Sintered Snowflake

Credit: Janet Wager

"My snowflakes are fragile. Will more glass make them stronger?”

My experiments for the e-book Low Temperature Kilnforming showed more frit does not make these sintered pieces stronger. Sintered pieces made from frit are made stronger with a combination of packing, time and temperature.

Packing

The amount of packing makes the whole more dense by linking more particles together. The most dense packing is done by putting down thin layers and packing each layer into the previous one. This allows more connections to be made between the pieces, which is important to the resulting strength.

Temperature

The temperature at which the frit is fired is important too. Firing at 650°C/1202°F will produce a weaker piece than one fired at 690°C/1274°F, if both are soaked for the same amount of time.

Time

Generally, the soak at the top temperature will be about two hours. The interplay of time and temperature are particularly important when sintering frit. The connections at the atomic level take more time to form at low temperatures than at high ones. The piece fired at 650°C/1202°F can be strengthened by extending the time to 4 hours instead of 2 hours, which will give it about two thirds of the strength of firing at 690°C/1274°F, for 2 hours.

Rates

The ramp rates need to be slow to achieve maximum strength. For example. A rate of 150°C/270°F to 690°C/1274°F needs a 2 hours soak, but a rate of 600°C/1080°F needs 6 hours to achieve the same strength. An alternative is to use a fast ramp to the strain point and then 50°C/90°F to the top temperature for a 3 hour soak.

Texture

The texture of the sintered piece will vary according to time and temperature. A firing at 690°C/1274°F for two hours will give a shiny top surface, but will be more textured underneath. A two hour soak at 650°C/1202°F will give a sugar grain appearance, but be weak. Extending the soak time to 4 hours will approximately double the strength. If you want to retain the sugar texture and have a strong piece, a long soak at a top temperature of about 650°C/1202°F for at least 3 hours will be needed.

Annealing

Sintered pieces need to be annealed for about 2.5 – 3 times longer than their thickness to be strong and the cool rates need to be related to the soak time to be as stress free as possible.

The e-book Low Temperature Kilnforming is available from:

Bullseye

Etsy

and me:  Stephen.Richard43@gmail.com

Observations on Some Suggestions about Annealing

There are writings from a teacher attempting to make glass fusing simple.  Unfortunately, glass physics and chemistry are very complicated.  Attempting to avoid these complications leads to failures and other difficulties as the practitioner progresses. 

Proper annealing is one of the fundamentals to achieving sound kilnforming results.  Some suggestions have been made by a widely followed person to “simplify” the understanding of the annealing process.  Discussion of the meaning and importance of annealing can be found in many places, including here.  

Annealing temperatures

It has been suggested that the annealing temperatures can be inferred from the CoE of the glass that is being used. Discussion of what CoE is and is not can be found here and here.

Annealing temperatures are not directly related to the expansion coefficient (CoE) of the glass.  This can be shown from the published annealing temperatures for different glasses organised by presumed CoE:

·        “CoE96”: Wisssmach 96 - anneal at 482°C;  Oceanside - anneal at 515°C

·        “COE94”: Artista - anneal at 535°C

·        “CoE 93”: Kokomo - anneal between 507°C and 477°C – average 492°C

·        “CoE 90”: Bullseye - anneal at 482°C; Wissmach90 - anneal at 482°C; Uroboros FX90 - anneal at 525°C

·        “CoE 83”:

o   Pilkington (UK) float - anneal at 540°C;

o   typical USA float - anneal at 548°C;

o   Typical Australian float - anneal between 505°C and 525°C, average 515°C

This shows there is no direct relationship between CoE and annealing temperature.  Do not be tempted to use a CoE number to indicate an annealing temperature.  Go to the manufacturer’s web site to get the correct information.

Temperature equalisation soak

Annealing for any glass can occur over a range of temperatures.  The annealing point is the temperature at which the glass can most quickly be annealed.  However, the glass cannot be annealed if it is not all at the same temperature throughout the substance of the glass.  It has been shown through research done at the Bullseye Glass Company that a temperature difference of more than 5°C will leave stress within the glass piece. To ensure good annealing, adequate time must be given to the temperature equalisation process (annealing). 

