KR20090107017A - Deposition of amorphous silicon films by electron cyclotron resonance - Google Patents

Abstract

A method is described for forming a film of amorphous silicon (a-Si:H) on a substrate by deposition from a plasma. The substrate is placed in an enclosure, a film precursor gas is introduced into the enclosure, and unreacted and dissociated gas is extracted from the enclosure so as to provide a low pressure in the enclosure. Microwave energy is introduced into the gas within the enclosure to produce a plasma therein by distributed electron cyclotron resonance (DECR) and cause material to be deposited from the plasma on the substrate. The substrate is held during deposition at a temperature in the range 200-600°C, preferably 225-350°C and a bias voltage is applied to the substrate at a level to give rise to a sheath potential in the range-30 to-105V, preferably using a source of RF power in the range of 50-250 mW/cm2 of the area of the substrate holder.

Description

Deposition of amorphous silicon films by electron cyclotron resonance

The present invention relates to a method of forming a film of amorphous silicon hydride (a-Si: H), hereinafter simply referred to as "amorphous silicon" by plasma deposition on a working surface. Typically, amorphous silicon hydride contains 3-20% of hydrogen, the presence of which serves to passivate the dangling bonds that are defective. More specifically, the present invention relates to silanes such as, for example, SiH ₄ , Si ₂ H ₆ or higher order oligomers in a process known as plasma-enhanced CVD. It involves the use of microwave energy to produce a plasma that uses electron cyclotron resonance to resolve. Other precursor gases that can be used to deposit amorphous silicon include molecules in which silicon is present in combination with one or more carbon, oxygen, or nitrogen, optionally hydrogen.

One particular area of concern with the use of amorphous silicon films is devices that convert solar energy into electrical power. Such amorphous silicon materials can also be used in electronic applications such as TFTs for displays.

In the technical field of exciting plasma with electron cyclotron resonance (hereinafter abbreviated as " ECR "), a rotational frequency of electrons in a static or quasi-static magnetic field is applied. Resonance is obtained when it is equal to the frequency of the accelerating electric field. This resonance can be obtained with respect to the magnetic field B at the excitation frequency f, and the relationship between the frequency f and the magnetic field B is as follows.

Where m is the mass and e is the charge of the electron.

Once the plasma is excited at the electron cyclotron resonance frequency, the electrons rotate in phase with the electric field when the ECR condition of Equation 1 is met to reach the critical energy required to dissociate or ionize the gas, Energy is continuously obtained from an external excitation source. In order to satisfy this condition, first, the electron is trapped in the magnetic field lines, i.e. its radius of gyration is sufficiently small with respect to the static magnetic field gradient with respect to the electron to a substantially uniform magnetic field during rotation. Secondly, the rotational frequency is relatively large with respect to the collision frequency between electrons and neutral elements such as atoms and / or molecules. In other words, the best conditions for exciting a plasma with electron cyclotron resonance can be expected to be obtained when the gas pressure is relatively low and at the same time the excitation frequency f is high, which also results in a high magnetic field strength B. Means.

The main problem with conventional divergent ECRs is that it is impossible to form a plasma of substantially uniform density over a large area. This means, for example, that it cannot be used to deposit a substantially uniform layer of material on a large size working surface. To overcome this problem, a technique known as distributed electron cyclotron resonance (DECR) has been developed, which includes devices that collectively generate a plasma having a substantially uniform density on a work surface, which is a plurality of Plasma excitation devices include devices in a network. Each of the plasma excitation devices consists of a wire applicator of microwave energy, one end of which is connected to a source for forming microwave energy, the other end of which is uniform and generates a magnetic field of intensity corresponding to the electron cyclotron resonance. The branches mate with at least one magnetic dipole to produce at least one surface. The dipole is installed at one end of the microwave application section to ensure electrons accelerated by electron cyclotron resonance oscillating between poles, and thus plasma diffusion located on the side of the dipole spaced from one end of the application section. Create a zone. Each of the excitation devices are distributed relative to each other, located adjacent to the work surface, and together form a uniform plasma with respect to the work surface.

Such a DECR device is disclosed in US Pat. No. 6,407,359 (corresponding to EP 1075168), a more detailed description of which is given below with reference to the drawings. As is evident from the figure, when viewed from a substrate, the excitation devices typically have the form of a rectangular array, and also include the particular case where the rectangle is square, whereby such devices often require matrix distributed electron cyclotron resonance, MDECR) device. However, the invention also shows that the excitation devices are arranged as non-rectangular two-dimensional networks, for example hexagonal networks, or the excitation devices are arranged in two parallel lines and the devices in one line are offset relative to one another. It can be appreciated that the present invention can be applied to a DECR device. One example of a hexagonal array is "Determination of the EEDF by Langmuir probe diagnostic in a plasma excited at T. Lagarde, Y. Arnal, A. Lacoste and J. Pelletier. ECR above a multipolar magnetic field "(Plasma Sources Sci. Technol. 10, 181-190, 2001). Such devices may also be located as circular arrays, semicircular arrays or near-array arrays. In some studies performed by the inventors, it should be noted that the depositions were performed by a central plasma excitation device surrounded by three or six devices, with the surrounding devices being located opposite to the magnet of the central device. With polarities, they are arranged in a triangular array or a hexagonal array, respectively. In addition, the present invention can be applied to DECR device devices that are not MDECR type. Thus, for example, it can be historically applied to a prior DECR reactor as compared to MDECR, which has a cylindrical shape and uses elongated antennas and magnets extending from the top to the bottom of the cylinder. This arrangement is described in Michel Moisan and Jacques Pelletier's "Microwave Excited Plasmas" (Elsevier, 1992), and is a cylinder-like substrate such as a tube or a plasma bipolar mean free path. It is suitable for uniformly coating the object specified by the small dimension (length, radius) in comparison to (see appendix 9.1 on page 269-271 of the above reference). This object may have a flat surface located in the center of the plasma and oriented perpendicular to the axis of the cylinder.

