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Glass Wafer vs Silicon Wafer: An In-Depth Comparison

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  • icon2 January 15, 2024
  • icon3 WaferPro
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At WaferPro, we provide specialized fabrication and processing services for both glass and silicon wafer substrates. But understanding the nuanced differences between these two ubiquitous materials is key to selecting the optimal option for your semiconductor, microelectronic, photonic, or biotech application.

This extensive guide examines all the major parameters including temperature sensitivity, electrical qualities, optical transmission, mechanical strength, pricing, and more. Read on to gain expert insights from WaferPro to inform your next wafer material decision.

Differences between glass wafer vs silicon wafer

Overview of Glass and Silicon Wafers

Semiconductor wafers provide the foundation for virtually every contemporary electronic system or device. Onto these thin yet resilient discs, intricate micro and nanoscale structures like integrated circuits, sensors, and optics can be constructed.

Silicon wafers dominate the electronics segment today, serving as the backbone not just of microchips but also MEMS, power devices, logic processors and other semiconducting components. Silicon’s innate conductive properties spring from its crystalline atomic lattice structure, enabling complex electronic architectures to be imprinted upon it.

Glass wafers encompass various amorphous glassy materials including borosilicate glass, aluminosilicate glass, and fused silica glass. They bring unique benefits to the table like optical transparency, chemical resistance, hermetic sealing capabilities, and electrical insulation.

Next we’ll do a deep dive across over 10 different wafer traits and attributes comparing glass against silicon wafers:

Temperature Sensitivity

Operating temperature ranges vary enormously between glass and silicon wafers, constituting a major differentiating factor.

Maximum Temperature Thresholds

  • Most glass wafers have much lower maximum working temperatures than silicon, posing challenges for high temperature processes.
    • Borosilicate glass - approximately 500°C
    • Fused silica glass - approximately 1000°C
  • Silicon wafers can reliably operate at temperatures exceeding 1000°C and up to around 1350°C.

So while silicon wafers offer ample headroom to spare for the hottest industrial or manufacturing environments, extreme care must be taken when utilizing glass wafers in processes over 500°C which exceeds the working range specifications for most glasses.

Thermal Conductivity

  • The thermal conductivity defines rate that heat travels through a material.
  • Silicon has a thermal conductivity around 130-150 W/mK depending on doping levels.
  • Glass wafer thermal conductivity varies by composition but ranges from 1.1-1.4 W/mK for fused silica up to ~1.7 W/mK for highly boron-doped aluminosilicate glass.

So silicon also transfers heat more rapidly compared to glass wafer materials. Combined with its higher working temperature threshold, this enables more efficient cooling profiles to be achieved with silicon wafers.

Thermal Expansion Coefficients

  • The coefficient of thermal expansion (CTE) determines the degree to which a material physically grows or shrinks in different temperature conditions.
  • Silicon wafers have very low CTE values ranging from 2.33 to 4.15 ppm/K.
  • Glass wafers can demonstrate up to ~10x higher CTEs depending upon exact composition:
    • Soda-lime glass: 8–9 ppm/K
    • Aluminosilicate glass: 4-8 ppm/K
    • Fused silica glass: 0.54 ppm/K

Large mismatches between CTEs of adjacent wafer and device layer materials induces mechanical stress which can cause physical warping or breakage. So silicon’s low CTE gives it better inherent temperature cycle resilience.

Summary - Temperature Considerations

  • Silicon wafer temperature credentials including higher working range, thermal conductivity, and lower CTE makes it better suited for high temperature manufacturing processes
  • Extreme care must be taken when utilizing glass wafers for processes exceeding 500°C
  • Fused silica glass wafers have the highest heat specifications among glass wafer materials

Next we examine the electrical profile differences between glass and silicon wafer substrates.

Electrical Properties

Glass Wafer Silicon Wafer Electrical Properties

While silicon earns its reputation from its nearly unrivaled electrical characteristics, glass wafers lend themselves more to electrical isolation use cases.

Conductivity & Semiconduction

  • Silicon wafers have excellent intrinsic electrical conduction abilities stemming from silicon’s fundamental semiconductor properties. Silicon sits in group IV of the periodic table with each Si atom having 4 valence electrons. Doped with elements like phosphorus or boron, silicon becomes either negatively-charged (N-type) or positively-charged (P-type). This allows for precisely controlled electrical currents.
  • Glass wafers are electrical insulators and do not conduct current well. The random amorphous structure of the glass state lacks ordered crystalline lattice arrangements of atoms. Disordered molecular structures inhibit free electron flows.