From the Bullseye research the following times are required for an adequate anneal soak:

6mm /   1/4"            60 minutes

[9mm /  3/8"           90 minutes]

12mm  / 1/2"          120 minutes

[15mm  /   5/8"       150 minutes]

19mm   / 3/4"         180 minutes

[ ] = interpolated from the Bullseye chart for annealing thick slabs

Anneal Cooling

There are suggestions that a “second anneal” can be used on important pieces.  Other than observing that all pieces are important to the maker, the suggestion should be investigated.  On looking into the idea, it is essentially a second soak at 425°C, which is slightly below the strain point, rather than controlled cool from the anneal soak temperature.

It is reported that the Corning Museum of Glass considers 450°C as the lower strain point – the temperature below which no further relief of strain is possible.  This means that any secondary soak must occur above 450°C rather than the suggested 425°C. Such a soak is unnecessary if the appropriate cooling rates are used. 

Cooling Rate

Except in special circumstances, the cooling rate needs to be controlled as part of the annealing process.  Soaking the glass at the anneal is not the completion of the annealing.  Most practitioners follow the practice of choosing a slow rate of cooling from the annealing soak to some point below the strain point rather than a rapid one with a soak at the strain point temperature.

Annealing is not just the soak time (which is there to equalise the temperature), it is about the rate of the annealing cool too. The rate at which you cool is dependent on the thickness of the glass piece and whether it is all of one thickness or of variable thicknesses.

Even thickness

                                         Cooling rate

Dimension      time (mins)     to 427°C to 371°C

6mm              60                 83°C       150°C

9mm              90                 69°C       125°C

12mm            120                55°C       99°C

15mm            150                37°C       63°C

19mm            180                25°C       45°C

                                        Cooling rate

Dimension      time (mins)     to 800°F   to 700°F

0.25"              60                 150°F       270°F

0.375"            90                 124°F       225°F

0.5"               120                100°F       178°F

0.675"           150                67°F         114°F

0.75"             180                45°F         81°F

Tack fused/ uneven thickness

If your piece is tack fused, you need to treat the annealing rate and soak as though it were twice the actual total thickness. This gives the following times and rates:

Tack fused

Dimension (mm)                                Cooling rate

Actual     Calculated       time (mins)    to 427°C   to 371°C

6            12                 120                55°C       99°C

9            18                 150                25°C       45°C

12          25                 180                15°C       27°C

15          30                 300                9°C         18°C

18          38                 360                6.7°C       12°C

Dimension (inches)                                Cooling rate

Actual     Calculated       time (mins)    to 800°F   to 700°F

0.25          0.5                 120                100°F       180°F

0.375        0.75               150                45°F         81°F

0.5            1.0                180                27°F          497°F

0.675        1.25               300                16°F         36°F

0.75          1.5                360                12°F          22°F

Contour fusing requires firing as though the piece is 1.5 times thicker.  Sharp tack or laminating requires 2.5 times the the actual thickness.

Fusing on the floor of the kiln

There is a further possible complication if you are doing your fusing on the kiln floor, or a shelf resting on the floor of the kiln.  In this case you need to use the times and rates for glass that is at least 3mm thicker than the piece actually is. 

Thus, a flat 6mm piece on a shelf on the floor would use the times and rates for 9mm: anneal soak for 90 minutes, anneal cool at 69°C to 427°C and then at 124°C to 371°C.  It would be safest if you continued to control the cooling to room temperature at no more than 400°C per hour.

But if it were a tack fused piece of a total of 6mm you would use the times and rates for 18mm.  This is using the rates for twice the total thickness plus the additional 3mm for being on the base of the kiln.  This gives the times and rates as being an anneal soak of 360 minutes and cooling rates of 7°C to 427°C and 12°C to 370, followed by 40°C per hour to room temperature.  Any quicker rates should be tested for residual stress before use.

