Considerations in Evaluating Used or Rebuilt
Zygo Metrology Lasers

Abstract

Introduction

The Test Arm may be a tool in a CNC milling machine, a stage in a semiconductor wafer stepper, a voice coil positioner in a hard drive servo writer, or any number of other precision devices. A single laser can be used with many independent measurement axes through the use of beamsplitters, separate interferometer optics and optical receivers, and associated digital processing channels.

The key attributes that make these lasers ideal for metrology applications are that they produce two frequency components 20 MHz apart that are linearly polarized, orthogonal, and oriented along the X and Y axes (horizontal and vertical) relative to the laser baseplate. The optical frequencies are highly stable and the corresponding wavelength (the actual "yard stick") thus should be as well. And, they remain stable for the life of the laser without any maintenance. (However, environmental factors like temperature, pressure, and humidity must to be taken into consideration as they have a significant effect on wavelength.)

Two views of a typical Zygo ZMI laser with its cover removed are shown below:

Left Side View of Interior of Zygo-7702C/E Laser

Right Side View of Interior of Zygo-7702C/E Laser

The heart of Zygo two-frequency lasers is a 3 to 4 mW HeNe laser tube that is presently made by or for Zygo. A bifilar-wound or thin-film heater is added to the laser tube, which is then mounted inside the gray metal enclosure visible below the HeNe laser power supply brick. The output beam exits toward the left, passing through the black beam sampler assembly to the AOM, reflects off of two turning mirrors to a spatial filter and then out to the right via beam expander telescope (hidden from view). Most Zygo lasers including the 7701, 7702, 7712/14, and 7722/24 use the same HeNe laser tube, though the optics may differ.

The custom HeNe laser tubes used by Zygo since the early to mid-1990s have an expected life of 50,000 hours. Before then Zygo used standard commercially available tubes from Aerotech (a former manufacturer of HeNe lasers) and possibly others. While the present Zygo tube is not available at every corner store :-), it is simply a random polarized HeNe laser tube that has been designed for long life and mode-flip-free behavior. A typical Zygo tube is shown below:

Zygo HeNe Laser Tube Used In 77XX Series ZMI Lasers

Its construction is essentially the same as a vanilla flavored HeNe laser tube used in countless other applications. There are no magnets, waveplates, or internal heaters to make it a truly special tube. Unlike HP/Agilent lasers [3,4], failures other than the tube reaching end-of-life are not that unusual and thus Zygo lasers may be pulled from service due to a problem elsewhere, usually on the controller PCB. Repair of the electronics is possible, but detailed documentation and schematics may not be available to any non-Zygo service organizations, rendering the newer digital control PCBs essentially unrepairable. There is a similar lack of information on the older analog control PCBs, but these at least could be reverse engineered in a straightforward, but very tedious way.

So, where there is an electronic problem, it may be possilble to repair the control PCB, but it may be easier to simply swap in a good control PCB from a similar ZMI laser with a bad tube. This eliminates any issues of optical alignment, but means that the settings may need to be adjusted on an analog control PCB, or that whatever the settings were on a digital control PCB will have to be close enough.

But where the tube is weak or dead, and the HeNe laser power supply and optical alignment have been ruled out as possible causes, then either a replacement tube needs to be installed, or the (good) electronics can be swapped into another chassis with a good tube but bad controller.

Commercially available standard tubes can be used since although the Zygo tube is custom, there is nothing really special about it as far as lasing is concerned. The main thing that makes it custom is the special thin film cathode deposited on the inside of the glass envelope which is the primary means of achieving the 50,000 hour life. The lasing parameters of a commercial HeNe laser tube with similar beam diameter, divergence, output power, and similar length will be close enough to permit such a tube to be installed in place of the original tube without requiring major optical adjustments except alignment. Suitable tubes are available from HeNe laser companies like Pacific Lasertec (PLT, formerly the HeNe division of Melles Griot):.

