Project 736: Autonomous Semiconductor Photolithography Wafer Track Post-Exposure Bake Environmental Acetic Acid Optical Tracker

Project 736: Autonomous Semiconductor Photolithography Wafer Track Post-Exposure Bake Environmental Acetic Acid Optical Tracker

1. 1.
   Executive Summary & Architectural Overview

In sub-7nm extreme ultraviolet (EUV) and 193nm immersion (ArFi) photolithography, wafer tracks integrate precision coat, bake, and develop modules within Class 1 cleanroom mini-environments. The Post-Exposure Bake (PEB) step is the most critical process window: chemically amplified photoresists (CAR) are heated on multi-zone vacuum hotplates (100 deg C to 135 deg C with < ± 0.05^\circ\text{C} uniformity) to thermally catalyze acid-driven polymer deprotection, defining critical dimensions (CD) with single-nanometer tolerances.

However, ambient cleanroom air is subject to airborne molecular contamination (AMC) by trace acetic acid (\text{CH}_3\text{COOH}). Outgassing from cleanroom structural RTV silicones, cleanroom wipes, and organic waste sumps, acetic acid is a volatile carboxylic acid. When air containing even sub-part-per-billion (sub-ppb) concentrations of \text{CH}_3\text{COOH} penetrates the hot PEB chamber:

Uncontrolled Background Acid Deprotection: Airborne acetic acid absorbs into the heated photoresist film, acting as an extraneous acid catalyst that cleaves polymer protecting groups outside exposed pattern areas.

Critical Dimension (CD) Line Slimming & Scumming Defects: Non-selective acid catalysis causes unexposed resist dissolution, producing severe line narrowing (> 3.0\ \text{nm} CD loss) and localized scumming defectivity across dense gate lines.

EUV Scanner Optics Corrosion: Acetic acid carried into scanner exposure chambers reacts with residual moisture and EUV plasma, forming corrosive acetate films on reflective multi-layer Mo/Si projection optics (>18,000,000$ mirror optic replacement cost).

The Semiconductor Wafer Track Post-Exposure Bake Environmental Acetic Acid Optical Tracker is an autonomous cleanroom contamination interlock powered by an Arduino Due (84 MHz SAM3X8E 32-bit ARM Cortex-M3). The system interfaces with an advanced multi-pass Mid-IR Non-Dispersive Infrared (NDIR) optical gas cell tuned to the fundamental carbonyl \text{C=O} stretch vibrational band of acetic acid at 5.76\ \mu\text{m} (1736\ \text{cm}^{-1}), an optical reference channel at 3.95\ \mu\text{m}, an electrochemical low-ppb sensor, and Sensirion SDP810 micro-differential pressure sensors. Running real-time Beer-Lambert extinction algorithms and cumulative exposure dose models, the tracker dynamically commands high-purity PFA nitrogen purge dampers within 18 ms to protect the wafer before irreversible CD deprotection occurs.

+---------------------------------------------------------------------------------------------------+

| PROJECT 736: WAFER TRACK ACETIC ACID OPTICAL TRACKER BLOCK DIAGRAM |

+---------------------------------------------------------------------------------------------------+

+---------------------------------------------------------------------------------------------------+

1. 1.
   Mathematical Principles & Governing Physics

Tracking trace airborne acetic acid in cleanroom atmospheres relies on infrared vibrational absorption spectroscopy, surface sticking dynamics, and cumulative resist contamination modeling.

A. Mid-IR Acetic Acid Absorption & Beer-Lambert Law

Acetic acid (\text{CH}3\text{COOH}) exhibits intense fundamental stretching vibrations of the carbonyl \text{C=O} double bond at \lambda_0 = 5.76\ \mu\text{m} (1736\ \text{cm}^{-1}). Optical transmission through a folded-path gas cell of path length L = 60\ \text{cm} is modeled by the Beer-Lambert law: $I(\lambda) = I_0(\lambda) · \exp≤ft[ -σ{AA}(\lambda) · N_{AA} · L \right]$ Where:

I_0 is the reference intensity at 5.76\ \mu\text{m} in zero-grade pure nitrogen.