Source for the annealing and cooling of fused glass

These times and rates are based on the table derived from Bullseye research, which is published and available on the Bullseye site.   It is applicable to all fusing glass with adjustments for differing annealing soak temperatures.

Annealing over multiple firings

It has been recommended by a widely followed person that the annealing soak should be extended each time subsequent to the first firing.  I am uncertain about the reasoning behind this suggestion. But the reasons for discounting it are related to adequate annealing and what is done between firings.

If the annealing is adequate for the first firing, it will be adequate for subsequent firings unless you have made significant alterations to the piece.  If you have added another layer to a full fused piece, for example and are using a tack fuse, you will need to anneal for longer, because the style and thickness have been altered.  Not because it is a second firing.  If you are slumping a fired piece, the annealing does not need to be any different than the original firing.

The only time the annealing needs to be altered is when you have significantly changed the thickness of the piece, or the style of fusing (mainly tacking additional items to the full fused piece).  This is when you need to look at the schedules you are planning to use to ensure your heat up is slow enough, that your annealing soak is long enough, and the cool slow enough for the altered conditions.

Determining the annealing point of unknown glass

You don’t have to guess at the annealing temperature for an unknown glass.  You can test for it.  It is known as the slump point test.

This test gives the softening point of the glass and from that the annealing point can be calculated.  This test removes the guess work from choosing a temperature at which to perform the anneal soak. The anneal temperature is important to the result of the firing.  This alone makes this test to give certainty about the annealing temperature worthwhile.

You can anneal soak at the calculated temperature, or at 30°C below it to reduce the anneal cool time.  This is because the annealing can occur over a range of temperatures.  The annealing occurs slowly at the top and bottom of the range. But is at least risk of "fixing in" the stress of an uneven distribution of temperature during the cool when the annealing is done at the lower end of the range.

Do not be fooled into thinking that CoE determines annealing temperatures.  Use published tables, especially the Bullseye table Annealing for Thick Slabs to determine soak times and cooling rates.  Use the standard test for determining the softening and annealing points of unknown glasses.

Further information is available in the ebook Low Temperature Kiln Forming and in Annealing Concepts Principles and Practice 

Revised 14.10.25

Annealing a Stressed Piece

An stress test strip and annealing witness between polarised filters.

If an unbroken fired piece shows stress that is known not to be from incompatibility, it is possible to fire and anneal again to relieve the stress.  If the stress results from incompatibilities, annealing again will not change the compatibility.  The process for stress testing is here. 

Conditions for doing this re-firing are:

• Slower heat up rates than usual for this thickness and profile are required. The glass is more than usually fragile and needs gradual heating. This avoids creating additional stress that may cause a break.

• Take the temperature up to the lower end of slumping temperature range - say 600 - 620C (1100 - 1150F) - and soak for 10 – 30 minutes depending on profile and thickness.  This ensures any existing stress is relieved and the glass is ready for the annealing.

• Reduce the temperature as fast as possible to the existing or new annealing temperature.

• Anneal for longer than previously. This can be for a greater thicknesses than the thickness and profile used for the stressed piece.  Most importantly, the anneal soak for the combination of profile and thickness needs to be followed.

• My experimentation has shown that the profile determines the additional amount of thickness that needs to be allowed for a sound anneal is as follows:

• Full flat fuse - fire for the thickness (i.e. times 1)
• Contour fuse -  fire for 1.5 times the thickest part
• Rounded tack fuse - fire for 2 times the thickest part
• Sharp tack/sinter - fire for 2.5 times the thickest part.

• Use the cool rates related to the anneal soak time. These are available from the Bullseye site for Celsius and Fahrenheit.  Too rapid a cool can induce temporary stress from differential contraction of the glass that is great enough to cause breaks, so follow the rates determined for this thickness and profile. 

• These rates are scientifically determined for all glass and especially for fusing glass and are inversely related to the anneal soak.  That means the longer the anneal soak, the slower the cooling rates need to be, and directly related to the soak length.  It does not matter which manufacturer's glass is being used, all the target times and temperatures should be followed, except the annealing temperature.