So for nearly any application requiring transistors, integrated circuits, computing, high speed signals, or other dynamic electrical functions - silicon reigns supreme over glass wafers.

Dielectric Strength

  • While poor electrical conductors under normal conditions, glasses demonstrate very high dielectric strength meaning they are extremely resistance to electrical discharge breakdown at high voltages.
  • Glass wafer materials generally have >10x greater dielectric breakdown thresholds than silicon. For example, fused silica glass can withstand applied electric fields beyond 1000 V/mm before experiencing arcing or discharge effects.
  • This makes glass an excellent choice for applications requiring electrical isolation and insulation properties under high DC or RF voltages. Common examples include RF antennas, power electronics, and high voltage capacitors.

So in summary on electrical traits:

  • Silicon wafer’s semiconducting nature enables dynamic ICs and electronics
  • Glass wafers provide outstanding electrical insulation even at very high operational voltages

Next we’ll explore the salient optical transmission differences between the two wafer types.

Optical Properties

Glass Wafer Silicon Wafer Optical Properties

Optical transparency spans an enormous range across wavelengths from deep ultraviolet (UV), through the visible spectrum, and continuing to far infrared (IR). The intrinsic optical transmission profiles of glass wafers versus silicon wafers also widely diverge, impacting their suitability for particular applications.

Visible Spectrum Transmission

  • Most silicon wafers are opaque to visible light. Visible photons have energies between 1.65 eV to 3.1 eV - insufficient to bridge silicon’s 1.12 eV bandgap energy. Instead, incoming visible photons get absorbed by valence electrons inside the silicon crystal lattice which then release the absorbed energy as heat.
  • Glass wafers demonstrate high transparency across the entire visible band ranging from violet 400nm wavelengths up to deep red 700nm light. Specific transmission percentage depends upon glass composition. High purity fused silica glass optimized for optics routinely achieves >90% transmission across visible colors.

So glass wafer transparency gives it inherent advantages for visible light-based applications including optics, displays, detectors, photovoltaics, and certain biotech devices.

Ultraviolet Transmission

  • Silicon wafer intrinsic absorption begins to drop markedly below ~350 nm into the near ultraviolet (UV) band and exhibits increasing transmission further into deep or vacuum UV (<200 nm).
  • Many glass wafer formulations maintain excellent transparency deep into the UV extending even below 150nm wavelengths for high purity fused silica substrates. For example, Corning 7980 fused silica wafers achieve >50% transmission down to wavelengths as short as 140nm. However, glasses containing higher alkali concentrations become absorbing or opaque for wavelengths under 350nm.

Therefore, glass wafers open up application spaces exploiting short UV wavelengths for processes like photoalignment, surface activation, photochemistry, or certain detectors and sensors.

Infrared Transmission

  • Silicon wafers demonstrate very good transmission for midwave infrared (MWIR) to longwave infrared light (LWIR) spanning from ~2μm to 12μm wavelengths. High resistivity float-zone (FZ) grown silicon wafers routinely achieve ~60% transmission or higher across this infrared band commonly leveraged for thermal imaging and spectroscopy applications.
  • Most glass wafers tend have lower maximal IR transmission levels topping out around ~90% or less for fused silica variants optimized for IR applications. However, silicon’s indirect bandgap structure means electrons can still interact with lower energy infrared photons. This allows detection of IR irradiation through electronic measurement principles not accessible to glass wafers lacking intrinsic infrared absorption mechanisms.

So in summary of key optics takeaways:

  • Glass wafer visible transparency enables light-dependent electronics like display panels, optics, photovoltaics and certain visible wavelength biosensors
  • UV transparency of some specialty glass wafers facilitates unique photonic and electronic processes below 350nm
  • Silicon wafers possess broad infrared light transmission especially from 2 to 12 micron region

Now let's move from photons to physicals - comparing basic density and mechanical attributes.

Density, Strength & Flexibility

Both silicon and glass wafers require customized handling equipment and extremely pristine environments purpose-built to accommodate their delicate physical makeup. But silicon does gain advantages from its inherently higher strength that stems from strong atomic bonds within its ordered crystal structure.

Density & Weight

  • With a density of 2.3290 g/cm3, silicon offers marginally higher density than most common glass wafers.
    • Borosilicate glasses - ~2.23 g/cm^3
    • Fused silica glasses - 2.20 g/cm^3
  • Consequently, silicon wafers have slightly greater weight than same-sized glass wafers although only by single digit percentages.
  • Nonetheless, weight reduction can become an important engineering consideration for very large wafers or in mobile/aero applications. Here glass wafers would offer a bit less ballast.