Used Zygo ZMI metrology lasers are also widely available. But many of these are already unusable due to low output power or bad electronics. Thus, finding one with both acceptable performance and adequate life expectancy requires a knowledge of what to look for and what tests to perform.

These lasers are often run 24/7 from the day they are installed until the day they die or fail preventive maintenence checks. Such lasers invariably find their way to eBay and unscrupulous sellers will either claim the "came from a working environment" or an inability to test them. The working environment claim may not be inaccurate, it's just that the laser was pulled because it was dead, not that the line was shut down! :) However, if a seller is reputable, has performed a few basic tests, and offers a warranty (even a relatively short one giving the buyer an opportunity to more fully test it), then a previously owned unit may be perfectly acceptable with low risk. Even where it has failed for other reasons like a bad HeNe laser power supply, a broken laser with a good tube may be easily repaired.

However, if it were possible to replace a bad then this opens up a third possibility with performance potentially equal to that of a new laser at a fraction of the cost. When done properly, the laser would perform essentially like new and have a decent life expectancy (though possibly not as long as that of the original long-lived custom Zygo tube).

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Used ("Previously Owned") Zygo ZMI Metrology Lasers

All Zygo lasers have a run-time meter of some kind. The older ones with analog control PCBs have a "mercury" column indicator inside the case near the HeNe laser power supply. The digital control PCB keeps track of run time in nonvolatile memory (NVRAM) which can be interrogated via the RS232 port. Assuming that the tube is original, and in the latter case that the control PCB hasn't been swapped, checking thes can give an idea of the expected remaining life of the tube.

When semiconductor fab lines shut down, the lasers often become available at various stages of their life. They appear on eBay and from many surplus dealers at costs ranging from $25 or less to several thousand dollars. However, almost any of these may be less than the cost of a rebuilt laser.

If it were possible to have confidence in the operating condition and life expectancy of a previously owned laser, it would represent a low risk alternative to either a new or rebuilt one.

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Rebuild Options

There are a few companies who will rebuild Zygo ZMI lasers by installing a new (but probably non-Zygo) tube assembly or sell you a laser with a rebuilt tube assembly. Although I have yet to see a typical cost, the amount of labor involved (more below) would suggest that it is a substantial fraction of the cost of a new laser. And there is some risk since depending on the quality and type of rebuild, the laser may not perform to spec or have a short life. A semiconductor wafer stepper (one of the most common applications of these lasers) is a very expensive piece of equipment often run around the clock. Downtime is costly, and errors in fabrication only found after wafers have been completed are even more costly. So it's not clear at what point the modest savings of a rebuilt laser installed once or twice over the entire life of the machine can be justified against the risk. Nonetheless, some large semiconductor companies are known to have seriously considered going this route and may be using rebuilt lasers in production.

The Zygo tube assembly consists of the actual glass HeNe laser tube and a bifilar-wound heater glued to its exterior.

Zygo Tube Assembly with Bifilar-Wound Heater

Note the very precise tube mount/centering technique - blobs of black RTV-Silicone! :)

A thin-film heater is also used in some Zygo lasers as shown below:

Zygo Tube Assembly with Thin-Film Heater

However, I while I have seen late model 7702s using a thin film heater along with a temperature sensor (not found with the wirewound heaters), I do not know if it is now standard. Some versions of the control PCB appear to be compatible with both types. The labor is certainly lower for a thin-film heater, though the cost of the heater itself is much greater. The one above is almost certainly not from a 7702 laser as the heater wires exit the wrong end and it lacks the temperature sensor.

Rebuilding a Zygo tube assembly can take two forms: regasing the original or replacement with a standard HeNe laser tube. Regasing is generally not an option both for technical reasons and due to the high cost compared to a replacement tube. So, we only concentrate on the replacement option.

The physical process of tube replacement is straightforward: Find a suitable random polarized HeNe laser tube about 9 to 10 inches long like the one shown below:

Spectra-Physics Random Polarized HeNe Laser Tube

However, note that these are not the typical barcode scanner tubes that may have similar size and appearance. They must have an output power of 3 or 4 mW, not the ~1 mW typically used in barcode scanners.