More information is available in my e-book Annealing Concepts, Principles, and Practice available from Bullseye, Etsy, and stephen.richard43@gmail.com

Cast Iron Mould Risks

There is a lot of concern about the safety of many products used in kilnforming, and much of it is based on hearsay. The best source for understanding the health and safety risks is Gregorie Rawls website.

Another, but more difficult to interpret, source is the SDS for each product.

Cast iron composition and safety

In this case the investigation is cast iron used as moulds. The first element is to know what cast iron is:

Cast iron is a class of iron-carbon alloys having a carbon content of more than 2% and silicon content around 1–3% with a melting point of 1,539°C (2,802°F). [Wikipedia]

The SDS gives the following information on Gray Cast Iron, the material of the cookware commonly used in the kiln:

• This material is rated as NOT HAZARDOUS by OSHA

• Appearance and Odor: Solid Mass, No Odor

• Specific Gravity: 7.86

• Boiling Point: 5000F

• 5 mg/m³ is the Time Weighted Average (TWA*) for fumes over an eight hour day. https://www.cdc.gov/niosh/idlh/1309371.html

These indicate there is no risk from fumes during casting firings as melting point will not be reached and the boiling point of cast iron is much higher than kilnforming kilns can reach.

The real risks are at room temperature, and are from the powder that may be created while grinding or smoothing the metal surface. The TWA* for cast iron dust is 10 mg/m³ over 8 hours. There are two alloy elements that also may be of concern – nickel and chromium. The amounts are low – chrome is from 0.5% to 2.5%, and even less nickel. The amounts are very low, giving little possible exposure.

The health concerns about using cast iron as a mould seems to be one of the misapprehensions of the amount of exposure, and therefore risk, that are common. The precautions are to have ventilation at source, use eye protection, and wear a N95 respirator.

The use of cast iron as a mould material

“Cast iron is a poor heat conductor compared to copper and aluminium, and this can result in uneven heating if a cast-iron pan is heated too quickly or… [unevenly].  Cast iron …[is] capable of storing more heat longer than... stainless steel pans. Slow heating... can lead to a more even temperature distribution. Due to the thermal mass of cast-iron utensils… they can retain heat for a long time...” Wikipedia.

This indicates that slower than usual ramp rates are advisable during the heat up to avoid breaking the cast iron through uneven heating.

Another thing to note is that the expansion rate [CoE] of between 106 and 114. The mould will contract more than glass, so preparing the mould with smooth sides and a sufficient draft is important to being able to remove the glass from the mould.

*Time Weighted Average (TWA) example:

"Rarely is exposure consistent throughout the day. Let’s say you are working in your studio for 8 hours grinding glass and exposure varies throughout the day… [Exposure varies in amounts]. The exposures throughout the day are averaged and the Time Weighted Average is determined. [In the example cited], … the OEL = 10 mg/m3 and the Time Weighted Average is 3.2 mg/m3, so actual exposure is below OEL (Occupational Exposure Limit)."    https://gregorieglass.com/general-information

Concealing Layers at the Edge

Credit: Sheri Coughlin

At tack fusing temperatures the layers of glass making up the piece are evident at the edge with two layer bases. 

 It is possible to conceal the layers at rounded tack fuses with a simple layup. If you cut the top piece 6mm larger than the bottom and place it carefully and evenly over the bottom, the top will form over the bottom, concealing the join of the two layers. This will work at all except sharp tack fusing levels.

A Sintering Project

Ready for firing

The project is to fire 6mm “balls” stacked 3 high onto a single sheet of clear glass without significant alteration to the base sheet or to the stacked balls. This creates a total thickness of 21mm. The proposal is to sinter the whole in one firing.