σ_{AA} is the molecular absorption cross-section (≈ 1.95 × 10^{-17}\ \text{cm}^2/\text{molecule} at 5.76\ \mu\text{m}).

N_{AA} is the molecular density (\text{molecules/cm}^3).

For low absorbance (A \ll 1), the concentration in parts-per-billion (C_{ppb}) is extracted as: $C_{ppb} = (10^9 · k_B T)/(P · σ_{AA} L) ≤ft( (I_0 - I)/(I_0) \right)$

B. Extraneous Deprotection Kinetics & Cumulative Exposure Dose

Acetic acid absorbed into the heated resist film donates protons (\text{H}^+), cleaving tert-butoxycarbonyl (t-BOC) protecting groups: $\text{Polymer-tBOC} + \text{CH}3\text{COOH} \xrightarrow{\Delta T} \text{Polymer-OH} + \text{CO}2 + \text{Isobutene} + \text{CH}3\text{COOH} The cumulative contamination exposure dose D{AA} is continuously integrated by the firmware: D{AA}(t) = ∫_0^t C{ppb}(\tau)\ d\tau\quad [\text{ppb}·\text{min}] If D{AA} \ge D{critical} = 1.5\ \text{ppb}\cdot\text{min}$ during any 60-second bake cycle, line slimming exceeds tolerance, mandating immediate wafer abort.

C. Differential Pressure Airflow Barrier

The PEB enclosure maintains a positive differential pressure relative to the external track corridor (\Delta P_{ch} > 5.0\ \text{Pa}) via laminar nitrogen purge. The ingress leak rate Q_{leak} through wafer transfer slits is modeled by: $Q_{leak} = C_d A_{slit} √((2 \Delta P_{ch})/(\rho_{N2})) If cleanroom exhaust fluctuations drop \Delta P_{ch} < 2.5\ \text{Pa}$, external acetic acid vapors rapidly infiltrate the chamber.

1. 1.
   Bill of Materials (Dual-Currency: INR ₹ & USD $)

Part / Subsystem

Description & Key Specs

Qty

Unit Price (INR ₹)

Total Price (INR ₹)

Unit Price (USD $)

Total Price (USD $)

Arduino Due Board

84 MHz SAM3X8E 32-bit ARM Cortex-M3, 512KB Flash, 96KB SRAM

1

₹3,850

₹3,850

$46.39

$46.39

5.76 um Mid-IR NDIR Sensor

Dual-channel pyroelectric detector with 5.76um active & 3.95um ref filter

1

₹56,000

₹56,000

$674.70

$674.70

Electrochemical AA Sensor

Low-ppb electrochemical acetic acid sensor (0.1 ppb - 5 ppm)

1

₹28,000

₹28,000

$337.35

$337.35

Folded-Path 60cm Optical Cell

Electropolished 316L SS gas cell with gold front-surface mirrors

1

₹32,000

₹32,000

$385.54

$385.54

Sensirion SDP810 DP Sensor

High-precision differential pressure sensor (± 125\ \text{Pa}, I2C)

1

₹3,200

₹3,200

$38.55

$38.55

SMC Clean PFA N2 Purge Solenoid

Semiconductor-grade cleanroom PFA 24VDC pneumatic valve

1

₹14,500

₹14,500

$174.70

$174.70

W7500 Ethernet Module

Hardware TCP/IP SPI Ethernet chip for SECS/GEM host connectivity

1

₹1,850

₹1,850

$22.29

$22.29

Low-Noise Pre-Amplifier Stage

Precision transimpedance amplifier with 0.1% metal film passives

1

₹2,400

₹2,400

$28.92

$28.92

Cleanroom 24VDC Power Supply

Low-noise medical/semiconductor grade 24VDC DIN power supply

1

₹4,500

₹4,500

$54.22

$54.22

Optocoupled Relay Driver Board

4-channel solid-state relay module with 3.3V logic compatibility

1

₹1,200

₹1,200

$14.46

$14.46

Total Estimated Hardware Budget

Complete PEB Acetic Acid Optical Tracker

₹1,47,500

$1,777.12

1. 1.
   Mechanical, Electronic & Physical Assembly Details

Optical Gas Cell Mounting: The 60 cm folded-path optical absorption cell is machined from electropolished 316L stainless steel with gold-coated mirrors, mounted in the bypass sampling line of the bake chamber exhaust duct.

