Radial flow reactor including glow discharge limitting shield

Abstract

An improved radio frequency (rf) powered radial flow cylindrical reactor utilizes a gas shield which substantially limits the glow plasma discharge reaction to a section of the reactor over the semiconductor substrates which are to be coated. The gas shield permits the use of higher rf input power which contributes to the formation of protective films that have desirable physical and electrical characteristics.

Description

BACKGROUND OF THE INVENTION

This invention relates to apparatus for depositing coatings on suitable substrates and, more particularly, to an improved radial flow reactor for coating semiconductor substrates utilizing laminar flow of reactant gases in a radial direction.

The invention will be described specifically with reference to use in depositing silicon-nitrogen films on silicon substrates although it should be evident that it is not limited thereto. The methods of depositing such films is described more fully and claimed in copending application of Houserreadial radial flow radio frequency (rf) powered reactor 10. Reactor 10 comprises a top plate section 12, a bottom plate section 14, and cylindrical side wall 16. Side wall 16 is connected to the top and bottom of plates 12 and 14 in a sealing relationship to define an evacuable chamber 24.

A first electrode 18, which is typically a circular metallic member, is coupled to an rf source 22 through an impedance matching network 20. Electrode 18 is illustrated as electrically isolated from top plate 12. A second electrode 26, which is typically a circular metallic member, comprises a top surface 28, which is adapted to support semiconductor substrates 30, a bottom portion 32, and an end portion 34. Heaters 36, which are typically contained with electrode 26, are utilized to heat the semiconductor substrates 3 30 to a preselected temperature.

A gas flow shield 38 is closely spaced to electrode 26 and essentially surrounds electrode 26 except for the portion of the top surface 28 thereof on which the semiconductor substrates wafers 30 are placed. A bottom portion 40 of shield 38 is essentially parallel to bottom portion 32 of electrode 26. A U-shaped end portion 42 of shield 38 surrounds the end portion 34 os of electrode 26.

A Plurality plurality of sheaths or tubes 44 communicate with the internal portion of chamber 24 extending through the bottom plate 14 and bottom portion 40 of shield 38 in a sealing relationship. Sheaths 44 are coupled at first ends thereof to a gas ring 46 which has a plurality of essentially equally spaced small apertures 48 therethrough. Gas ring 46 exists in the cavity between the bottom portion 32 of electrode 26 and the bottom portion 44 of gas shield 38. Sheaths 44 are connected by second ends thereof to a common sheath (tube) 50 which has a control valve 52 connected in series therewith.

A sheat sheath 54 communicates with the interior of chamber 24 and extends through 14 and 38 in sealing relationship and contacts electrode 26. Electrode 26 has a central region generally at 56 which defines an aperture therethrough. Sheath 54 extends to this aperture and terminates at the top surface 28 of electrode 26. The other end of sheth sheath 54 is coupled to vacuum pumps 58 that are used to evacuate the interior of chamber 24.

The reactant gases required to coat the semiconductor substrates wafers 30 contained within chamber 24 are introduced into tube 50 and flow as indicated by the arrows.

An rf glow discharge reaction is caused to occur within chamber 24 between electrodes 18 and 26 when the rf source 22 is activated and appropriate gases are introduced into chamber 24 through 50. Gas shield 38 is typically spaced 1/4 inch or less from electrode 26. This close spacing substantially inhibits the glow discharge reaction which occurs between electrode 18 and the top surface 28 of electrode 26 from occurring around end portion 34 and bottom portion 32 of electrode 26. This serves to intensify the rf glow discharge reaction immediately above semiconductor substrates 30. In addition, the gas shield 38 permits the effective use of higher input rf power than is possible without the shield. Without shield 38 there is a tendency for the gases introduced into the chamber 24 to react below electrode 26 and therefore to dissipate before reaching semiconductor substrates 30. Thus, without the shield 38 the increasing of rf power beyond a certain point is not particularly helpful in intensifying the glow discharge reaction above the substrates 30 where it is important that the reaction occur.

