Case Study

F712T0056
Casjo, Copyright 2020, PI (Physik Instrumente) L.P., www.pi-usa.us

Photonic Array Device Assembly
BackgroundPI’s Fast Multichannel Photonic Alignment (FMPA) technology was introduced
in 2016. It quickly amassed prestigious honors like the R&D 100 Award andthe Lightwave Innovations Award for its revolutionary ability to perform
multiple alignment processes in parallel, for example:••For elements whose positional optimization depends on otherelements. Examples include waveguide structures whose inputs,outputs and channels can interact, such as short waveguides whoseinputs and outputs “steer” each other. Other examples includelenses or diffractive elements in imaging or resonator assemblies
whose mutual positions are interrelated. FMPA can perform these
optimizations in one go, yielding a global optimization that is often
100 times faster than previously possible.
For alignments in multiple degrees of freedom. A ubiquitousexample in Silicon Photonics (SiP) is any array device, wherealignment motions in 𝛳𝛳 Z (common notation for rotation about
the optical axis) cause dealignment in XY. Previously, any
corrective motion in 𝛳𝛳 Z necessitated a pause and a realignment
in XY, meaning array alignments required a lot of time. FMPA’s
parallel alignment capability allows the full device alignment tobe performed in one quick step, since the compensation of thegeometric errors due to 𝛳𝛳 Z motions occurs in parallel with them.

This rapid reduction of global alignment time has proven to have a profound impact on test and assembly cost for photonic
products. It is well established in industry literature that alignment is the largest contributor, overall, to product cost 1.
Since alignment must be performed many times across multiple process steps in the test and assembly of photonics
devices, FMPA’s dramatic reduction in process time is highly leveraged.

The Toolmaker Market is Just Now Emerging for Photonics

There are similarities between the Silicon Photonics industry and the young semiconductor industry of the mid-1980s.
Today, you can build a fab just by issuing purchase orders, but that was not the case 35 years ago. Consequently, theplayers of that era--such as IBM, which dominated the semiconductor industry of that era--built their fabs around bespoke
tools constructed by systems integrators who were often captive or internal.
The situation is similar in Photonics today. There has not been enough toolmakers to satisfy demand, and there has been
little incentive for them to progress beyond legacy automation principles dating to the short-lived Photonics boom of 19972002. That boom was driven by one dominant end application: long-haul telecommunications, supplanting satellite-based
approaches. That is quite unlike today’s broadly diverse end-application base, which sees exponentiating data consumption
spanning needs as diverse as social networks, personal health, Big Data, the Internet of Things and even selfies. Still, thepainful memories of that era linger and have contributed to hesitation by potential new toolmaker entrants. That is finally
1 See, for example, “Automated High-Throughput Assembly for Photonic Packaging”, Barwicz et al, Photonics Summit, Cadence, 2017,https://www.cadence.com/content/dam/cadence-www/global/en_US/documents/company/Events/summits/photonics/fortier-2017.pdf and “Processbased cost modeling of photonics manufacture…”, E. Fuchs et al, J. Lightwave Tech., 2006, https://www.semanticscholar.org/paper/Process-based-costmodeling-of-photonics-the-cost-a-Fuchs-Bruce/125e24b2e2e71860f088526441ee5ce16e6ce42c

Case Study

F712T0056
Casjo, Copyright 2020, PI (Physik Instrumente) L.P., www.pi-usa.us
starting to change with entrants like FormFactor with its groundbreaking Cascade wafer probers and Tegema with its
Integrated Photonics Assembly tools, but it has meant that Photonics players and their contract manufacturers have oftenchosen to design and build their own production tools, just as IBM did in the ‘80s.

Production Example

It is necessary to protect the identity of most PI customers, so we will refer to this rather typical user as Substantial
Photonic Company, or SPC. SPC manufactures networking hardware. Their devices incorporate photonic chips withmultiple I/O channels, along with other elements such as lasers, photodiodes and fiber arrays. After probing 2, the chips aresingulated and proceed to packaging.
Attaching the fiber arrays to the chips is one of the fundamental steps in SPC’s packaging process and will be our example
here. For the reasons elaborated in the last section, SPC chose to build its own tooling. Not incidentally, this enabled themto leverage the latest alignment technology for this process (and others): PI’s FMPA.
In this and most other SiP alignments, the quantity that must be optimized is the optical power traversing the multiple
channels of the device 3. We tend to see three ways of conveying the signal from devices to PI’s controllers during test andassembly alignment:•••

