Indian Journal of Engineering & Materials Sciences Vol. 7, October-December 2000, pp. 303-309

LPCVD and PECVD silicon nitride for microelectronics technology

B C Joshi , G Eranna, D P Runthala, B B Dixit, 0 P Wadhawan, P D Yyas

Central Elec tronics Engineering Research Institute, Pil ani 333 031 , India

Received 25 February 2000; accepted 12 September 2000

Silicon nitride deposition by chemical vapour depositi on (CYD) based techniques like low pressure CYD (LPCYD) and plasma enhanced CYD (PECYD) is described in thi s paper. The technological advantages of silicon nitride deposition by these two techniques, developed at CEERI, are discussed in detail. Applications of LPCYD nitride films for LOCOS, composite gate structures for MNOS and MOS devices are highlighted. The importance of PECYD nitride films for diffu­sion masking of compound semiconductors, and for passivation in Si , GaAs. and InP devices are demonstrated. Process pa­rameters of LPCYD and PECYD nitride deposition have been optimized for various substrate conditions depending on the technolog ical requirements. Materi al properties are being explored for various micromachining acti viti es, which includes diaphragm , cantilever, and beam formations.

Chemical vapour deposition (CYD) techniques are most frequently used as a standard tool for a large variety of depositions in semiconductor processing. These depositions are carried out in a wide variety of process conditions at deposition temperatures varying from 100 to 1000°C and pressures from atmospheric pressure to about 50 mTorr. The energy for the gas phase chemical reaction is supplied thermally, by photons (photochemically), or by a glow discharge. Depositions using CYD process produce relatively uniform films with better control on the stoichiometry.

The most widely used materials for semiconductor processing are polycrystalline silicon (poly-Si), si licon dioxide, low pressure CYD for stoichiometric silicon nitride, and plasma enhanced CYD for near stoichiometric silicon nitride. Silicon nitride has an important application in device processing due to its ability to withstand thermal shock and sustained elevated temperature exposure. Structures of silicon nitride are usually produced by powder metallurgy methods for bulk applications. Thin film coatings are generally done by CYD techniques that involve mixtures of gases, usually SiHJNH3, SiClJNH3 or SiH2CI 2/NH 3 by gas phase reactions I. Nitride can easily be patterned with photoresist and etched in a CFJOz-based plasma or in a more severe process, in H3P04 at 180°C where the film etch rates are of the order of 100 A/min.

The thermo-chemi cal gas phase reactions at high temperatures, between 700-900°C, yield stoichiometric si licon nitride. These nitride films are

being used as insulating films on the semiconductor devices because they are amorphous, dense, stable against oxidation, and show good barrier against moisture penetration. Stoichiometric si licon nitride is used as a mask for the LOCOS process, where active areas are defined on an integrated circuit and also for device isolation. Presently, this stoichiometric nitride is very popular, to define very complicated and delicate microstructures, in the field of sensors and micromachining.

Because of the involvement of high temperature, of the order of 775°C, for the deposition of silicon nitride, the application of these films is limited. But by applying the plasma it is also poss ible to deposit the nitride films at lower temperatures, below the melting point of aluminum and gold, which are commonly used as interconnection metals in device fabrication . These plasma deposited films are normally amorphous and contain 20-30 atomic % of hydrogen. The applications of plasma nitride films have spread rapidly mainly because of its low temperature depositions. This includes a passivation layer deposition to protect active area from contamination from diffusion of sodium, protection of interconnecting lines from scratches and mechanical damage after the completion of the device fabrication. These films are the dielectric of choice for use in amorphous silicon thin film transistors for active­matrix liquid crystal display. It is not uncommon to employ these nitride films as intermetallic dielectrics in multilevel metallization for high-density integrated ci rcuits.