The vacuum pumps of FIG. 1 are selected to be compatible with a high gas flow rate of approximately 2 liters per minute minutes at greater than 1 mm pressure. A 150 cfm Leybold-Heraeus roots blower backed with two 17 cfm mechanical pumps running in parallel were found to be sufficient to achieve the needed high gas flow rate. Additional pumping capacity comprising a cryopanel and a 400 l/s vacuum pump located below an isolation valve (not illustrated) in the reactor 10 of FIGS. 1 and 2 is utilized to initially pump the reactor 10 and 100 to a base pressure of ∼10-6 mm.

In operation, semiconductor substrates 30 are loaded on support surface 28. The reactor 10 is then sealed, closed, and pumped down to 10-6 mm. The heaters connected or part of the electrodes 26 are turned on and the semiconductor substrates are heated to approximately 275°C The vacion Vacion isolation valve is closed and reactant gases are admitted to the reactor and the roots blower valve is opened again. A dynamic pressure of approximately 600μ is established in the reactor with the input gases flowing at the desired flow rates. Thereafter the roots blower valve is throttled to the desired pressure. The rf power source is now activated to the desired power level.

A fully functional reactor very similar to the reactor 10 of FIG. 1 was constructed with sections 12, 14 and 16 all being stainless steel and electrode 18 being aluminum. Two tubes 44 are utilized and the gas ring utilized is a tubular member having a diameter of 5 inches. The spacing between the U-shaped section 42 of gas shield 38 and the end portion 34 of electrode 26 is approximately 1/4 inch. The spacing between the top surface 28 of electrode 26 and electrode 18 is approximately 1 inch.

Another fully functional reactor similar to that of reactor 10 of FIG. 1 was constructed with sections 12 and 14 made of aluminum and sidewalls 16 made of pyrex. Since section 12 is electrically isolated from sections 14 and 16, it is not necessary that the electrical connection from the impedance matching network 20 to electrode 18 be electrically isolated from top plate section 12. In this reactor the gas ring is a cylindrical member having a diameter of approximately 14 inches. There are approximately 120 gas outlet apertures of 40 mils each in diameter equally spaced around this gas ring. The spacing between the end portion 34 of electrode 26 and the U-shaped portion 42 of gas shield 38 is approximately 1/8 of an inch. The distance between surface 28 of electrode 26 and electrode 18 is approximately 1 inch.

Referring now to FIG. 3, there is illustrated a flow diagram of reactant gases that may be used in the reactor of FIGS. 1 and 2. Sources of silane (SiH₄) in a carrier gas argon (Ar) 1000, ammonia (NH₃) in a carrier gas argon (Ar) 1100, carbon tetrafluoride (CF₄) 1200, and oxygen (o₂) (O₂) are connected through a separate one of valves 1400, 1500, 1600 and 1700, respectively, to separate flow meters 1800, 1900, 2000 and 2100, respectively, and then through separate leak valves 2200, 2300, 2400, and 2500, respectively. The outputs of leak valves 2400 and 2500 are both connected through a valve 2900 to a reaction chamber 2700. Reaction chamber 2700 can be the chamber 24 of FIGS. 1 and 2. The outputs of leak valves 2200 and 2300 are both connected to mixing chamber 2600. Mixing chamber 2600 is in communiction with reaction chamber 2700 through valve 2800.

The reactant gases SiH₄ and NH₃ mix in the mixing chamber 2600 and then pass through valve 2800 into reaction chamber 2700. During the time of depositing inorganic films on semiconductor substrates, valves 1600, 1700, 2400, 2500 and 2900 are closed and valves 1400, 1500, 2200, 2300 and 2800 are open.