Optical monitor tap. Here a portion of the optical throughput through the device or channel can be directlyaccessed. Often this is leveraged to perform assorted metrologies on the device during test and assembly, such as
spectroscopy. Inclusion of a high-bandwidth optical power meter is straightforward in these circumstances. PI’s
FMPA controllers incorporate high-speed analog inputs for the purpose. So, an optical power meter with ananalog output is needed. Select meters like PI’s own F-712.PM1
are optimized for fast alignment, providing the necessary fast
signal together with a logarithmic response, yielding many
benefits including eliminating auto-ranging (which would
otherwise need to be disabled) and providing especially extensive
capture capability due to the multi-decade dynamic range.
Photocurrent tap. Here the optical throughput through thedevice or channel can be measured by an on-device photodiodeor similar element. The resulting photocurrent is accessible to theoutside world. Connecting this to a high-bandwidthtransimpedance amplifier provides the signal that can FMPA’salignment rapidly optimize. An external photodiode input on theF-712.PM1 meter can provide this easily, together with thebenefits of the wide dynamic range from its logarithmic response.
Figure 1. F-712.HU1 Fast Multi-Channel
RSSI port. Some devices’ optical signal can only be measured byAlignment
Micro-Robot. This is an assembly of ainterrogating their Received Signal Strength Indicator port,
fast, high-resolution closed-loop Nanocube®
typically via I2C or SPI. In a production alignment workstation, apiezo nanopositioner and a long-travel, 6-DOF
simple microcontroller (such as an Arduino) is regularly employedhexapod driven by brushless DC servo-motors
to perform these queries rapidly in a loop, converting thenumeric response to each query into a voltage to drive the FMPA
process.

2 See https://www.youtube.com/watch?v=izM33rfVVc4 for FormFactor’s introduction to photonic wafer probing.

3 In principle, FMPA can optimize virtually any hill-shaped figure-of-merit that can be measured or calculated with sufficient speed, including things likeextinction ratio for polarization-sensitive alignments, calculations of image sharpness for imaging optic assembly, or calculations of beam quality for laser
assembly. In addition, PI controllers allow virtually any fast, measured quantity to be used as a servo input, allowing automatic positioning to achieve adesired set-point.

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Case StudyF712T0056
Casjo, Copyright 2020, PI (Physik Instrumente) L.P., www.pi-usa.us
SPC uses a mix of these in their applications. To perform the fiber-array attachment, SPC chose PI’s F-712.HU1 FMPA
system (Figure 1), deploying more than three dozen of these at their contract manufacturer. The alignment of the fiber
arrays to the silicon photonic chip proceeds as follows:
1.

2.

3.
4.

The fiber array is brought into near proximity to the chip. SPC’s production processes are highly controlled, so
their initial Z gap is repeatable to a few microns. Note: Other customers often must set or validate the Z gap. Thiscan be accomplished via several possible methods:
a. Vision. The field of machine vision has been broadly democratized in recent years, with the advent ofexcellent, USB-connectable cameras that can reduce the need for costlier “pro-grade” machine vision
cameras and frame grabbers. Powerful and accessible vision frameworks like http://OpenCV.org andintelligent cameras like http://OpenMV.io make quick work of device-presence detection, edge detection,
squaring and gap setting.
b. Contact. FMPA systems are available with optional force sensors with milligram-level sensitivity. Somecustomers use these to bring devices into contact, establishing the Z=0 condition. Then the motion
system can be commanded to back off to a desired gap.
c. Optical geometry. A succession of area scans can be performed at increasing Z, allowing the beam fan-out
to be observed and the absolute separation calculated. Since PI FMPA systems incorporate built-in areascan functionality and this is often used for device qualification, this can be a cost-effective approach.
d. Waist finding. In some waisted applications such as with a lensed fiber, the FMPA system can perform a
gradient search in Z to quickly determine the optimum Z position. An XY gradient search in parallel willkeep the alignment on-beam in cases where the mechanical and optical axes are not parallel.
e. Proximity sensor. Some device situations can accommodate a proximity sensor such as a capacitiveprobe, allowing the Z gap to be set directly.
f. OTDR. Some device situations can utilize optical time-domain reflectometry to set the gap between twoelements.
In SPC’s application, first light can generally be immediately detected in the first channel of the array thanks to the6-7 decade dynamic range of the F-712.PM1 meter. If not, the NanoCube® can find it by performing an area scanover its full 100x100µm range in typically 300msec or less. This is a built-in command in its controller and includes
runtime options such as automatic Gaussian fitting. Similarly, the hexapod (which shares nearly identical scan-andalignment functionality and commands with the NanoCube®) can perform an area scan over ranges from microns
to millimeters. The two mechanisms can even scan in parallel, speeding scans to tens of millimeters with scan
resolutions down to submicron scales.
Once first light is detected in the first channel, the NanoCube® can be commanded to optimize lock-onto thecoupling with a continuous gradient search. This will fine-align and track the coupling in real time.
Since the hexapod can rotate about any desired point in space, its pivot point is set near the optical axis of the firstchannel. The hexapod can then be commanded to perform a 𝛳𝛳 Z optimization on the 2nd channel 4. Note that theXY lock-on of the first channel automatically accommodates misplacements of the rotational centerpoint, allowing
both alignment processes to proceed in parallel— FMPA’s key advantage. Similarly, optimizations of 𝛳𝛳 X /𝛳𝛳 Y canoccur at the same time for devices requiring them.

Total time for steps 1-4 is approximately one second. Glue is then applied; the XY tracking corrects any disturbance fromthis and accommodates drift during the early phases of epoxy polymerization.
System options can include provisions for light-curtain and other safety interlocks, and soft limits are a standard feature for
implementing forbidden zones to prevent collisions.

4

The identical approach is used for devices with a U-shaped loop-back channel, as often found for facilitating assembly alignment.

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