Scheduling for a sinter firing needs to be done as though 2.5 times the thickest part – in this case 52mm, or 2 inches

It is slightly more risky to do this in two firings, than one, in my opinion. A suggested schedule for sintering frit using Bullseye was:

• 100ºC /180ºF — 482ºC /900ºF, 60'  =5.8 hrs

• 40ºC /72ºF — 593ºC/1100ºF,10'      =2.8

• 20ºC /36ºF — 665ºC /1230ºF,30     =4.1

• Skip to anneal temperature, soak for 6 hours =6.5

• 6.7ºC /12ºF — 427ºC /800ºF,0'       =8.2

• 12ºC /22ºF — 371ºC /700ºF,0'        =4.7

• 40ºC /72ºF — room temperature,0’ =8.8

• Off                       =40.9 hours total or 1.7 days

This was annealing as for 38mm/1.5 inches thick. Annealing for 50mm/2” thick would need about 112 hours or 4.6 days.

However this schedule was not successful – the pieces were only lightly stuck together. Thinking about why, led to the proposal that the soak time and temperature were not long or high enough to give adhesion between the pieces.

A second attempt used a faster ramp rates to higher temperatures.

• 200°C /360°F – 540°C /1004ºF, 30’ =3.2 hrs

• 60°C /108°F -625°C /1157ºF, 30’     =1.92

• 30ºC /54ºF - 685ºC /1265ºF, 120’    =4.0

• skip to anneal temperature and soak/hold for 4 hours (as for 25mm/1”)

• 15ºC /27ºF – 427ºC /800ºF, 0’        =3.67

• 27ºC /49ºF – 370ºC /700ºF, 0’        =2.11

• 90ºC /162ºF – 50ºC /122ºF, 0’       =3.56

• Off

•  = a minimum total of 18.5 hours plus natural cooling of the kiln

This schedule used a:

• faster first ramp to a higher (540ºC /1004ºF) first soak

• a faster (60ºC /108ºF, which is 150% of the previous) rate to the lower slump temperature (625ºC /1157ºF)

• the same relative reduction (50%) in rate to a higher temperature (685ºC /1265ºF)

• a shorter (120’) anneal soak

• and consequently faster cooling rates, which showed no stress after firing

The whole structure held together and was sound. There was no apparent change in the size of the individual 6mm balls.

This difference in scheduling is an illustration of how time and temperature can be interchanged.

It also shows that size matters when sintering pieces together. Higher temperatures and more time are required for dots and balls than for frit.

More information is available in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and stephen.richard43@gmail.com

Elevation of Moulds

Is it necessary to elevate slumping moulds above the shelf? 

I first heard of the need to elevate moulds from a Bhole representative about 2007. I ignored it, but didn't get around to testing until working on my e-book Low Temperature Kilnforming.

That work showed there is a larger difference in air temperature above and below the unsupported mould than the supported one. But that difference is much smaller than between the air temperature and the glass.

At 150°C/270°F per hour the maximum difference in the temperature under the mould between the elevated and on-the-shelf mould at top temperature was 41°C/74°F while the air temperature difference was 126°C/227°F higher than under the elevated mould.  Many of the tests showed less difference than the maximums given here.

By reducing the ramp rate from 150°C/270°F per hour to 120°C/216°F, the under mould to above mould differential was reduced by a quarter. I didn't test beyond that. But it would appear that slower rates of 100°C/180°F and less will reduce that differential.

The graph also shows that there is a large difference between what the pyrometer reads than the mould temperature of the slump. Slower ramp rates produce an air temperature much closer to the mould temperatures.

Shortly into the rapid cool towards anneal soak and cool only minor temperature difference showed between elevated and on-the-shelf moulds throughout the anneal soak and anneal cool.

These details make it clear to me that elevating moulds is completely unnecessary with slow ramp rates. This of course, fits with the low and slow mantra that many of us promote. However elevating the mould will not harm the slump.

One caution, though. Damp. Wet, or heavy moulds must be supported to avoid breaking the shelf. So I advocate placing these moulds on the floor of the kiln with 2cm posts, rather than on the shelf. I don't know if it is necessary. I haven't tested it. But I do know that moulds in this condition will break the shelf without significant separation between the two.