Replacing the tube is not nearly as easy as swapping the entire tube assembly in HP/Agilent lasers. But the glass tube itself can be extracted without using extraordinary means to remove any potting compound as there is none. It simply clamps within the gray enclosure using the two sets of brackets visible in the photo and those blobs of RTV-Silicone. Removal then requires taking off the front panel and beam sampler block, disconnecting the heater cable and unsoldering the anode wire.

Prepare the new tube by adding a heater with similar electrical characteristis to that of the original, which can be wire-wound or a thin-film type. Or, if the original was the latter, it might be possible to simply peel the old one off and transfer it to the new tube. Then install the replacement tube assembly in the Zygo chassis in reverse order from removal. Next, the output must be precisely aligned to pass the un-shifted mode through the output optics and the AOM must be adjusted to pass the shifted mode as well. If the tube is carefully centered when installed in its enclosure, this process will be simpler. Finally, the control PCB may need to be adjusted to match the amplitude of the sampled modes for stabilization, or alternatively, their intensity can be adjusted with a filter inside the beam sampler.

However, where a Zygo chassis is available with a good tube but bad control PCB, swapping in good electronics is even easier and quicker.

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Potential Issues

• Beam profile: The normal Zygo beam profile is probably described best as a Gaussian that is cut off. The 3 mm and 9 mm beams have the same shape as the 6 mm beam, but 1/2 and 3 times the diameter, respectively. The easiest thing to do is to compare the beam profile of the candidate laser with a genuine one. For many applications, the exact beam profile is not that important and was simply selected based on the desired interferometer optics. But where long measurement distances are involved, for example, getting the maximum benefit from a larger diameter beam may be required.

  The beam profile on a used laser should not have changed unless someone attempted to adjust it or swap optics to convert from one beam diameter to another (e.g., from 6 mm to 3 mm).

  Since the beam originates internally from a pinhole, a rebuilt Zygo should also have a similar beam profile to the original.

• Mode alignment, balance, and purity:. Test the alignment by rotating a polarizer in front of the laser and confirming that the beat signal from a high speed photodiode or optical receiver (virtually) totally disappears when aligned with the X or Y axis. Test the mode balance by comparing the optical power through the polarizer when oriented with the X and Y axes. The two values should be within 20 percent of each other for a 7701 or 5 percent of each other for a 7702. For a rebuilt laser, errors in mode alignment depend on how accurately the HeNe laser tube is oriented when it is was originally mounted, and this may be related to mode purity. Mode balance can also change since that is a function of the RF drive to the AOM and its alignment. Unequal mode balance will reduce the available optical MEAS signal level relative to laser output power but have no other effect. Errors in mode alignment or purity will result in interference between F1 and itself, and F2 and itself in the interferometer optics. The result will be low frequency level shifts of the envelope of the detected signal from the photodiode in the optical receiver. This may result in jitter in the MEAS signal.

  For a much more in-depth treatment of issues relating to off-axis modes, see reference [5].

  None of these is likely to change in a significant way over the useful life of the laser except possibly mode balance, which is affected by the RF level to the AOM and thus electronic component drift. However, when the output power declines to well below the Zygo spec'd minimum, amplitude ripple in the output power due to laser current ripple from the HeNe laser power supply can interfere with a clean beat frequency signal.

• Rogue modes: Unwanted longitudinal modes are extremely unlikely to be present. They won't develop in a used laser and it simply is not possible for them to be present in a rebulit laser that has locked correctly since there isn't enough space for a long enough tube to support additional longitudinal modes!

