nitride hard mask
**Hard Mask** is a **thin inorganic film used as an etch mask in place of or in addition to photoresist** — providing superior etch resistance for deep etches, enabling tighter CD control, and allowing photoresist to be removed without disturbing the pattern below.
**Why Hard Masks?**
- Photoresist: Poor etch selectivity vs. many materials (SiO2, Si, metals).
- Thick resist needed for etch depth → poor depth-of-focus, wider CD.
- Hard mask: 10–50nm inorganic film → excellent selectivity, thin profile, tight CD.
**Common Hard Mask Materials**
- **Silicon Nitride (Si3N4)**: Excellent etch selectivity vs. SiO2 and Si. Used for STI, contact, poly gate.
- **Silicon Oxide (SiO2)**: Hard mask for Si etching, TiN gates.
- **TiN**: Used as hard mask for high-k/metal gate etch, good mechanical hardness.
- **SiON**: Intermediate properties, doubles as ARC (anti-reflection coating).
- **Carbon (a-C)**: Amorphous carbon — extreme etch resistance, used at 7nm and below.
- **SiC or SiCN**: Low-k etch stop and hard mask in Cu dual damascene.
**Trilayer Hard Mask Stack (< 10nm)**
```
Photoresist (top)
SiON (SHB — spin-on hardmask)
Amorphous Carbon (ACL — bottom anti-reflection + etch mask)
Target material
```
- Thin resist patterns SOC/SOHM layer.
- SOHM transfers to ACL by O2 plasma (resist gone, ACL patterned).
- ACL transfers pattern to target (ultra-high selectivity).
**CD Improvement**
- Resist CD ± 3nm — transferred to hard mask by anisotropic etch.
- Hard mask CD ± 1–1.5nm (after etch trim).
- Net CD improvement from resist to final pattern via hard mask.
**Process Flow**
1. Deposit hard mask.
2. Coat photoresist.
3. Expose and develop resist.
4. Etch hard mask (opens pattern in hard mask).
5. Strip resist (O2 plasma — hard mask survives).
6. Etch target layer using hard mask.
7. Strip hard mask (selective to target).
Hard mask technology is **the enabler of deep, aggressive etches in advanced CMOS** — without hard masks, the sub-5nm features and high-aspect-ratio contacts of modern transistors would be impossible to pattern reliably.