304 INDIAN J ENG. MATER. SCI., OCTOBER-DECEMBER 2000

Different Techniques to Deposit Silicon Nitride Films

There are mainly two deposition techniques used to deposit the silicon nitride films: low pressure CYD (LPCYD) and plasma enhanced CYD (PECYD). Both the techniques are equally popular and widely used for different applications. LPCYD is a slow deposition process, where as PECYD depositions are done at relatively low temperatures with higher deposition rate. Recently it is reported that by using porous silicon sufficiently high thick films, up to 13 !lm thick, can be grown by first converting the films into porous form and subsequently treating them with ammonia at high temperatures2

. Table 1 summarizes the techniques used for the deposition of silicon nitride. Depending on the application, one has to choose the right process for the deposition.

LPCVD system

Silicon nitride system (MIS Thermco, USA) is used to deposit precisely controlled thin films of silicon nitride. These films are deposited by the gas phase chemical reactions of ammonia and dichlorosilane at an elevated temperature (775°C). Diffusion of reactant gases is greatly enhanced by maintaining a vacuum of the order of 0.2-0.3 Torr in the reaction chamber3

• Schematic diagram of the LPCYD system is shown in Fig. l.

Diffusion furnace with Analock series temperature controller, and gas control system with three gas loops are used for this purpose. Flow of gases is controlled

separately by mass flow controllers (MFC) . The vacuum system includes chemical series pumps (MIs AIcatel, France). To obtain a good uniformity of the films, the temperature of the three heating zones was maintained differently as follows . If Td is the deposition temperature then the temperature profile of the three zones should be:

at the inlet,. center, and exhaust-end of the reactor chamber respectively. The profile will compensate the temperature fluctuations at the inlet of the reaction chamber and also give uniform deposition rate at the end side of the tube. The thickness of the films was measured by using profilometer. The variation of the thickness within the wafer was ±2% and from wafer to wafer ±3% in a batch.

PECVD system

A dual chamber configuration of PECYD system (MIs Plasma Lab., UK) was used to deposit all our silicon nitride films. The standard 13 .56 MHz plasma excitation frequency is being used to maintain the plasma conditions. The substrate is kept face-up on the bottom electrode. The deposition of silicon nitride is best achieved by co-reaction of nitrogen, ammonia and silane gas mixtures. The source gases are showered from the top electrode and are pumped out from the bottom side of the lower electrode. It is a cold wall system in which the deposited electrode is heated by a resistive heating inside the susceptor. The

Tablel - Different techniques to deposit / grow silicon nitride films

Deposition/Growth process Process Temperature(°C) Film thickness

Silicon nitride (LPCYD)

Silicon nitride (PECYD)

Silicon nitride (non-CYD)

Pr ess ur e Sensor

SiH4 + NH3SiCI2H2 + NH3

SiH4 + NH3 + N2SiH4 + N2

Porous Silicon + NH3

r===~ I 3- Zone Re si st~n(e Hea te r

Gas Inl e t

700 - 900650 - 750 2000 A 200 - 350200 - 350 1.0 11m

1100 13.0 11m

RF Power Gas Mixture In . ill Perf or ~ t e~ Elec trode

Silic on Wa f er .

Vacuum Pump Vac uum Pump

Fig. I - Schematic diagram of a low-pressure chemical vapour vapour deposition system

Fig. 2 - A typical plasma-enhanced parallel plate chemical deposition reactor:

JOSHI et al. : SILICON NITRIDE DEPOSITON BY LPCYD AND PECYD 305

chamber and the electrode are relatively cold which sets up a temperature gradient causing less number of particles to condense on the chamber walls. A typical PECYD parallel plate deposition system is shown in Fig. 2.

The process of PECYD silicon nitride films has been developed by optimizing process parameters such as reactants ratio (silane:ammonia = Rx) and its flow, chamber pressure, RF power and deposition temperature. The growth rate and properties of these films are very strongly dependent on the above said parameters4

.

The typical optimized process parameters are: deposition temperature (=2S0°C), pressure (=0.4 Torr), silane to ammonia ratio (Rx=l :8), RF power

2 0

(=0.12 W/cm ). The growth rate can vary 400 A/min to 800 A/min depending on the gases flow . Lowering Rx will lead to particulate formation and also lead to silicon-rich conditions. Fig. 3 shows the deposition rate of the nitride films with RF power and also with different gas ratios. However, the substrate temperature has certain effect on the film deposition rates.