After one or more deposition runs, inorganic films form on the electrodes 18 and 26 and on other areas in the reactor of FIGS. 1 and 2. To clean off the films, the heaters and rf source of FIG. 1 are turned on and valves 1600, 1700, 2400, 2500 and 2900 are all opened, and valves 1400, 1500, 2200, 2300 and 2800 are all closed. The films deposited on internal parts of the reactor are cleaned by the resulting rf glow discharge reaction (the reactant gases being CF₄ and O₂) and a new set of semiconductor substrates can then be placed in the reactor for deposition of protective films thereon.

Advantageously all interconnecting tubing connecting the sources of gases illustrated in FIG. 3 to the reactor of FIGS. 1 and 2 are made of stainless steel to insure these connections are essentially leak-free. This essentially prevents any but the desired gases from entering the systems during the deposition operations. Essentially pure sources of SiH₄, NH₃, and Ar could be easily substituted for the SiH₄ in Ar and NH₃ in Ar sources.

In the first set of operating conditions described below the reactor was essentially as illustrated in FIGS. 1 and 2 without the gas shield and with electrode 18 in electrical contact with top plate 12. Side wall 16 is pyrex in this case. The following operating conditions were utilized to deposit protective films having the denoted characteristics on semiconductor substrates;

______________________________________
1st Operating
Condition     2nd Operating
(using apparatus
Condition
of FIGS. 1 & 2
(using apparatus
without gas shield)
of FIGS. 1 & 2)
______________________________________
Reactant gas   SiH4 /NH3 /Ar
SiH4 /NH3 /Ar
SiH4      1.25%         1.70%
NH3       1.56%         2.39%
Ar             97.19%        95.91%
Total gas      2000          2320
flow (SCCM)
Pressure in    1000          950
reactor (μ)
Substrate      330 deg. C.   275 deg. C.
temperature
(degrees C.)
Tuned RF       60            250
power (watts)
(reflected power = ∼ 0)
Thickness      1.1           1.1
of deposited
layer (μ)
Stress in      1-2 (tension) 1-5 (tension)
resulting layer
(109 dynes/cm2)
Etch rate in   175           180
BHF (Angstroms
per min.)
Density (GCM-3)
2.4           2.55
Composition of 1.1           1.05
resulting
layer (Si/N)
Refractive Index
2.15          2.05
Cracking       400           450
resistance
(deg. C. to which
substrates with
deposited layers
could be raised
without cracking)
Adhesion of    Good          Good
deposited layer
Step Coverage of
Very good     Very good
deposited layer
Scratch resistance
Good          Good
Dielectric constant
6.9           6.4
Breakdown strength
3.4           3.9
(106 V/cm)
Resistivity at 5 × 1018
4 × 10 13
2 × 106 V/cm
(ohm/cm)
______________________________________
3rd Operating 4th Operating
Condition     Condition
(using apparatus
(using apparatus
of FIGS. 1 & 2)
of FIGS. 1 & 2)
______________________________________
Reactant gas   SiH4 /NH3 /Ar
SiH4 /NH3 /Ar
SiH4      1.78%         1.78%
NH3       2.25%         2.25%
Ar             95.97%        95.97%
Total gas      2320          2320
flow (SCCM)
Pressure in    950           950
reactor (μ)
Substrate      275 deg. C.   275 deg. C.
temperature
(degrees C.)
Tuned RF       300           400
power (watts)
(reflected power = ∼ O)
Thickness (μ)
1.1           1.1
Stress in      1-2 (compression)
1-2
resulting layer              (compression)
(109 dynes/cm2)
Etch rate in   125           75
BHF (Ang-
stroms per min)
Density (GCM- 3)
2.75          2.90
Composition of 0.8           0.75
resulting
layer (Si/N)
Refractive Index
2.00          1.94
Cracking       550           550
resistance
(deg. C. to
which substrates
with deposited
layers could be
raised without
cracking)
Adhesion of    Good          Good
deposited
layer
Step Coverage  Very good     Very good
of deposited
layer
Scratch        Good          Good
resistance
Dielectric     6.8           5.8
constant
Breakdown      5.0           8.1
strength
(106 V/cm)
Resistivity    3 × 1015
5 × 1019
at 2 × 10 6 V/cm
(ohm/cm)
______________________________________

