Extreme Ultraviolet Lithography (EUV): 13.5 nm Light Sources and Pellicle Physics

Extreme Ultraviolet Lithography (EUV): 13.5 nm Light Sources and Pellicle Physics

Printing semiconductor transistor gate pitches down to $2\text{ nm}$ architectural nodes requires shifting optical lithography exposure wavelengths from $193\text{ nm}$ deep-UV (ArF excimer lasers) to $13.5\text{ nm}$ Extreme Ultraviolet (EUV) light—a regime situated at the border between ultraviolet radiation and soft X-rays.

Because all atmospheric gases and optical glasses absorb $13.5\text{ nm}$ photons completely, EUV lithography systems (such as ASML Twinscan EXE/NXE series) must operate under ultra-high vacuum ($<10^{-7}\text{ mbar}$) using pure reflective mirror optics.

1. Generating 13.5 nm EUV Photons via Laser-Produced Plasma (LPP)

Generating consistent EUV photons is one of the most demanding engineering triumphs in history:

1. Molten Tin Droplet Generator: A piezo injector dispenses $50{,}000$ molten tin ($Sn$) droplets per second ($25,\mu\text{m}$ diameter) travelling at $80\text{ m/s}$.
2. Pre-Pulse Laser Strike: A low-energy pulsed $CO_2$ laser strikes each droplet, flattening it into a microscopic pancake disk.
3. Main Laser Blast: A $20\text{ kW}$ pulsed $CO_2$ laser ($10.6,\mu\text{m}$ wavelength) pulverizes the flattened tin target into a high-density plasma heated to $400{,}000\text{ Kelvin}$. Highly ionized tin ions ($Sn^{8+}$ to $Sn^{12+}$) undergo atomic transitions, radiating atomic photons at $13.5\text{ nm}$ into $2\pi$ steradians.

       Laser-Produced Plasma (LPP) EUV Source Scheme
        [ Droplet Generator ] ---> Molten Tin Droplet (50 kHz)
                                       |
        [ Pre-Pulse Laser ]  --------> (*) Flattens to Pancake
                                       |
        [ Main CO2 Laser ]   --------> (###) Superheats into 400k K Plasma
                                       |
                     +-----------------+-----------------+
                     |    13.5 nm EUV Radiation Emitted  |
                     v                                   v
             [ Ellipsoidal Collector Mirror (Mo/Si Multilayers) ]

2. Distributed Bragg Reflector (DBR) Mirror Optics

Because refraction through lenses is physically impossible at $13.5\text{ nm}$, EUV optics utilize Bragg reflection mirrors composed of 40 to 50 alternating nanoscale layers of Molybdenum ($Mo$) and Silicon ($Si$):

• Layer Thickness: $Mo = 2.8\text{ nm}$, $Si = 4.1\text{ nm}$ (total periodic bilayer $d \approx 6.9\text{ nm} \approx \lambda/2$).
• Constructive interference reflects at most $70%$ of incident light per surface.
• Through a typical optical train with 11 successive mirror reflections, only $\sim 2%$ of initial EUV source light reaches the silicon wafer resist!

3. Carbon Nanotube EUV Pellicle Survivability

Photomasks must be shielded from airborne contamination particles. However, the protective membrane (pellicle) must withstand intense EUV radiation while transmitting $>90%$ of light:

• Operating under vacuum, pellicles cannot cool through air convection; they dissipate hundreds of watts of absorbed optical heat exclusively through blackbody thermal radiation ($>1000^\circ\text{C}$).
• Free-standing carbon nanotube (CNT) networks and ultra-thin polycrystalline silicon membranes ($<30\text{ nm}$ thickness) provide the requisite high optical transmission combined with thermomechanical endurance.

def compute_euv_system_transmission(num_mirrors=11, reflectivity_per_mirror=0.695, pellicle_trans=0.90):
    '''
    Computes total optical power throughput from source to photoresist.
    '''
    mirror_train_trans = reflectivity_per_mirror ** num_mirrors
    # Double-pass through pellicle (to mask and reflected back)
    total_transmission = mirror_train_trans * (pellicle_trans ** 2)
    return mirror_train_trans, total_transmission

mt, tt = compute_euv_system_transmission()
print(f"Mirror train throughput: {mt*100:.2f}% | Final wafer plane delivery: {tt*100:.2f}%")

Extreme Ultraviolet engineering underpins the fabrication of modern high-density microprocessors, squeezing tens of billions of silicon transistors onto fingernail-sized dies.