Physical Properties of the Silicon Nitride Films The physical properties of LPCYD and PECYD

silicon nitride films are shown in Table 2. The nitride films are amorphous in nature and stoichiometrically

different because of the hydrogen ion content. LPCYD films contain less amount of hydrogen than PECYD films. The deposition temperature basically decides the percentage of hydrogen in the deposited films. The film etch rate, in buffered hydrofluoric acid (BHF), mainly depends on the hydrogen concentration and typically PECYD nitride films etch faster when compared to the LPCYD grown silicon nitride films . Chow et al. 5 compared plasma silicon nitride layers from different commercially available reactors and concluded that the etch rate of plasma

500-

400

20

DEP. TEMP. ~ 300'C PRESSURE = 0.5 TORR

40 60 80 100

Total RF Po",er(Watts)

Rx l:l

Rxl:2

Rxl :8

120 140

PECVD SiJN4 DEPOSmON RATE AS A FUNcnON OF RF POWER

160

Fig. 3 - Deposition rate of PECYD silicon nitride films as a function of RF power

Table 2 - Properties of LPCYD and PECYD silicon nitride films

Parameter

Deposition temperature (0C)

Film composition

Atomic % of hydrogen

SilN ratio

Solution etch rateBuffer HF (at 25°C)49% HF (at 23°C)85% H)P04 (at 155°C)85% H)P04 (at 180°C)

Plasma etch rate92% CF4, 8% O2, 700 Watt

IR absorption spectraSi-N bondSi-H bond

Refractive index

Density (g/cm3)

Dielectric constant

Resist ivity (Q-cm)

Dielectric strength (106 V /cm)

Energy gap (eY)

Stress (109 dynes/cm2)

Na+ penetration

H20 penetration

LPCYD

700-800

Si JN4 (H)

4-8

0.75

10-15 Nmin80 Nmin15 Nminl20 Nmin

600 Nmin

- 830 cm'l-

2.01

2.8 - 3.1

6.0 - 7.0

1015_10 17

10.0

5.0

10.OT

< 100 A

Zero

PECYD

200-400

SiNxHy

20 - 30

0.8 - 1.2

200-3C0 NminI500-3000 NminlOO-200 A/min600-1000 A/min

1000 A/min

- 830 em' 12200 cm' l

1.8 - 2,5

2.4 - 2.8

6.0 - 9.0

< 1015

5.0

4.0 - 5.0

2.0 C - 5.0 T

< 100 A

Near Zero

306 INDIAN J ENG. MATER. SCI., OCTOBER-DECEMBER 2000

nitride layers in BHF solution is a function of the total amount of 'hydrogen incorporated in the layer during deposition.

The deposited nitride films were characterized by studying surface morphology with optical microscope, Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM); film thickness with Nano-Spec®, etch rate with wet chemical etchants and reactive ion etching (RIE); refractive index with Ellipsometry, chemical composition with Auger Electron Spectroscopy (AES) and Secondary Ion Mass Spectrometry (SIMS) .

The structural and electrical properties of plasma silicon nitride layers depend on the consumption of the layers and, therefore, on the deposition conditions. Table 3 lists the typical properties of the nitride films. Samuelson and Mar6 measured the SiiN and Si-HlN­H ratio for plasma silicon nitride layers deposited under different conditions, and they concluded that the Si-N/N-H ratio is directly correlated to the refractive index of the deposited layer, where as the SilN ratio does not have a simple correlation with the

refractive index. Figs 4 and 5 are the micrographs scanned on the film surface using Atomic Force Microscope in I-micron range. The photographs indicate the close packing of these films without any micro-pores within them. This corroborates the film amorphous behaviour. The film surface roughness is about 5.0 A for LPCYD nitride and 3.8 A for PECYD films. The film surface roughness shows the topography of the film and the control over the deposition techniques 7. The overall thickness variation, macroscopically on a 2-inch wafer is about ±2% in case of LPCYD films and ±3% in PECYD films.