The tuned rf power indicated for each of the above operating conditions was read from a meter on the rf power supply. It is to be appreciated that the effective rf input power density between the electrodes of a reactor is a function of the geometry of the electrodes and the spacing therebetween. The reactors utilized with the above operating conditions have a circular top electrode having a radius diameter of 14 inches. Electrode 18 was separated from the electrode 26 by approximately 1 inch. A reactor with different type of or size of electrodes and different spacing between electrode electrodes would require an appropriately different input rf power in order to produce films on semiconductor substrates with essentially the same characteristics as described herein.

The first operating condition is useful for depositing protective films on semiconductor substrates which utilize aluminum metallization. The aluminum metallization can easily withstand temperatures at and above the 330°C used. The second through fourth operating conditions can be used with semiconductor substrates which have aluminum or gold with titanium, palladium and gold beam leads since the temperature utilized is below that at which titanium and palladium and gold interact.

Cracking of the protective films allows moisture and inpurities (i.e., sodium) to attack the surface of the semiconductor substrates and thereby destroy the circuitry contained thereon. It is therefore very important that protective films be as crack-resistant as possible.

The fourth operating condition results in films which are substantially stoichiometric silicon nitride (Si₃ N₄) and which contain essentially no other organic combinations or argon incorporation. The physical characteristic of the resulting Si₃ N₄ film are superior to Si₃ N₄ films produced by chemical vapor deposition (CVD) processes in that they are much less susceptible to cracking than the CVD produced Si₃ N₄. The reason for this is that the silicon nitride films resulting from operating condition four have relatively low compressive stress and not the relatively high tensile stress of the CVD produced films.

It is important to note that in all of applicants' operating conditions careful precautions were taken to limit the presence of nitrogen (N₂) or oxygen (O₂) in the reactor during the glow discharge reactions, it has been determined through experimentation that the addition of even small amounts of N₂ (up to 2%) or O₂ (up to 0.2%) in the reactant gas mixture can significantly adversely affect the characteristics of the resulting films. The addition of only 2% nitrogen to the reactant gases resulted in an order of magnitude increase in tensile stress of the resulting film, and an increase in the BHF etch rate of over 7 times. The addition of only 0.2%, O₂ to the reactant gases resulted in a 7-times increase in the BHF etch rate.

Using the second operating conditions as a standard, the effects of varying the five main process parameters, namely, (A) Gas pressure, (B) Total gas flow, (C) Pressure, (D) Substrate temperature and (E) RF input power into the reactor, were studied. The graphs illustrated in FIGS. 4, 5, 6, 7 and 8 each illustrate on the abscissa one of the variables denoted above, and on the respective ordinate axis some of the resulting characteristics of the film deposited on semiconductor substrates.

A. Gas Composition

The graph of FIG. 4 illustrates the effect of increasing SiH₄ concentration (1.4≦% SiH₄ ≦1.9; 0.5≦SiH₄ /NH₃ ≦0.9) in the reacting gases. These gas compositions were achieved by adjusting the flowmeters for 3%SiH₄ -Ar and 5%NH₃ -Ar to various complementary settings so as to keep the total flow constant.

As expected, increasing the SiH₄ concentration in the gas led to a corresponding linear increase in the Si/N ratio in the film (from ∼1.0 to ∼1.2), and a linear increase in the refractive index (from ∼1.9 to ∼2.2). For the lowest SiH₄ concentration used, (SiH₄ /NH₃ =0.52), the film density was found to be relatively low (∼2.3 gcm-116³ gcm-3), and the BHF etch-rate was corresponding correspondingly high (250 angstroms/min.). With increasing SiH₄ /NH₃ ratio, the film density ρ showed a broad peak (ρ≈2.55 gcm-3) for 0.58≦SiH₄ /NH₃ ≦0.79. The ρ decreased again at SiH₄ /NH₃ ∼0.9; however, this was not accompanied by corresponding increase in BHF etch-rate, presumably because the films now had a much higher Si content (Si/N∼1.2). The film σ, which was always tensile, showed a peak at SiH₄ /NH₃ ∼0.6, which is located at a slightly lower SiH₄ concentration than that for the peak in ρ.