Hardness & Toughness

  • Silicon wafers have high hardness of ~1,350 Vickers exceeding aluminosilicate glass (~700 Vickers) and fused silica glass (~500-800 Vickers depending on OH concentration).
  • Silicon also demonstrates greater fracture toughness resisting propagation of cracks and chips.
  • So glass wafers must contend with being more frangible where fracturing, chipping, or splintering during handling becomes concerns especially as dimensions scale upwards.

Flexural & Tensile Strength

  • The flexural strength quantifies maximum bending force a material can withstand before permanent deformation and damage.
  • Silicon wavers have excellent flexural strengths exceeding those of glass wafers by potentially 5-10x in some cases. So glass is much more prone to bend-induced breakage.
  • Similarly, silicon wafers enjoy 2-4x times higher tensile strengths compared to glass - measure of resistance stretched or pulled without fracturing.

In summary of physical traits:

  • Silicon wafer’s superior hardness, flexural & tensile strengths makes it more resilient to both physical shock and normal handling stresses
  • Glass wafer fragility requires greater care and precaution during movement, storage and processing

Next up - how do these materials withstand exposure to chemicals and extreme environments?

Environmental & Chemical Resistance

Semiconductor fabrication environments readily expose wafers to high heat, intense UV radiation, plasma discharge, and potent chemical formulations during essential processes like etching, cleaning and photoresist stripping. Understanding wafer material vulnerabilities determines operational reliability.

Chemical Resistance

  • Glass wafers made from high purity fused silicas demonstrate outstanding chemical resistance and almost universal inertness. High silica content glasses withstand virtually all acids and organic chemicals - retaining high strength even upon total immersion.
  • Silicon wafers conversely exhibit extremely poor tolerance to chemicals. Silicon gets rapidly etched by acids and bases which damage surfaces by introducing unwanted electrical leak and carrier recombination sites.

So for applications involving fluid exposure or electrolytic environments, glass wafers deliver unparalleled assurance. Examples include biofluidic devices, electrolyte sensors, electrophoresis, or electroosmotic pumps.

Plasma & UV Resistance

  • The powerful energetic photons, ions and reactive neutral species inside industrial plasma etching tools can gradually erode and roughen wafer surfaces through processes like ion sputtering.
  • Silicon wafers develop surface roughness values under 10-15Å when exposed to hundreds of etching plasma cycles - still suitable for subsequent nanoscale lithography patterning
  • Glass wafers on the otherhand demonstrate almost no measureable change across over 24 hours of continuous O^2 plasma irradiation.

Similarly, prolonged exposure to other highly energetic radiation like ultraviolet (UV) or electron beams results in markedly less physical and chemical damage on glass versus silicon wafer substrates.

Temperature Cycling Resistance

  • Repeated cycles between extreme high and low temperatures induces structural stress and fatigue effects in wafer materials.
  • As described previously, silicon wafers have inherently better temperature cycling endurance thanks to close CTE matching between silicon films and substrates.
  • Glass wafers are prone to accelerated crack development and propagation driven by CTE mismatches across interfacing material layers.

In summary, while silicon marginally outperforms glass for thermal cycling reliability, glass wafer chemical inertness enables unique application environments incompatible for silicon like biomedical devices, sensors, MEMS, and microfluidics.

Now that functionality has been extensively covered, let's examine the crucial pinnacle of decision making - how much is this going to cost?

Pricing Comparison

Pricing often weighs as a prime factor when choosing between glass or silicon wafers. But processing add-ons like polishing, cleaning, and quality validation quickly compound costs above base wafer prices. Below we break down typical price ranges by wafer size across low and high test grade options:

2 Inch Wafers

Wafer TypeGradeAvg Price Range
Borosilicate GlassPrime$10 - $40
Fused Silica GlassPrime$20 - $50
SiliconPrime$10 - $40

4 Inch Wafers

Wafer TypeGradeAvg Price Range
Borosilicate GlassPrime$15 - $45
Fused Silica GlassPrime$20 - $50
SiliconPrime$15 - $30

6 Inch Wafers

Wafer TypeGradeAvg Price Range
Borosilicate GlassPrime$ 20- $50
Fused Silica GlassPrime$25 - $60
SiliconPrime$20 - $40

As evidenced from the pricing breakdowns, glass wafers - especially higher purity fused silica variants - range from comparable cost up to potentially 20-50% higher prices compared to equivalent silicon wafers. Cost differences expand further once incorporating dielectric coatings, CMP smoothing steps, or custom laminations.

