Technical
Article
An introduction to light source design for semiconductor
test
Martin Buck
Senior Test Development Engineer
www.ams.com
My first introduction to a light sensor was as a student at school. I had to use a light dependent resistor (LDR) to automatically turn on a light bulb at dusk. I’m sure this is a familiar circuit to many
readers.
Moving forward a few years I went to technical college. There I was introduced to slotted optical
switches used as incremental rotary encoders in mechanical computer mice. This type of mouse
was used extensively in the 1980’s and 90’s until being replaced by the optical mouse. Little did I
realise back then that light sensors would explode into a multitude of applications.
To give you some idea of the range of optical sensors available I have picked a few devices from
our company’s extensive portfolio. AS7000 is a fully integrated health and fitness biosensor designed for wearable devices. TMG3992, TMG3993 and TMG4903 are a range of gesture modules
for touchless control in a wide range of applications from mobile handsets to automotive. TSL2572
is an ambient light sensor which approximates the response of the human eye, enabling features
such as dynamic display brightness on mobile handsets and TV’s. Color sensors are also available; these can be used in industrial process control, medical diagnostic equipment and LED control
to name a few. Proximity detection can be added to a number of these sensors and is also available separately on devices like TSL2672. Proximity sensing allows functions such as energy saving
to extend battery life of portable devices.
The increased complexity of the modern light sensor creates unique challenges in the world of semiconductor test. To test the optical performance of the sensor we will need a light source. So the
question is where do we start? If we were testing a memory cell or an analog to digital converter
(for example) the techniques used are well documented, however, there are virtually no references
relating to the design of light sources for testing. The reason is that the solutions used are closely
guarded secrets.
In this article I will discuss some of the key datasheet parameters relating to light emitting diodes
(LED’s). These form the basis of many light sources used in test. An understanding of these parameters will enable you to start the design of your source.
It is worth noting that there is commercial software available to aid with light source design. Such
software may or may not be available to you. It is still possible to design a suitable light source
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without the use of such software by making the light source easily modifiable.
Lifetime
In general LED’s do not abruptly fail after a certain number of hours use (as would be the case for a
filament bulb). Due to this there can be a misconception that an LED lasts forever. In reality the
output of the LED (like every luminaire) will reduce over time. In the visible light spectrum the light
emitted from the source is referred to as luminous flux and is quoted in lumens (lm). Lumen maintenance is the term used to describe the remaining luminous flux at a point in time. A number of LED
manufacturers have adopted the Illuminating Engineering Society (IES) L70 standard to quote lifetime. The L70 standard gives a lifetime based on the number of hours elapsed before the lumen
output drops to 70% of its original level. The L70 level was chosen because the human eye cannot
detect the difference in a light’s output until it has reduced by 30%.
An additional misconception is that an LED’s output will reduce linearly over time. In fact during the
first few hundred hours of the LED’s life the luminous intensity can increase or decrease. Whilst
these fluctuations may not be detectable by the human eye a light sensor may well react to them. It
is therefore important that luminous intensity is maintained during testing. It should be noted that
lifetime is not always quoted on a datasheet. If it is quoted the standard used may not be L70 so
this should be checked closely.
Temperature
Luminous intensity is also linked to operating temperature. Changing temperature can easily double or half the relative luminous intensity. The following diagram shows data from a Kingbright
KPTR-3216SURCK red LED:
Temperature variation also has an effect on the dominant peak wavelength of the LED. This could
cause an issue for the device under test (DUT). The photodiode that is under test will have a certain spectral response. Changes to the peak wavelength could result in a higher or lower than expected result due to this spectral response.
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Whilst we must consider the ambient operating temperature we also have to consider the heat generated by the LED device itself. It may surprise you that the radiant efficiency (optical power out divided by electrical power in) is within the range of 5 to 40% for LED’s meaning up to 95% of the
power is lost as heat. Keeping the LED drive current as low as possible will help, as will only leaving the LED on for a short period but inevitably there will be a certain current needed to sufficiently
illuminate the DUT. This may necessitate the use of a heat sink or other thermal management techniques.
Maintaining Luminous Intensity
During device test the luminous intensity has to be maintained using optical feedback. In its simplest form this could be a Light to Voltage sensor such as ams’ TSL250R product. The LED current
can then be controlled to ensure that the light to voltage sensor provides a fixed output voltage.