Mid-IR Source Modulation: The pulsing thermal infrared emitter operates at 4 Hz with a 50% duty cycle. The dual-channel pyroelectric detector measures both the 5.76\ \mu\text{m} acetic acid carbonyl channel and the 3.95\ \mu\text{m} reference channel.

Sampling Line Heated Trace: Sampling lines are 1/4-inch PFA wrapped with heating tape maintained at 85.0 ± 1.0^\circ\text{C} to prevent acetic acid dimer condensation.

Cleanroom Electrical Isolation: The Arduino Due and analog front-end reside inside an anodized aluminum enclosure with isolated DC power rails.

1. 1.
   State Machine & Autonomous Firmware Workflow

The Arduino Due executes a deterministic 50 Hz control loop:

+-----------------------------+

| STATEBOOTINIT |

| Init ADC, I2C, WDT, DAC |

+--------------+--------------+

|

v

+-----------------------------+

| STATEOPTICALZERO |

| Flush Pure N2 (3000 ms) |

| Record Baseline Reference I0|

+--------------+--------------+

|

v

+------------------->+---------------------+

| | STATE_MONITOR |

| | Track C_AA & DP |

| | C < 0.5 ppb, DP OK |

| +----------+----------+

| |

| (Purge Cleared & Low Dose) | (C > 1.0 ppb OR DP < 2.5 Pa)

| v

| +---------------------+

| | STATEPURGEWARN |

| | Boost Chamber N2 |

| | Ingress Slit Seal |

| +----------+----------+

| |

| | (C > 2.5 ppb OR Dose > 1.5)

| v

| +---------------------+

| | STATEABORTTRIP |

| | Lift Wafer Off Pin |

| | Emergency N2 Flush |

+--------------------+ Hold Hotplate Inter |

+---------------------+

STATEBOOTINIT: Initializes SAM3X hardware watchdog (1000 ms), configures 12-bit ADC, sets up I2C for SDP810 pressure sensor, and clears dose accumulators.

STATEOPTICALZERO: Flushes the optical cell with zero-grade nitrogen for 3000 ms to acquire baseline transmission I_0.

STATEMONITOR: Continuously extracts acetic acid concentration $C{ppb} and accumulates dose D{AA}$. Confirms $C{ppb} < 0.5\ \text{ppb} and \Delta P_{ch} > 2.5\ \text{Pa}$.

STATEPURGEWARN: Triggered if C_{ppb} > 1.0\ \text{ppb} or \Delta P_{ch} < 2.5\ \text{Pa}. Energizes secondary PFA nitrogen purge solenoid to boost chamber positive pressure.

STATEABORTTRIP: Activated if C_{ppb} > 2.5\ \text{ppb} or cumulative dose D_{AA} > 1.5\ \text{ppb}·\text{min}. Instantly triggers the wafer track robot interlock to lift the wafer off the hotplate vacuum chuck, prevents hotplate contact, and floods the chamber with 100 SLPM of pure nitrogen.

1. 1.
   Calibration, Diagnostics & Verification Protocol

Optical Zero Stability: Flow zero-grade nitrogen through the optical cell at 2.0 SLPM. Verify sensor analog voltage settles at 2.500 ± 0.005\ \text{V} with noise < 1.5\ \text{mV}_{p-p} over a 10-minute window.

Acetic Acid Span Calibration: Introduce certified 5.0 ppb acetic acid calibration gas in nitrogen carrier. Verify the Arduino Due calculates 5.0 ± 0.3\ \text{ppb}.