However, for applications where glass wafer properties uniquely enable device performance and silicon remains unviable, their premium cost gets readily justified.

Summary - Glass vs Silicon Wafers Pricing

  • Base glass wafers range from similar 20-50% higher costs than bare silicon wafers
  • Final pricing highly application-dependent based upon custom fabrication and test grade requirements
  • Silicon enjoys inherent wafer cost advantages from massive economies-of-scale in electronics manufacturing

Concluding Thoughts: Glass Wafer vs Silicon Wafer

The glass versus silicon wafer decision ultimately comes down to your application needs and priorities - there is no universally superior option. Performance, temperature resistance, chemical tolerance, mechanical properties and pricing all enter the equation. To summarize differences:

Silicon wafer strengths include:

  • Temperature tolerance over 1000°C
  • Semiconducting electrical operation enabling transistors and ICs
  • Hardness and mechanical durability
  • Potentially lower pricing

Glass wafer advantages encompass:

  • Optical transmission spanning UV to IR wavelengths
  • Electrical insulation plus high dielectric breakdown thresholds
  • Excellent chemical resistance retention even under direct soak conditions
  • Enables unique fluidic, microfluidic and electrophoretic devices

Of course, wafer embedding or hybrid glass-silicon solutions open up even greater possibilities combining strengths from both realms.

Or explore other exotic wafer substrates like GaN, SiC, AlN or GaAs taking semiconductor capabilities into new extremes.

Leverage WaferPro's Glass Wafer & Silicon Wafer Expertise

WaferPro offers rapid custom wafer slicing, dicing, grinding, polishing and inspection services spanning semiconductor, MEMS, photonic, optic, sensor and biotech applications. Skip the upfront capital investment of buying your own wafer manufacturing tools - leverage WaferPro’s established glass and silicon expertise and quality systems backed by two decades delivering precision wafer services worldwide.

Get Your Custom Wafers Quote Today

Still unsure if glass or silicon makes sense for your next wafer-based devices or products? Contact WaferPro's engineering team to walk through your exacting specification, geometries, volume, and budget. Request a Custom Quote to get the conversation started and benefit from our wafer material application insights. Describe your project's unique needs and challenges to determine the ideal material solutions from our glass and silicon inventory.

FAQs: Glass vs Silicon Wafers

Still have some lingering questions? Check out answers to some of our most frequently asked questions comparing glass and silicon wafers below:

Are glass wafers compatible with silicon device fabrication lines and tools?

In most cases, no. Glass wafers require specialized equipment, handling, and processes tuned specifically for amorphous and brittle substrates. Most semiconductor fabs are purpose-built for rugged silicon wafers upto 300mm diameter. Glass wafers fabrication is normally segregated into smaller specialty lines with heavy focus on contamination control.

Can both materials withstand rapid thermal processing (RTP)?

Silicon wafers have no issues with common RTP heat ramp velocities exceeding 400°C/sec to 1000°C peaks. The highly turbulent convection currents inside RTP chambers however strictly preclude glass wafers due to propensity for particulate generation and wafer breakage.

What are silicon-on-glass (SOG) wafer solutions?

Also referred to as silicon-on-insulator (SOI) wafers, SOG configurations embed silicon device layers onto glass wafer substrates gaining benefits from both materials. This allows fabrication of things like RF antennas, power devices and sensors providing electrical insulation yet leveraging silicon’s electronic properties.

Is wafer thickness uniformity better for glass or silicon?

Monocrystalline silicon sourced from large diameter ingots can achieve average TTVs of less than 1μm across entire 300mm wafers. Competing glass wafer manufacturing methods currently struggle to deliver less than 5-10μm thickness variations. However rapid advances are enabling sub-micron glass wafer uniformities.

Which material gets used for more costly large diameter wafers?

While glass wafers remain niche, 300mm/12” production is almost exclusively silicon where massive capital investments have scaled mature fabrication processes to this large wafer size. The lower material demand and technical immaturity for glass makes large diameter formats currently uneconomical.

What wafer shape options exist besides round discs?

Along with circular wafers, WaferPro provides specialized glass and silicon wafer slicing/dicing into squares, rectangles, hexagons, annuli rings and custom shapes. Common applications include microfluidic inlets, MEMS resonators, quartz crystal blanks, photomasks and microdisplay tiling.

Contact WaferPro's engineers to explore how specialty glass or silicon wafers can enable your next microfabrication application.

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