Required Illuminance
So we are aware of the importance of maintaining the luminous intensity but what illuminance is
needed in the first place? Well part of the answer is dependent on the sensitivity of the DUT. It’s
best to first choose an LED which has a luminous intensity higher than needed by the DUT. Ensure
that the chosen LED has a standard footprint size. The reason for this is that if there is a problem in
debug (for example more illumination is required) we can swap to a different LED easily.
Having established which LED we want to initially work with we have to check the viewing angle in
the datasheet. Each data sheet is a little different but generally you find a plot like this:
The plot shows the relative intensity in relation to the angle the LED is viewed at. 0o is perpendicular to the LED. In the example shown the LED has a viewing angle of 120o. This is defined as the
angle where the measured light intensity is 50% of its maximum value. In practice for our DUT we
want the intensity to be the same over as wide an angle as possible. From the viewing angle plot
we can determine the maximum angle where 100% relative intensity is maintained. In this case the
angle is approximately 10o.
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Having established the angle we now need to consider the area that needs to be illuminated. Nowadays we are required to test more than one DUT at a time so the illuminated area can be quite
large. A further complication is if the DUT has multiple photodiodes. This could be the case for
many encoders. Unless the photodiode is to be in saturation during testing an even illuminance
over the whole photodiode area will be required.
So, as an example, let’s consider illuminating a circle having a diameter of 2cm using the Kingbright
KPTR-3216SURCK LED referenced earlier. The LED provides 100% relative luminous intensity up
to an angle of 10o. We need to establish how far from the DUT the light source would need to be.
Simple trigonometry gives us the answer:
d
Ø
r
Distance (d) = Radius (r) / tan (angle of interest (Ø) )
d = 1cm / tan 10o = 5.67cm
So we now know what distance our LED must be from the DUT. We also know the sensitivity of the
DUT to light and therefore the illuminance needed. The problem is that illuminance at the dut
(measured in lux) is inversely proportional to the squared distance of the object from the light
source. This is a particularly wordy way to say that the further away the light source is the dimmer it
appears. Fortunately there is a simple calculation to determine the illuminance based on the luminous intensity quoted on the datasheet:
lx = cd / d2
Where lx = Illuminance in Lux, cd = Luminous Intensity in Candela and d = measuring distance in
meters:
From the KPTR-3216SURCK datasheet we take the typical luminous intensity which is 80mcd. We
already know the distance needed to illuminate the area is 5.67cm therefore:
Illuminance = 80mcd / 0.05672 = 24.884 lx
Compare this value to being only 1mm from the DUT:
Illuminance = 80mcd / 0.0012 = 80k lx
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To give you with some idea of the magnitude of this difference 100k lx is direct sunlight. A cloudy
day could be around 20k lx. In an office we would have around 500 lx and maybe 50 lx in a dimly lit
passageway.
In our example if 24.884lx was insufficient we have two choices. The first is to increase the drive
current but this has an effect on the operating temperature and could shorten the life of the LED.
The second option is to switch LED type to one having higher luminous intensity. If we were to
choose a different LED it must be ensured that the angle calculated is still valid.
A word on diffusers
In this article we have used examples consisting of only one LED. In reality the source could be an
array of LED’s. An array may be needed for a number of reasons, for example a single LED might
not provide sufficient illumination on the DUT therefore more LED’s are needed. Another example
is where the calculated distance between DUT and LED becomes impractical.
Overlapping light patterns created by more than one LED may cause a problem for the DUT. It
could be the case that the array creates “bright spots”. One way to combat this effect (if it is a real
problem during test) is to use a diffuser. There are numerous types of diffuser available, all will result in some level of transmission loss of the light.
When using a diffuser the distance between LED to diffuser and diffuser to DUT must both be calculated. For the distance between the diffuser and DUT this is calculated by using the diffusing angle of the desired diffuser (instead of the desired angle of the LED).
Many optical component suppliers will have diffuser kits. These typically have a screw mount making it simple to experiment with different types of diffuser and diffusing angle. Furthermore spacers
can be easily manufactured to adjust the distance between LED, diffuser and DUT.