While most of our work has involved operating conditions in which the ratio of silane to ammonia was between 0.5 and 0.9, which is believed the preferred range, it may be feasible to deposit useful protective films with ratios outside this range.

B. Gas Flow

The graph of FIG. 5 illustrates the effect of increasing the total gas flow on the other variables of the process. The total gas flow was varied in the range 1.0 to 2.5 liters min-1, with the SiH₄ /NH₃ ratio constant at 0.71(%SiH₄ =₁.70). It may be seen from FIG. 5 that increasing the flow led to a higher deposition rate (from 120 to 200 angstroms/min), a greater refractive index, and a larger Si/N ratio in the film (from 0.8 to 1.05). For this range of film composition, the film density seems to have a dominant effect on the BHF etch rate; a broad maximum in ρ corresponds to a broad minimum in the etch rate. The tensile stress descreases decreases with increasing flow; this is probably the result of a higher film purity (with respect to possible nitrogen/oxygen contamination) as the flow is increased.

C. Pressure

The graph of FIG. 6 illustrates the effect of increasing pressure on the other variables of the process. The average pressure during film deposition was varied from ∼700 to 1000μ (±25μ). As shown in FIG. 6, increasing the pressure also led to a higher deposition rate, whereas the density and the BHF etch rate did not change much. The refractive index decreased linearly This generally (i.e., for pressures ≧750μ) correlates with a decrease in the Si/N ratio in the film.

D. Substrate Temperature

The graph of FIG. 7 illustrates the effects of varying the temperature of the semiconductor substrates on the other variables of the process. The limited range of substrate temperatures studied (200°≦T_s ≦.Badd.300°".Baddend. 300° C.) was influenced by the desire to stay below temperatures at which Pd-Au interdiffusion (in Ti/Pd/Au metallization) becomes excessive. As shown in FIG. 7, T_s (substrate temperature) has a pronounced effect on the BHF etch rate, which decreases almost exponentially with increasing T_s. The decrease in BHF etch-rate is associated with a linear increase in the film density, ρ and in the refractive index, n. Thus, for films deposited at 200°C the BHF etch rate was 700 angstroms/min, the density was ∼2.3 gcm-3 and the refractive index was ∼1.85. Interestingly, these films also had a rather large Si/N ratio (∼1.2) and a high tensile stress (7×10⁹ dynes cm-2). With increasing T_s, both σ and the Si/N ratio in the film displayed a shallow minimum at ∼250°C; however, a higher T_s of 275°C was preferred because it led to films with yet greater density (2.55 gcm-3) and somewhat lower etch-rate without an excessive increase in σ.

E. RF Input Power

The graph of FIG. 8 illustrates the effect of increasing the rf input power. Tuned rf input powers were investigated in the range of 100 to 350 watts (reflected power=0). For this series of experiments, the SiH₄ /NH₃ ratio was kept constant at 0.8, and SiH₄ at 1.81. For increasing rf power, there was found to be a rapid and linear increase in the film ρ (weight-gain measurement., using 1μ thick films) from 2.2 gcm-3 at 100 watts to 2.8 gcm-3 at 350 watts. Films (1μ) thick with lower density had a distinct yellowish tinge to them when deposited on Al-metallized devices, whereas those with densities ≧2.4 gcm-3 appeared to be grayish and more truly transparent. Both the film ρ σ and BHF etch-rate showed a bimodal behavior at ∼275 watts. Below this power level the stresses were very low tensile (∼0.5×10⁹ dynes cm-2) and the etch-rates were relatively high (275 to 325 angstroms/min). At rf powers ≧300 watts, the stresses, which had been tensile, became compressive (1-2×10⁹ dynes cm-2) and the BHF etch rates were relatively low (<150 angstroms/min). Significantly, the refractive index showed a decrease with increasing rf power. The refractive index, film composition, and film density have been correlated using the Lorentz-Lorentz equation.