• Mode flipping: In HP/Agilent metrology lasers, only a single Zeeman-split longitudinal mode is lasing. However, in Zygo lasers, there are a pair of orthogonally polarized longitudinal modes lasing, parked approximately equidistant on either side of the neon gain curve. However, which one is on the high side and which one is on the low side is not a result of anything fundamental and during warmup they basically march through the gain curve as the tube expands. Once the feedback is enabled for locking, a specific polarization will be forced to the high side, and the orthogonal polarization will be on the low side. Only one of these modes is used in 7701/2/12/14, and only one at a time in 7722/24 lasers.

  Under some conditions, the two modes can flip polarization instantaneously. They will then be on the wrong side of the neon gain curve. In a well behaved tube, this won't happen spontaneously, but may occur due to back-reflections. But in some cases, they can flip with no obvious cause and such events may happen hours apart. However, should one occur when the laser is in use, the optical frequency will jump up or down by about 750 MHz (~1.5 ppm) and the feedback will then cause the modes to drift back to their normal position over a few seconds. This will result in either (hopefully) a loss of signal error or (worse) an error in the measurement data which could go undetected.

  The HeNe laser tube must be thoroughly tested prior to installation to assure that spontaneous mode flipping does not occur, but this affliction can even be found in some Zygo lasers.

• Stick-slip noise: Zygo takes great pains to make sure that as parts of the HeNe laser tube expand with temperature, the cavity length won't suddenly change as a result of something being restrained by friction, and then suddenly breaking loose changing the distance between the mirrors. Both parts inside the tube, and its mounting, can contribute to stick-slip noise. If a slip were to happen while being used for a measurement, there would be a transient error that might not even be caught by the electronics. Once fully warmed up, this is not likely to occur, so it may not be an issue unless the laser it put into service soon after it has locked.

  Rebuilt lasers are more likely to suffer from this problem due to both the use of laser tubes that haven't been designed to eliminate it, and due to the possible use of rigid adhesives or other factors in the physical mounting of the tube in the Zygo enclosure.

• Laser output glitches: Aside from electronic problems like bad solder joints in the HeNe laser power supply or wiring, this is not likely to happen with a replacement (non-Zygo) tube. However, after having been run for 10s of thousands of hours (or with some, much less) original Zygo tubes may exhibit momentary dropouts and restarts of the laser tube resulting in a corresponding momentary loss of beam. The cause is believed to be imperfect electrical contact between the thin-film cathode coated on the inside of the glass and the cathode end-bell. Successive events may occur within minutes - or days. The duration is order of milliseconds and my result in a mode flip after the glitch as well as described above. But the glitch alone will result in measurement errors if not flagged as an error. Unlike stick-slip noise, these generally do not go away after warmup.

  Since these dropout occur infrequently, and are virtually imperceptible by eye anyhow, the only way to test for them is to run the laser while monitoring the beam or laser tube current using a data acquisition system with a sample rate of at least 50 s/s. But again, this really only applies to tubes manufactured by Zygo, usually after 10s of thousands of hours of use. Dropouts are not likely with the types of tubes used to rebuild Zygo lasers, nor the Aerotech tubes found in really old Zygo lasers.

• Life expectancy: The original Zygo HeNe laser tubes are specially processed to result in an expected life of 50,000 hours. Where a new conventional HeNe laser tube has been installed, the lifetime will depend on its construction and quality. Typical commercial HeNe laser tubes have an expected life of 10,000 to 20,000 hours with some going to 40,000 hours.

• Beam alignment: Beam alignment in Zygo lasers is determined by the position of the internal pinhole and beam expander telescope, neither of which should change either in a used or rebuilt laser.

• Time to lock (READY LED on solid): This is typically 10 to 15 minutes for a healthy Zygo laser and should not change significantly.

• Beat (REF) frequency: Since this is determined by a crystal oscillator, it should be unchanged.

• Absolute optical frequency/wavelength: It is not known if Zygo laser tubes use isotopically pure He and Ne, which along with fill pressure are what mostly determine the optical frequency. However, normal machine calibration procedures should factor in any change.

• Short term optical frequency/wavelength stability: With a replacment tube running at the same tube current as the original, the stability should be similar.