Real life example
One project I was working on had a linear photodiode array to test. The photodiode array formed
part of a triangulation sensor. This type of array can also be used in scanners and printer edge detection. ams has a number of standard products in this area such as TSL140 and TSL141 although
in this case the product was an application specific design.
In a triangulation sensor the module emits focused light on to an object. The light is then reflected
back from the object to the sensor. In order to determine the correct position of the object the photodiodes in the array must all respond equally. The maximum mismatch between photodiodes
could be 1% or less depending on the positional accuracy needed.
The light source used is shown overleaf:
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EEPROM containing light
source identification, calibration
factors etc.
635nm Red LED array
Light to Voltage Sensors
You can see that the light source consists of an array of surface mount LED’s to illuminate a large
area during testing. There are 3 light to voltage sensors for redundancy (ensuring that should a
sensor be damaged it can be easily detected using the measurement from the other two). During
development the LED’s were changed to a different type. This was done as the original LED’s only
gave the required illuminance when driven close to their maximum current. This generated far too
much heat; additionally the sensors were changed as the initial ones became saturated making accurate control of the source impossible.
After all the initial issues with the light source were resolved data was collected from a real device.
The data is shown here:
200
1
180
0,9
160
0,8
140
0,7
120
0,6
100
0,5
80
0,4
60
0,3
40
0,2
20
0,1
Photodiode Mismatch (%)
Photodiode response (mV)
Photodiode Array Results
Output Voltage
Mismatch
0
0
1
21
41
61
81
101
121
Photodiode number
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The graph shows that across the array there is a drop of around 3% in the output voltage (well
within the expected performance). Neighbouring pixels have a maximum mismatch of 0.3% which
is again well within specification.
In production we actually test two DUT’s in parallel. The light source illuminates both DUT’s at the
same time. We have to ensure that the results from both sites are comparable. Any issues with the
light source such as bright or dark spots would create a correlation issue. The following chart
shows the results from one photodiode in the array measured across 9 different devices. Each device was tested 50 times.
Photodiode response (mV)
Site To Site Comparison
220
210
200
190
180
170
160
150
Device 1
Device 2
Device 3
Device 4
Site 1
Device 5
Device 6
Device 7
Device 8
Device 9
Site 2
The chart clearly shows excellent correlation between the two sites. The correlation between sites
for all photodiodes in the array was equally good.
Having one device interface board (DIB) for testing is not sufficient for full production. Therefore
several boards need to be manufactured which means there are several light sources too. To ensure good correlation between boards the light source must be calibrated before being allowed into
production. The process involves using samples with a known response to light, the current to the
new light source is then adjusted to give the required output. Once the current is known the voltage
from the light sensors on the light source are recorded and used to control the source for all future
runs.
The following chart shows 10 devices looped 10 times each. In total there are four runs, two were
performed on the first production board, two were performed on the new board. The result shows
exceptionally good correlation between both boards.
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Test Board Comparison
230
220
210
Response (mV)
200
190
180
170
160
150
140
Device 1
Device 2
Device 3
Board 1 - Run 1
Device 4
Device 5
Board 1 - Run 2
Device 6
Device 7
Board 2 - Run 1
Device 8
Device 9
Device 10
Board 2 - Run 2
Summary
Designing a light source to test light sensors requires us to consider a number of factors. Without
expensive software to design the source we have to consider making the solution easily adaptable.
Where possible the design should use a standard LED footprint. In some applications we may need
to consider housing the source in a cylinder. This allows screw mount spacers and diffusers to be
easily added.
We must be aware of the need to control the light source output during use. This can be achieved
with a suitable feedback sensor within the design. Current should be kept as low as possible to reduce the amount of heat generated by the source. Should excessive heat be generated then suitable thermal management will be needed to prevent damage to the LED or causing a shift in dominant wavelength.
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Biography
Martin Buck began working in the semiconductor industry over 20 years ago after received a First
Class BEng(hons) degree in Electrical and Electronic Engineering from the University of Plymouth.
Martin has had roles in failure analysis, quality assurance, product engineering and test development (among others). Since 2005 he has been working for ams based in the UK as a senior test development engineer. The UK office was founded by Martin and three colleagues. He has worked on
a wide range of products in several sectors including consumer electronics, medical electronics and
industrial electronics.
For further information
ams AG
Martin Buck
Senior Test Engineer
Tel: +44 1752 754911
[email protected]
www.ams.com
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