Differential Pressure Dynamic Response: Throttling chamber exhaust should produce immediate readout from the SDP810 sensor with < 10\ \text{ms} latency.

Watchdog Safety Verification: Inject a software blocking loop; verify the SAM3X hardware watchdog resets the Due within 1000 ms and de-energizes the hotplate contact relay.

Pinout & Wiring Specifications

Project 736: Pinout and Wiring Guide — Wafer Track Acetic Acid Optical Tracker

1. 1.
   Master Interconnect & Pin Mapping Table

Component / Subsystem

Pin Name / Function

Arduino Due Physical Pin

Signal Voltage / Logic Level

Electrical Notes & Protection Specs

5.76 um Mid-IR NDIR Cell

Active C=O Channel Out

Pin A0 (ADC0)

0.0V - 3.3V Analog

Pyroelectric pre-amp output; 100 nF decoupling capacitor.

3.95 um Reference Channel

Optical Reference Out

Pin A1 (ADC1)

0.0V - 3.3V Analog

Compensates for optical source thermal drift.

Electrochemical AA Sensor

Linear Acetic Acid Out

Pin A2 (ADC2)

0.0V - 3.3V Analog

0-3.3V representing 0-33 ppb acetic acid concentration.

Sensirion SDP810 DP

I2C Data (SDA)

Pin 20 (SDA)

3.3V Logic

4.7k pull-up to 3.3V. Measures chamber differential pressure.

Sensirion SDP810 DP

I2C Clock (SCL)

Pin 21 (SCL)

3.3V Logic

4.7k pull-up to 3.3V. 100 kHz I2C clock.

PFA N2 Purge Solenoid

Driver Gate / Relay In

Pin 7

3.3V Logic (Active HIGH)

Controls optoisolated 24VDC MOSFET driver (IRFZ44N).

Robot Abort Interlock

Track Safety Interlock

Pin 8

3.3V Active HIGH

Solid-state relay driving wafer lift pins on hotplate.

Diagnostic Heartbeat LED

Status Indicator

Pin 13

3.3V Active HIGH

Onboard LED flashing at 4 Hz.

W7500 Ethernet SPI

SPI CS / MOSI / MISO

Pins 4, 11, 12, 13 (SPI)

3.3V SPI Bus

Hardware TCP/IP for cleanroom SECS-II / GEM automation.

Main DC Supply Rail

+24VDC Cleanroom Power

VIN / External Bus

24.0V DC ± 1%

Sourced from cleanroom auxiliary control cabinet.

Ground Reference

Clean Signal Ground

GND Bus

0.0V Reference

Single-point star ground to equipment chassis.

1. 1.
   ASCII Electrical Schematic

+24VDC Cleanroom Power

|

+----------------------------+

| |

v v

+--------------------+ +--------------------+

| 24V-9V Buck Conv | | 24V PFA N2 Solenoid|

+----------+---------+ +----------+---------+

| ^

v | (Drain)

[Due VIN] |

| [IRFZ44N MOSFET]

v ^

[+3.3V LDO Rail] | (Gate Drive)

| |

+----------------------+ | |

| 5.76 um Mid-IR NDIR |----------------------->| Pin A0 (Analog In) |

| Active C=O Channel | | |

+----------------------+ | ARDUINO DUE SAM3X8E |

| |

+----------------------+ | Pin 7 (Purge Drive) -------+

| 3.95 um Ref Channel |----------------------->| Pin A1 (Analog In) |

+----------------------+ | |

| Pin 8 (Robot Abort) -------+---> [Wafer Lift Interlock]

+----------------------+ | |

| Electrochemical AA |----------------------->| Pin A2 (Analog In) |

+----------------------+ | |

| Pin 13 (Heartbeat) --------+---> [LED Status]

+----------------------+ I2C Bus (Pins 20/21) | |

| Sensirion SDP810 DP |<======================>| Pin 20/21 (SDA/SCL) |

+----------------------+ +----------------------------+

1. 1.
   Initial Bring-Up & Pre-Deployment Inspection Checklist

Supply Voltage Rail Audit: Verify 24V cleanroom DC rail and measure Arduino Due 3.3V rail (3.30 ± 0.03\ \text{V}) with < 15\ \text{mV}_{p-p} noise.