Low Temperature Kilnforming e-book is available from Bullseye  and Etsy and is applicable to all fusing glasses.

Shotgun Annealing

 Shotgun annealing is chosen when the annealing temperature is unknown or uncertain. The name comes from the characteristic spread of the shot pellets to include the target.

To follow this process, pick highest relevant anneal temperature. We know soda lime glass has a range from about 540°C/1004°F to 470°C/878°F. Unless you are firing float glass (which anneals between 540°C/1004°F and 520°C/968°F), you can start the anneal cycle at 520°C/968°F and continue it to 470°C/878°F (a 50°C range). The rate to be used is determined by the amount of time required to anneal the piece according to thickness.

To be safe, a shotgun anneal will need double the time to go through the chosen range that a normal anneal soak requires.

• A 6mm/0.25” full fused piece would normally need an hour soak. So the shotgun anneal rate would be 25C/45F per hour over a 50°C/90°F range.
• A 12mm/0.5” full fused piece would normally need a two hour soak. This implies a rate of one quarter of the range or a cool rate of 12°C/22°F over the range.
• A 6mm/0.25” tack fused piece would need to be fired for twice its thickness, so as for 12mm/0.5”.

Annealing times for different profiles and thicknesses are given in this blog post:  and in this ebook.

If the glass is really unknown or older than fusing glass, a wider shotgun anneal range should be used. This gives a temperature range of 540°C/1004°F and goes to 470°C/878°F, or a range of 70°C/126°F. There is still a requirement for the shotgun process to be double the normal anneal soak.

• So for a 6mm/0.25” full fused piece two hours are required to go through the range, or 35°C/63°F per hour.
• A 12mm/0.5” full fused piece and a 6mm/0.25” tack fused piece will need a rate that takes 4 hours to go through the range, or 18°C/32°F per hour.

Once the slow fall of temperature through the range is complete, there should be a one hour soak to ensure the temperature has been equalised throughout the reduction in temperatures. This is applicable to pieces 12mm/0.5” thick. Thicker pieces need a longer soak at this point.

The final part of the anneal is cooling at a rate appropriate for the thickness and profile. E.g.:

• A 6mm/0.25” full fused piece would be cooled at 83°C/150°F to 427°C/800°F, and then at 150°C/270°F to 370°C/700°F or lower.
• A 12mm/0.5” full fused piece needs a two hour soak, so the cooling rates are determined by that, i.e., 55°C/99°F per hour to 427°C/800°F and then at 99°C/178°F per hour to 370°C/700°F or lower.

There is an alternative process which is used to determine the annealing temperature of an unknown glass. Once the anneal temperature is determined for a glass, there is no need for a shotgun anneal process. This is known as the slump point test. 

Much more on the principles and practices of annealing can be found in my e-book. Annealing Concepts, Principles and Practice from Bullseye, Etsy and stephen.richard43@gmail.com

Avoiding Grinding Lines

 

Sometimes in a completed piece a grey line appears between pieces or at the edge of the whole piece. Most often these are the result of grinding the edges to fit with each other, which leads to devitrification.

In scoring and breaking glass to fit into a fusing project, there are often adjustments needed to the pieces for a good fit, so processes need to be used to minimise grinding.

The obvious preventive is accuracy in scoring and breaking to get a good fit without adjustments. This reduces the need for changing the shape.

Many people use their grinders to make the adjustments required though. Most grinders use a medium or coarse grinder bit. This leaves many pits in the edges of the glass, requiring a significant amount of cleaning to remove glass dust from them. The general practice is to place any ground piece immediately into water to avoid the glass particles drying into the pits. This is rarely sufficient to avoid devitrification, because the surface of the edges are not smooth, enabling devitrification to grow on those rough areas.

Fine grinding heads are available and should be used when adjusting sizes for kilnforming. These need to be cleaned just as for the coarser grits. The results of grinding with a fine grit bit (or to 400 grit with other tools) is usually enough when clean to avoid devitrification.

