greek cross
**Greek cross** is a **sheet resistance measurement pattern** — a symmetric four-point probe structure shaped like a plus sign (+), providing more accurate sheet resistance measurements than Van der Pauw structures through improved geometry.
**What Is Greek Cross?**
- **Definition**: Plus-shaped (+) test structure for sheet resistance measurement.
- **Design**: Four arms of equal length extending from central square.
- **Advantage**: Symmetric geometry improves measurement accuracy.
**Why Greek Cross?**
- **Accuracy**: Symmetric design reduces measurement errors.
- **Repeatability**: Consistent geometry improves reproducibility.
- **Standard**: Widely adopted in semiconductor industry.
- **Simple Analysis**: Straightforward resistance calculation.
**Greek Cross vs. Van der Pauw**
**Greek Cross**: Symmetric, more accurate, requires specific geometry.
**Van der Pauw**: Works for arbitrary shapes, less accurate.
**Preference**: Greek cross preferred when space allows.
**Measurement Method**
**1. Current Injection**: Apply current through opposite arms.
**2. Voltage Measurement**: Measure voltage across other two arms.
**3. Resistance**: R = V / I.
**4. Sheet Resistance**: R_s = (π/ln2) × R × correction factor.
**Design Parameters**
**Arm Length**: Typically 10-100 μm.
**Arm Width**: Typically 1-10 μm.
**Central Square**: Small compared to arm length.
**Symmetry**: All four arms identical.
**Applications**: Sheet resistance monitoring of doped silicon, silicides, metal films, polysilicon, transparent conductors.
**Advantages**: High accuracy, good repeatability, symmetric design, standard method.
**Limitations**: Requires specific geometry, larger than Van der Pauw, sensitive to arm width variations.
**Tools**: Four-point probe stations, automated test systems, semiconductor parameter analyzers.
Greek cross is **the preferred sheet resistance structure** — its symmetric geometry provides superior accuracy compared to arbitrary Van der Pauw shapes, making it the standard for semiconductor process monitoring.