  The optical frequency tends to decrease with use. For a used laser, it's never likely to be so low as to not meet Zygo specs for optical frequency, but this, too, can be used as a guide to the health of the laser.

• Long term optical frequency/wavelength stability: All HeNe lasers drift in optical frequency as they age, mainly due to the decrease in gas pressure inside the tube. A rebuilt laser should be similar to an original in this regard as well.

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Acceptance Testing Summary

Confirm that the +/-15 VDC power supplies have the required specifications (voltage accuracy and current ratings). It would also be desirable to run everything on a power conditioner and/or constant voltage (e.g., Sola) transformer to rule out incorrect or noisy power as a cause of unexpected behavior, should any occur. And, of course, if the cabling isn't idiot-proof, double check the connections BEFORE applying power!

1. Starting: Except for the 5501B, a new or rebuilt laser should produce an output beam almost immediately, though many used lasers will require a few seconds to a minute or more to come on (though they will typically restart instantly if power is interrupted).

   A laser that takes a long time to start but runs reliably may be perfectly acceptable, especially in applications where the laser is then run continuously for days or longer. However, starting can be hard on the HeNe laser power supply, and the Zygo controller and/or measurement display electronics may give up after a fixed amount of time, and produce a non-recoverable hard error.

2. Running: Once the laser starts, it should remain on until power is removed. Any dropout, sputtering, or flickering is a cause to reject the laser unless corrective action is taken. But this is usually beyond the capabilities (or desires!) of the end-user. It's essential to monitor the laser status until at least when it locks ("Stable" LEd on) since marginal tubes may only start misbehaving after they warm up.

3. Locking: Most of the Zygo lasers typically in 10 to 15 minutes (Stable LED on solid). A bit less or a bit more doesn't matter, but a very long lock time could indicate other underlying problems, and may result in a hard error from some measurement electronics.

   A healthy used or rebuilt laser should lock in about the same time as a new laser.

4. Locked output power: The optical output power from the front of the laser with the normal (large) aperture should exceed the minimum specification for the particular laser. For most of these, it's 400 µW. For original Zygo laser tubes, the output power may increase or decrease slightly after the laser locks and over the next hour or so, especially for used lasers with higher mileage tubes. Much of this change is due to a change in mirror alignment due to thermal effects. Output power from 7701s and 7702s with new tubes will typically be over 600 uW. For 7712/14s, 2 mW.

   Note that a measurement of output power should only be considered accurate once the laser has locked and READY is on solid. Before then, it may vary by 25 percent or more due to mode sweep, especially for a high mileage tube whose output power has declined significantly compared to its value when new.

5. Beam profile: The normal beam profile for Zygo lasers is along the lines of the center portion of a Gaussian with the sides cut off, not the typical full Gaussian TEM00 spatial mode of a common HeNe laser. The beam profile won't change in a used laser, though the variation from center to edge will tend to increase as the power declines. But a rebuilt laser with a conventional tube may not have matched optics, so almost anything is possible, though the most likely result is no visible difference compared to the genuine Zygo laser.

   Two possible effects of a sub-optimal beam profile will be to decrease MEAS signal amplitude and make interferometer alignment more critical. For many applications, the exact beam profile may not matter, but for some equipment, there may be specific installation tests that will fail if the beam profile isn't close to the original from Zygo.

6. Mode alignment: When a polarizer (sheet polarizer or polarizing beam splitter cube such as from an HP interferometer) is rotated in the output beam, the MEAS signal from an optical receiver should be present at all orientations except in an angular range centered around the X and Y axes with respect to the laser baseplate.

7. Mode balance: The output power in the X and Y polarized modes should be within about 20 percent of each-other for a 7701; 5 precent for a 7702 or 7712/14 after the laser has been on for several hours. (They drift somewhat during warmup.) While a modestly larger difference won't really affect performance beyond slighlty decreasing the useful MEAS signal level, it may be an indication of sloppy adjustments (or lack of knowledge with respect to adjustments!) during a rebuild. Also use a polarizer to check the appearance of the H and V polarized components of the beam. Both should be fairly symmetric, though perhaps not quite perfect.