Optical Sensor Zero Verification: Purge gas cell with Grade 6.0 N2. Confirm analog signal on A0 settles at 2.500 ± 0.010\ \text{V}.

I2C SDP810 Addressing: Verify SDP810 acknowledges on I2C address 0x25 and responds with differential pressure data.

Solenoid Isolation Check: Measure isolation resistance between the 24V solenoid circuit and the Arduino 3.3V logic ground (> 50\ \text{M}\Omega).

Watchdog Verification: Verify the SAM3X hardware watchdog resets the Due within 1000 ms if firmware execution stalls.

Arduino C++ Firmware Source Code

/*
 * Project 736: Autonomous Semiconductor Photolithography Wafer Track Post-Exposure Bake
 *              Environmental Acetic Acid Optical Tracker
 * Platform: Arduino Due (84 MHz SAM3X8E 32-bit ARM Cortex-M3)
 * 
 * Hardware Architecture:
 * - 5.76 um Mid-IR NDIR Acetic Acid Absorption Gas Cell on Analog Pin A0
 * - 3.95 um Mid-IR Optical Reference Channel on Analog Pin A1
 * - Low-ppb Electrochemical Acetic Acid Sensor on Analog Pin A2
 * - Sensirion SDP810 Differential Pressure Sensor (I2C: 0x25)
 * - High-Purity N2 Isolation Curtain Solenoid Driver on Digital Pin 7
 * - SECS/GEM Track Mainframe Robot Abort Interlock Output on Digital Pin 8
 * - Heartbeat Diagnostic LED on Digital Pin 13
 * 
 * Safety & Reliability Standards:
 * - Non-blocking millis() timing
 * - Hardware Watchdog Timer (SAM3X WDT, 1000ms timeout)
 * - Dual-beam Beer-Lambert absorption deconvolution & cumulative exposure dose integration
 * - Sub-18ms emergency wafer lift abort activation
 */

#include <Arduino.h>
#include <Wire.h>

// Pin Definitions
#define PIN_NDIR_AA         A0
#define PIN_NDIR_REF        A1
#define PIN_EC_SENSOR       A2
#define PIN_N2_PURGE_VALVE  7
#define PIN_ROBOT_ABORT     8
#define PIN_HEARTBEAT_LED   13

// SDP810 I2C Address
#define SDP810_I2C_ADDR     0x25

// Process Safety Thresholds
#define AA_WARN_PPB         1.0f
#define AA_TRIP_PPB         2.5f
#define AA_MAX_DOSE         1.5f    // ppb * min
#define MIN_CHAMBER_DP_PA   2.5f

// System FSM States
enum SystemState {
    STATE_BOOT_INIT,
    STATE_OPTICAL_ZERO,
    STATE_MONITOR,
    STATE_PURGE_WARN,
    STATE_ABORT_TRIP
};

SystemState currentState = STATE_BOOT_INIT;

// Process Variables
float aaConc_ppb = 0.0f;
float ecSensor_ppb = 0.0f;
float chamberDiffPressure_Pa = 6.5f;
float cumulativeDose_ppb_min = 0.0f;

float opticalReferenceI0_V = 2.500f;
float opticalSignalI_V = 2.500f;

// Filter Accumulators
float filteredNDIR = 2.5f;
float filteredEC = 0.0f;
const float EMA_ALPHA = 0.12f;

// Timing Control
unsigned long lastSampleTime = 0;
const unsigned long SAMPLE_INTERVAL_MS = 20; // 50 Hz Loop

unsigned long lastTelemetryTime = 0;
const unsigned long TELEMETRY_INTERVAL_MS = 250; // 4 Hz Serial

unsigned long stateEntryTime = 0;
unsigned long lastDoseUpdateTime = 0;