Another way to eliminate the appearance of grinding lines is to avoid grinding altogether. If adjustments are necessary, groze the glass to shape. The slight irregularities will be accommodated by the movement of the glass during contour and full fuses. Larger spaces between glass can be filled with powder of the same colour. If the joint has different colours, use the powder of the darker or denser colour as it will fill the gaps with less evidence.

Glass in Microelectronics

 Passivation involves depositing a protective material onto the surface of metals or metal alloys to enhance their resistance to environmental factors and glass is an excellent choice for this.

Image source: iStock

In the fast-paced world of semiconductor manufacturing, where precision and reliability are paramount, choosing a suitable passivation material is critical to ensuring the optimal performance of electronic devices. Among the library of viable materials, glass has gained significant attention for its unique properties and versatility. This article looks at how glass is used for passivation and what properties make it highly suitable for the job.

Understanding Passivation in Semiconductors

Before unpacking the specifics of glass as a material for passivation, it is essential to understand the concept of passivation in semiconductor manufacturing. Passivation involves depositing a protective material onto the surface of metals or metal alloys to enhance their resistance to environmental factors.

The layering material can be organic or inorganic and should exhibit excellent electrical insulation and strong substrate adhesion, as well as block the ingress of chemical species. In the case of semiconductors, passivation is crucial to preventing degradation and ensuring long-term reliability.^1,2

Why Use Glass for Passivation?

Glass has emerged as a compelling choice for passivation due to its unique combination of properties. For example, glass can be formulated in numerous ways, with common types including Pb-Si-Al, Zn-B-Si, and Pb-Zn-B. This allows manufacturers to produce glass capable of meeting low and high-voltage electrical specifications; matching the coefficient of thermal expansion of semiconductor materials; and meeting the low temperature processing requirements.^3,4

Glass is chemically durable and thus can provide an inert barrier against external elements, such as moisture and contaminants, which might otherwise compromise the semiconductor’s performance. Moreover, the high transparency of some glasses, such as borosilicate glass, makes them ideal for applications with critical optical properties, such as photovoltaics. This transparency enables efficient energy transmission and absorption, contributing to the overall performance of semiconductor devices and solar cells.^5,6

How are Semiconductors Passivated?

Glass can be deposited onto semiconductors in a variety of ways. Choosing methods for passivation depends on factors such as the semiconductor device’s specific requirements, the passivation layer’s desired properties, and the overall manufacturing process. Methods for achieving glass passivation in semiconductor manufacturing include:⁷

• Chemical vapor deposition (CVD), including plasma-enhanced CVD (PECVD)
• Physical vapor deposition (PVD), including E-beam deposition
• Sputter Coating
• Atomic Layer Deposition (ALD)

In manufacturing, the process of glass passivation is frequently succeeded by chemical procedures, such as the etching of contact windows or the electrolytic deposition of contacts. These procedures may pose a threat to the integrity of the glass.

The chemical resistance of different passivation glasses varies significantly and serves as a crucial factor in determining the suitable glass type and the accompanying etching process.⁸

Comparing Glass to Other Materials

While various materials can be used for passivation, glass stands out for its exceptional stability over temperature, humidity, and time. Literature searches reveal a lack of head-to-head comparisons with other common passivation materials; however, general comparisons can be drawn.⁶

Amorphous silicon (a-Si) films utilized in solar cells present numerous advantages. These include a lower deposition temperature, in contrast to the temperatures commonly employed in cell manufacturing. However, it is essential to note that a-Si films exhibit sensitivity to subsequent high-temperature processes, which are frequently necessary in industrial manufacturing technology.⁹

Similarly, AlO_x passivation films can be applied at relatively low temperatures but can be limited by slow deposition speeds when using specific application methods. This can generate problems for high-throughput techniques, such as solar cell production.⁹

Polyimide, a common passivation material lauded for its strength and thermal stability, is also susceptible to moisture absorption. This can impact the strength and dielectric properties of the protective coating, risking the integrity of the semiconductor.¹⁰