8. Mode purity: If the mode alignment and balance are acceptable, mode purity should be decent.

9. REF signal: An optical receiver should produce a clean stable waveform (squarewave) with crisp sharply delineated tops and bottoms and rising and falling edges.

10. REF frequency: This should be very close to 20 MHz.

11. MEAS signal (stationary): This should be the same as the REF signal, above - clean and stable.

12. MEAS signal (moving): The MEAS waveform from the optical receiver should be clean, just like REF. Movement of the "Target" will result in the period/frequency of the waveform changing, but at any instant, it should have no fuzz or other indication of instability. Misaligned, impure, or rogue modes can result in both amplitude and duty cycle changes, with the most obvious result being fuzzy rising and/or falling edges (depending on the scope triggering).

13. Optical frequency: For a rebuilt laser, knowing the optical frequency is probably not very important as long as any machine calibration procedure takes the corresponding variation in wavelength into consideration. The optical frequency for rebuilt lasers may be quite different from the original, especially those using conventional tubes.

    For a used laser, the optical frequency relative to a similar known laser that is new, has seen little use, or has been run for a known amount of time, can provide another indication of how much use it has seen. The difference in optical frequencies for a healthy laser will typically be only a few MHz, while one near end-of-life may be lower by 15 MHz or more.

    It's not clear that knowing the absolute optical frequency is of much added value for any of these lasers, except possibly to use it as a reference in testing other similar lasers in the future, and to track changes as the tube ages. So, comparing with a low mileage Zygo laser is as good as comparing with an iodine stabilized HeNe laser.

14. Mode flipping and other transient errors: Unless very severe, this will require running the laser with some means of monitoring its output over many hours. It may be possible to use the measurement electronics of the equipment to do this, but they may not even catch some glitches. So, it is probably best to use a stand-alone data acquisition system to monitor the optical power of the H and V components of the laser output at a rate of at least a few samples per second. If possible, have the software flag any sudden changes that occur. A similar anomaly in both H and V is due to something happening to the beam the laser tube produces. An anomaly in one polarized component, with a partial opposite effect in the other, is due to a problem in the AOM or its driver. In addition to mode flipping, stick-slip effects may produce similar errors. And, of course, electronic problems like bad solder connections or cables can also result in similar symptoms. In some cases, the laser will turn on the "Service" LED, but probably not for a momentary error that is corrected.

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Discussion and Conclusions

And of course, with any laser that has been modified without Zygo's strict quality control, there would be some risk, so rigorous adherence to a weekly or monthly test and calibration regiment would be essential in identifying and tracking any changes in performance over time.

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1. Zygo Corporation. Go to: "Stage Position (OEM)" or search for "ZMI".

2. General information on Zygo metrology lasers and systems: Sam's Laser FAQ chapter "Commercial HeNe Lasers", sections starting with: Zygo HeNe Lasers.

3. Agilent Technologies. Search for a specific model laser or system, or "metrology lasers". Their Web site has specifications for all current lasers and systems but little if any on older models like the 5501B that they consider obsolete and no longer support. There is also extensive technical information on all aspects of Agilent metrology systems and components.

4. General information on HP/Agilent metrology lasers and systems: Sam's Laser FAQ chapter "Commercial HeNe Lasers", sections starting with: Hewlett-Packard/Agilent HeNe Lasers.

5. In depth treatment of measurement anomolies due to rogue or off-axis modes: "An investigation of two unexplored periodic error sources in differential-path interferometry", Tony Schmitz and John Beckwith, Precision Engineering, volume 27, issue 3, July 2003, pages 311-322.

6. Companion document: Considerations in Evaluating Used or Rebuilt Hewlett Packard/Agilent Metrology Lasers.

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