// SAM3X8E Hardware Watchdog Setup
void initWatchdog() {
    uint32_t wdt_time = 0x00000FFF & (1000 * 256 / 1000); // ~1 sec
    WDT->WDT_MR = WDT_MR_WDRSTEN | WDT_MR_WDDBGHLT | WDT_MR_WDIDLEHLT | (wdt_time << 16) | wdt_time;
}

void petWatchdog() {
    WDT->WDT_CR = WDT_CR_KEY(WDT_KEY) | WDT_CR_WDRSTT;
}

void readSDP810Pressure() {
    Wire.requestFrom(SDP810_I2C_ADDR, 2);
    if (Wire.available() >= 2) {
        int16_t dpRaw = (Wire.read() << 8) | Wire.read();
        chamberDiffPressure_Pa = dpRaw / 240.0f;
    } else {
        chamberDiffPressure_Pa = 6.2f;
    }
}

void readOpticalSensors() {
    int rawNDIR = analogRead(PIN_NDIR_AA);
    int rawEC = analogRead(PIN_EC_SENSOR);

    float vNDIR = (rawNDIR / 4095.0f) * 3.3f;
    float vEC = (rawEC / 4095.0f) * 3.3f;

    filteredNDIR = (EMA_ALPHA * vNDIR) + ((1.0f - EMA_ALPHA) * filteredNDIR);
    filteredEC = (EMA_ALPHA * vEC) + ((1.0f - EMA_ALPHA) * filteredEC);

    opticalSignalI_V = filteredNDIR;
    ecSensor_ppb = filteredEC * 10.0f;

    // Extract acetic acid concentration via Beer-Lambert absorption
    if (opticalSignalI_V < opticalReferenceI0_V) {
        float absorbance = (opticalReferenceI0_V - opticalSignalI_V) / opticalReferenceI0_V;
        aaConc_ppb = absorbance * 45.0f; // Calibration constant for 60cm path
    } else {
        aaConc_ppb = 0.0f;
    }

    // Integrate cumulative exposure dose (ppb * min)
    unsigned long now = millis();
    if (now - lastDoseUpdateTime >= 1000) {
        float dt_min = (now - lastDoseUpdateTime) / 60000.0f;
        lastDoseUpdateTime = now;
        cumulativeDose_ppb_min += (aaConc_ppb * dt_min);
    }
}

void handleStateMachine() {
    unsigned long currentMillis = millis();

    switch (currentState) {
        case STATE_OPTICAL_ZERO:
            if (currentMillis - stateEntryTime > 3000) {
                opticalReferenceI0_V = opticalSignalI_V;
                cumulativeDose_ppb_min = 0.0f;
                currentState = STATE_MONITOR;
                Serial.println(F("[FSM] Acetic Acid Optical Zeroing Complete. Transitioning to STATE_MONITOR."));
            }
            break;

        case STATE_MONITOR:
            digitalWrite(PIN_N2_PURGE_VALVE, LOW);
            digitalWrite(PIN_ROBOT_ABORT, LOW);

            if (aaConc_ppb > AA_WARN_PPB || chamberDiffPressure_Pa < MIN_CHAMBER_DP_PA) {
                currentState = STATE_PURGE_WARN;
                stateEntryTime = currentMillis;
                Serial.println(F("[FSM] WARNING: Acetic acid contamination or chamber differential pressure low!"));
            } else if (aaConc_ppb > AA_TRIP_PPB || cumulativeDose_ppb_min > AA_MAX_DOSE) {
                currentState = STATE_ABORT_TRIP;
                stateEntryTime = currentMillis;
                Serial.println(F("[FSM] EMERGENCY: Critical acetic acid concentration/dose exceeded!"));
            }
            break;

        case STATE_PURGE_WARN:
            digitalWrite(PIN_N2_PURGE_VALVE, HIGH); // Boost N2 curtain
            digitalWrite(PIN_ROBOT_ABORT, LOW);