Finally, it should be understood that the specific embodiment of the invention described is merely illustrative of the general principles and various modifications thereof are feasible, including change in dimensions, geometry, and materials. Moreover, it should be evident that other gases may be utilized to form films of other materials.

Claims

1. coating apparatus Apparatus comprising:

an evacuable chamber;

a member located in said chamber having a support region adapted to support at least one object and a central region which defines an aperture therethrough;

means for facilitating a radio frequency discharge within the chamber adjacent the object to form a flow glow discharge plasma from reactant gases introduced into the chamber;

exhaust means in communication with the centrally disposed aperture in the member; and

coating gas-vapor feed means located adjacent the support region of the member such as that gas-vapor feed is induced to flow across the support region of the member in a radial flow and out through the exhaust means, the coating gas-vapor feed means being adapted to substantially inhibit any glow plasma discharge reaction from occurring until reactant gases introduced into he the apparatus through the coating gas-vapor feed means reach the support region of the member.

2. Apparatus for coating a substrate comprising:

an evacuable chamber;

a member located in said chamber having a top region which is adapted to hold the substrate and a central region defining an aperture therethrough;

means for generating a radio frequency discharge within the chamber adjacent the substrate to form a glow discharge plasma from reactant gases introduced into said chamber;

means for establishing a radial flow from an outer portion of the member which is adapted to hold the substrate toward and out of the aperture of the reactant gases suitable for forming the plasma and coating on the substrate;

means in communication with the aperture for exhausting the gases; and

means for substantially confining said plasma glow discharge to a zone which encompasses the top region of the member and extends substantially vertically upward therefrom.

3. The apparatus of claim 2 further comprising heating means contained within the chamber for heating the substrate to a preselected temperature.

4. The apparatus of claim 2 wherein the means for generating a glow discharge includes substantially parallel electrodes within the chamber.

5. The apparatus of claim 4 wherein the chamber is electrically coupled to one of the electrodes.

6. Apparatus comprising:

an evacuable chamber having an input port and an output port;

a member located in said chamber having a support region adapted to support at least an object;

means for facilitating a radio frequency discharge within the chamber adjacent the support region of the member located therein; and

gas feed means in communication with the input port which includes shielding means disposed so as to substantially confine the radio frequency discharge to a portion of the chamber adjacent to the support region of the member.

7. A radial flow reactor comprising a chamber adapted to be evacuated, there being disposed within said chamber:

a first electrode;

a second electrode including a supporting region in spaced relation with said first electrode and adapted, in cooperation with said first electrode, to create a plasma discharge between said electrodes; and

gas feed means for flowing a gas suitable for use in a plasma discharge reaction process in radial directions across the supporting region of said second electrode; and

CHARACTERIZED IN THAT

the gas feed means includes shielding means disposed around portions of the second electrode for substantially confining the plasma flow discharge at the second electrode to the supporting region thereof. 8. A reactor as in claim 7 wherein said gas is introduced into said chamber adjacent to the side of said second electrode opposite to the side thereof facing said first electrode, said second electrode having an aperture through a central portion of said supporting region, and including exhausting means connected to said aperture and being effective for exhausting the space immediately surrounding said supporting region; and

CHARACTERIZED IN THAT

said shielding means defines a path for the flow of gas from where it is introduced into said chamber to immediately adjacent said supporting region, said shielding means shielding said path from said first electrode for preventing a plasma glow discharge along said path. 9. A reactor as in claim 8 further CHARACTERIZED BY heating elements included within the second electrode.