package of the miniature atomic clock developed by this group.
20
Figure 2. 8: Physics package assembly of the CSAC developed by De Natale et al. Image taken from [79].
Finally, the MACQS (Miniature Atomic Clock and Quantum Sensors) project started on July 2009, with the objective to fabricate a Double Resonance (DR) rubidium atomic clock. This project aimed to use alternative approaches (i.e. using Rb instead of Cs in the reference cell, and using DR instead of CPT as interrogation scheme) for atomic clock fabrication, without need to be smaller or better with respect to the already cited systems. In a DR atomic clock the presence of a large microwave cavity (dimensions ranging several cm) is needed, so the miniaturization level of the final system will not be comparable with the already cited clocks, based on MEMS technologies, and which use CPT as interrogation scheme. Nevertheless, the objective of the MACQS project was to introduce new fabrication techniques and innovative designs to batch-fabricate the parts of a DR atomic clock. The batch-fabrication allows to reliably and modularly producing the different components, therefore drastically decreasing the production cost of the system. In this well-defined size (mesoscale, like the size of a DR atomic clock), ceramic technologies such as Low-Temperature Co-fired Ceramic [80]–[83] (see the following for more details about this technology) offer several advantages, such as ease of 3D structuration, capability for manufacturing suspended heaters for local temperature control and possibility to easily integrate active elements such as sensors.
The MACQS project aimed to use LTCC technology for the packaging of the atomic clock, in particular to design LTCC heaters for temperature stabilization of the cell and of the VCSEL. This was something that was not attempted before the beginning of this project, even if another European project, the MAC-TFC (MEMS Atomic Clocks for Timing and Frequency Control & Communication) [84], [85] started almost in parallel with the MACQS project, and one of its objectives was to use LTCC technology for the packaging of an atomic clock.
21