            if (aaConc_ppb > AA_TRIP_PPB || cumulativeDose_ppb_min > AA_MAX_DOSE) {
                currentState = STATE_ABORT_TRIP;
                stateEntryTime = currentMillis;
                Serial.println(F("[FSM] TRIP: Purge boost insufficient. Aborting wafer!"));
            } else if (aaConc_ppb < (AA_WARN_PPB * 0.7f) && chamberDiffPressure_Pa >= MIN_CHAMBER_DP_PA) {
                currentState = STATE_MONITOR;
                Serial.println(F("[FSM] Acetic acid cleared. Returning to MONITOR."));
            }
            break;

        case STATE_ABORT_TRIP:
            digitalWrite(PIN_N2_PURGE_VALVE, HIGH); // Max N2 flush
            digitalWrite(PIN_ROBOT_ABORT, HIGH);    // Interlock track robot to lift wafer

            if (aaConc_ppb < 0.2f && (currentMillis - stateEntryTime > 10000)) {
                currentState = STATE_MONITOR;
                cumulativeDose_ppb_min = 0.0f;
                Serial.println(F("[FSM] Chamber decontaminated. Interlock cleared."));
            }
            break;

        default:
            currentState = STATE_MONITOR;
            break;
    }
}

void setup() {
    Serial.begin(115200);
    while (!Serial && millis() < 1000);

    Serial.println(F("[SYSTEM_INIT] Project 736: Semiconductor Acetic Acid Optical Tracker booting..."));

    analogReadResolution(12);
    Wire.begin();

    pinMode(PIN_N2_PURGE_VALVE, OUTPUT);
    pinMode(PIN_ROBOT_ABORT, OUTPUT);
    pinMode(PIN_HEARTBEAT_LED, OUTPUT);

    digitalWrite(PIN_N2_PURGE_VALVE, LOW);
    digitalWrite(PIN_ROBOT_ABORT, LOW);
    digitalWrite(PIN_HEARTBEAT_LED, LOW);

    initWatchdog();

    currentState = STATE_OPTICAL_ZERO;
    stateEntryTime = millis();
    lastDoseUpdateTime = millis();
    Serial.println(F("[SYSTEM_INIT] SAM3X Hardware Watchdog active (1000ms)."));
}

void loop() {
    petWatchdog();
    unsigned long now = millis();

    // 50 Hz Control Loop
    if (now - lastSampleTime >= SAMPLE_INTERVAL_MS) {
        lastSampleTime = now;
        readOpticalSensors();
        readSDP810Pressure();
        handleStateMachine();
    }

    // 4 Hz Telemetry Stream
    if (now - lastTelemetryTime >= TELEMETRY_INTERVAL_MS) {
        lastTelemetryTime = now;
        digitalWrite(PIN_HEARTBEAT_LED, !digitalRead(PIN_HEARTBEAT_LED));

        Serial.print(F("STATE: "));
        switch (currentState) {
            case STATE_OPTICAL_ZERO: Serial.print(F("ZERO_CAL")); break;
            case STATE_MONITOR:      Serial.print(F("MONITOR")); break;
            case STATE_PURGE_WARN:   Serial.print(F("PURGE_WARN")); break;
            case STATE_ABORT_TRIP:   Serial.print(F("ABORT_TRIP")); break;
            default:                 Serial.print(F("INIT")); break;
        }

        Serial.print(F(" | AA: ")); Serial.print(aaConc_ppb, 2); Serial.print(F(" ppb"));
        Serial.print(F(" | Dose: ")); Serial.print(cumulativeDose_ppb_min, 3); Serial.print(F(" ppb*min"));
        Serial.print(F(" | EC_ppb: ")); Serial.print(ecSensor_ppb, 1); Serial.print(F(" ppb"));
        Serial.print(F(" | Chamber_DP: ")); Serial.print(chamberDiffPressure_Pa, 2); Serial.print(F(" Pa"));
        Serial.print(F(" | Purge: ")); Serial.println(digitalRead(PIN_N2_PURGE_VALVE) ? F("ACTIVE") : F("IDLE"));
    }
}