Applications of Glass Passivation

Passivation glasses demonstrate outstanding performance in wafer passivation and encapsulation processes, providing advantages to a diverse range of semiconductor devices, including:⁸

• Thyristors
• Power transistors
• Diodes
• Rectifiers
• Varistors

Glass also has applications in solar cell passivation. In a recent study, researchers developed a method for enhancing borosilicate glass (BSG) passivation using high temperatures before lowering the temperature to accommodate the metallization process. In doing so, they notably improved the solar cell’s efficiency.¹¹

In another study, phosphosilicate glass (PSG) was found to significantly enhance the practical lifetime of minority carriers and improve the overall performance of solar cells, particularly in structures involving nanocrystalline silicon and crystalline silicon.¹²

Mo-Sci’s Expertise in Glass Thin Films

Fueled by the increasing prevalence of smart devices and advancements in the automotive and aerospace sectors, the semiconductor passivation glass market is anticipated to grow consistently in the next few years.³

Mo-Sci’s expertise lies in leveraging the unique properties of glass to create tailored solutions, ensuring the reliability and performance of many applications, including glass seals and glass coatings. Contact us for more information.

Krista Grayson

References and Further Reading

1. Pehkonen, S.O., et al. (2018). Chapter 2 – Self-Assembly Ultrathin Film Coatings for the Mitigation of Corrosion: General Considerations. Interface Science and Technology. doi.org/10.1016/B978-0-12-813584-6.00002-8
2. Lu, Q., et al. (2018). Chapter 5 – Polyimides for Electronic Applications. Advanced Polyimide Materials. doi.org/10.1016/B978-0-12-812640-0.00005-6
3. Reliable Business Insights. [Online] Semiconductor Passivation Glass Market – Global Outlook and Forecast 2023-2028. Available at: https://www.reliablebusinessinsights.com/purchase/1365249?utm_campaign=2&utm_medium=cp_9&utm_source=Linkedin&utm_content=ia&utm_term=semiconductor-passivation-glass&utm_id=free (Accessed on 05 January 2024).
4. Schott. [Online] Passivation Glass. Available at: https://www.schott.com/en-hr/products/passivation-glass-p1000287/technical-details (Accessed on 05 January 2024).
5. Zhong, C., et al. (2022). Properties and mechanism of amorphous lead aluminosilicate passivation layers used in semiconductor devices through molecular dynamic simulation. Ceramics International. doi.org/10.1016/j.ceramint.2022.07.191
6. Hansen, U., et al. (2009). Robust and Hermetic Borosilicate Glass Coatings by E-Beam Evaporation. Procedia Chemistry. doi.org/10.1016/j.proche.2009.07.019
7. Korvus Technology. [Online] The Revolution of PVD Systems in Thin Film Semiconductor Production. Available at: https://korvustech.com/thin-film-semiconductor/ (Accessed on 05 January 2024).
8. Schott. Technical Glasses: Physical and Technical Properties. Available at: https://www.schott.com/-/media/project/onex/shared/downloads/melting-and-hot-forming/390768-row-schott-technical-glasses-view-2020-04-14.pdf?rev=-1
9. Bonilla, R.S., et al. (2017). Dielectric surface passivation for silicon solar cells: A review. Physica Status Solidi. doi.org/10.1002/pssa.201700293
10. Babu, S.V., et al. (1993). Reliability of Multilayer Copper/Polyimide. Defense Technical Information Centre. Available at: https://apps.dtic.mil/sti/citations/ADA276228
11. Liao, B., et al. (2021). Unlocking the potential of boronsilicate glass passivation for industrial tunnel oxide passivated contact solar cells. Progress in Photovoltaics. doi.org/10.1002/pip.3519
12. Imamura, K., et al. (2018). Effective passivation for nanocrystalline Si layer/crystalline Si solar cells by use of phosphosilicate glass. Solar Energy. doi.org/10.1016/j.solener.2018.04.063