AES depth profiles of silicon nitride layers on monocrystalline silicon show the presence of oxygen at the interface between silicon and silicon nitride and at the surface. It is further shown that the Si-HlN-H ratio is also linearly correlated with the refractive index. The total amount of hydrogen bonded to silicon and nitrogen is about 2x1022 (cm-3

) or -20 alo, almost independent of the gas-phase composition as measured by infrared absorption spectroscopy. The

Table 3 - Propel1ies of PECYD silicon nilride films

S.No. Ratio(Rx) Etch Refractive in- FTIR peaks Si- SIMS SilN ratio rate(Nmin) dex N, Si-H, N-H

1

2

3

4

I: 1

I : 2

I: 4

I: 8

400

500

650

900

Fig. 4 - Atomic force microscope pictures of LPCYD silicon nitride films deposited at 780°C using dichlorosilane and ammonia chemistry

1.96

1.90

1.88

1.80

bonds

83022003300

83022003300

0.918

0.88

0.75

0.70

Fig. 5 - Atomic force microscope pictures of PECYD silicon nitride films deposited at 300°C using silane and ammonia chemistry

JOSHI et af. : SILICON NITRIDE DEPOSITON BY LPCVD AND PECVD 307

Si-H/N-H ratio appeared to be only a function of the SilN ratio of the as-deposited layer for a large variety of deposition conditions8

. Fig. 6 indicates the Auger spectrum of the PECVD films where the ratio of SilN is roughly about 0.78. This is very close to the actual expected composition of 0.75. Fig. 7 indicates the film FTIR spectrum where different vibrational modes of Si-H, Si-N, N-H, Si-O, Si-Si are identified. Presence of these vibrational bonds shows the compound formation of the participating gaseous species and indicates the presence of the expected stoichiometric compositions. The presence of Si-H disappears, as shown in Fig. 8 when the films are annealed at high temperature where hydrogen escapes from the bulk nitride. This is an indication that the hydrogen is trapped within the bulk film and very loosely bonded. Fig. 9 indicates the SIMS profile, of the PECVD films, of the film to different atomic species of Si , N, SiN, O2, and H2. From the figure, the

N

Si

so 210 370 530 690 850 1010 1130

ilocrBY <eV) - -

Fig. 6 - Auger surface analysis of the PECVD deposited silicon nitride of the film. The SilN ratio is about 0.78

3S

20

1>

4000 3SOO 3000 2SOO 2000 I soo 1000 SOO

w.vcnumI>cn

Fig. 7 - FTIR of PECVD as-deposited silicon nitride showing different bonds

atomic concentrations of the Nand H atomic ratios are same throughout the scanned area. However, the O2 presence is noticed on the film surface but slowly disappears inside the body.

At room temperature, high modulus materials, such as silicon nitride exhibit linear-elastic behaviour at lower strain, and transition abruptly to brittle-fracture behaviour at higher strains. Intrinsic stress depends on thickness, deposition rate (locking in defects), deposition temperature, ambient pressure, method of film preparation, type of substrate used (lattice mismatch), incorporation of impurities during growth. The stress turns from tensile in stoichiometric films to compressive in silicon-rich films. The RF power of a plasma-enhanced CVD deposition influences stress, e.g., a thin film may start out tensile, but decreases with increase in RF power and finally becomes compressive with further increase in the power. PECVD nitride films are usually compressive due to

l5

30

15

· 'ou ~. )000 25000 n .. 'IOU SOl

'w'ntnlldlbers

Fig. 8 - FTIR of PECVD silicon nitride films annealed at high temperature in nitrogen ambient

Time <s)

Fig. 9 - SIMS analysis of PECVD deposited silicon nitride films

308 INDIAN J ENG. MATER. SCI., OCTOBER-DECEMBER 2000

Fig. 10-Cantileveretched in (100) silicon using LPCYD silicon nitride

the presence of hydrogen atoms in the lattice. By thermal annealing, the lattice hydrogen escapes out of the film and the film turns towards tensile.

Stacked silicon nitride-silicon dioxide layers are of interest in several applications. While they form the backbone of NMOS memory technology, they have also been studied for such applications as storage dielectrics in DRAMs and as interpoly dielectric in floating gate memories9

. In these applications, the possibility of using a physically thicker stack with the same capacitive coupling as a thinner thermal oxide layer is attractive. These stacks can be designed for lower leakage at the same average field compared to either silicon dioxide or silicon nitride alone JO

• In scaled MNOS memories, an oxide-nitride-oxide layer is useful in reducing the effect of hole injection from the gate.

New ventures of silicon nitride into the world of micromachining

Silicon nitride has entered into a New World of micromachining as a basic construction material for di fferent cantilevers, beams, and diaphragms II. Because silicon nitride is very strong, hard, and chemically inert, the stress in the film can be controlled by changing the SilN ratio during the LPCYD deposition process. A large number of chemicals that are used as etchants do not attack the nitride. Hence, a th in film of nitride is sufficient to use as an excellent etch stop in building small microsystems particularly for harsh chemical s l2

•

Fig. 10 indicates the cantilever structure, where it is etched in (I 00) silicon using KOH etchant.

Fig. II - SEM micrograph of 41 0 ~lm deep microchannels etched at 85°C in ( ItO) silicon usi ng LPCYD deposited silicon nitride layer over thermal Si02 layer

For prolonged KOH etching, a high density LPCYD silicon nitride acts as a mask. LPCYD nitride serves better for this purpose as the etch rate of this film is less than 1.0 A/min, a 400 A layer of LPCYD nitride is sufficient to mask against KOH etchant to etch a wafer of 600 )..lm. The etch selectivity of Si/Si3N4 was found to be better than 104 in 7 M KOH at 80°C. Fig. 11 shows the pictorial view of silicon wafer etching used to etch the channels for laser diode packaging. It indicates the deep etched channels in the (110) silicon wafer by using LPCYD nitride as a masking layer. The film is stable even after the deep etch of 410 )..lm and exposing the film for KOH solution for 6 hours at 85°C.

Nitride films are typically under a tensile stress of about IxI09 Pa. If in the overall processing of the device, nitride deposition does not pose a problem, KOH emerges as the preferential anisotropic etchant.

Conclusions

In this paper, we have shown the importance of silicon nitride and its deposition by llsing LPCYD and PECYD techniques. The film stochiometry, composition , etch rate, AES and its applications in different fields are highlighted and shown the importance in the device applications and also in the new emerging field of micromachining.

Acknowledgements

Authors thank Y K Dwivedi and K Rangra for the micrographs on channel etching and cantilever formations in silicon. We also wish to acknowledge

JOSHI et al. : SILICON NITRIDE DEPOSITON BY LPCYD AND PECYD 309

the help of A K Sharma for nitride film sample scanning, characterization and recording by AFM.

References Sherman A, Chelllical vapor deposition for microelectrollics: prillciples, technology, and applicatiolls (Noyes Publication, New Jersey), 1987.

2 Madou M, Fundalllelllais of lIlicrofabricatioll (CRC Press, New York), 1999.

3 Dong D, Irene E A & Young D R, J Electrochenl Soc, 125 ( 1979) 819.

4 Sherman S, Wagner S, Mucha J & Gottscho R A, J Electro­chelll Soc, 144 (1997) 3198.

5 Chow R, Lanford W A, Ke-Ming W & Rosier R S, J Appl Phys, 53 (1982) 5630.

6 Samuelson G M & Mar K M, J Electrochelll Soc, 129 (1982) 1982.

7 Martinez F L, Prado A del , Martil I , Bravo D& Lopez F J, J Appl Phys, 88 (2000) 2149.

8 Claassen W A P, Yalkenburg W G J N, Habraken F H PM & Tammingo Y, J Electrochem Soc, 130 (1983) 2419.

9 Ma T P, IEEE Trails Electronic Devices, 45 (1998) 680. 10 Giridhar R Y, Electrochemical Society Meeting, 1988.

II Isomae S & Tamaki Y, J Electrochelll Soc, 146 (1997) 340. 12 Storgaard-Larsen T & Leistiko 0, J Electrochelll Soc, 144

(1997) 1505.