Although DECR technology offers the possibility of depositing materials at high deposition rates, it has been found difficult to obtain high quality materials when depositing in this way, particularly with solar cells. We have found that the desired combination of specific properties can be achieved through precise control of the two factors, the ion energy and the temperature of the substrate. Properties found to be particularly important in relation to these two factors include bandgap (preferably low bandgap), density (preferably high density), disorder parameter (preferably low level), and Surface roughness (a low level of roughness is preferred). The first three of these properties are in turn related to the hydrogen content, and the preferred range of hydrogen content (3 to 20%) contributes to the achievement of the desired values of these properties. Appropriate combinations of ion energy and substrate temperature have been found to allow the growth of layers with high optical and electrical qualities, which can be used in thin film solar cells because they optimize the capture of sunlight and promote the transfer of sufficient charge. have.

According to the present invention, there is provided a method of forming an amorphous silicon (a-Si: H) film on a substrate by vapor deposition, the method comprising placing a substrate in an enclosure, the film precursor at a flow rate within the enclosure. Drawing a gas (film precursor gas), extracting unreacted and decomposed gas from the enclosure to provide a low pressure in the enclosure, generating a plasma using a distributed electron cyclotron resonance, and Applying microwave energy to a gas in the enclosure such that material is deposited on the substrate, the substrate being maintained at a temperature in the range of 200 to 600 ° C. during deposition, and having a sheath in the range of -30 to -150V. A bias voltage is applied to the substrate to increase to the level of sheath potential.

Preferably, the sheath potential is in the range of -35 to -85V. Preferably, the temperature is in the range of 225 to 350 ° C.

Reference is made here to the "hot electron confinement envelope." The definition of "hot electron confinement envelope" first requires the definition of "hot electron component zones." Fast) are the regions in which the main electrons are confined, which are regions in which the electrons vibrate between two adjacent magnet poles of opposite polarity. magnet poles), or poles of two adjacent magnets (hereinafter referred to as “inter-magnet poles”), in which the region satisfies the adiabatic approximation condition (with respect to magnetic field gradient) Lamor radius is small), electrons get energy across regions that satisfy the ECR coupling condition.

The magnets and the hot electron confinement zones define a hot electron confinement envelope. This is the volume of the envelope of the array of magnets, extending parallel to the magnetic axes of the magnets in both directions at a distance that the inter-magnet zones (if present) extend beyond the ends of the magnets, and the zones within the magnets It extends perpendicularly to the magnetic axes of the magnets in all directions at a distance extending beyond the outward facing surfaces.

The invention is further described in accordance with the accompanying drawings.

1 is a front view schematically showing a plasma manufacturing apparatus used to carry out the present invention.

2 is a plan view of an example of the type of apparatus of FIG. 1.

3 shows the charge distribution and the resulting voltage in the region extending from the plasma main body to the wall in contact with the plasma.

4 schematically shows the configuration of the ion energy E _ion .

5 is a graph showing the linear relationship between the induced DC bias component of the substrate voltage and the RF power applied to the substrate.

6A to 6D are graphs showing the relationship between roughness, disorder parameter, ε _i (max), and L _d with respect to the sheath potential when silane is used as the film precursor gas.

7A-7C are graphs corresponding to FIGS. 6A-6C, where disilane is used as the film precursor gas.

8A-8C are graphs corresponding to FIGS. 6A-6C, where microwave energy is pulsed at high frequency (10 kHz) and three different energy levels (500 W, 1000 W, and 250 W).

9A-9D are graphs corresponding to FIGS. 6A-6D, where microwave energy is pulsed at lower frequencies.

10A-10E are graphs showing the effect of substrate temperature on roughness, disordered parameter, ε _i (max), band gap and diffusion distance L _d .

1 and 2 show an apparatus for generating a plasma over a substrate on which a film is deposited. The apparatus, as schematically shown, is sealed to match devices (not shown in FIG. 1) that draw in gas and pump out the unreacted and separated gas (via exit 11). The enclosure comprises an enclosure (1), wherein the devices depend on, for example, approximately 10 ⁻² to 2 × 10 ⁻¹ Pascals depending on the nature of the gas and the excitation frequency. Keep it at the same desired value. However, gas pressures of 10 ⁻² Pa or less (eg, lowered to 10 ⁻⁴ Pa), or higher than 2 × 10 ⁻¹ Pa (higher to 5 × 10 ⁻¹ or even higher than 1 Pa or higher) Can be used. For example, pumping can be performed by a 1600 l / s Alcatel Turbo-molecular pump, which extracts gas from the enclosure.

By control of a mass flow controller (MFC), gas is drawn into the enclosure from a suitable gas source, for example a pressurized gas cylinder. The gas may include, for example, SiH ₄ as a film precursor gas, or may include one of the other gases described above in connection with the deposition of amorphous silicon. In addition to the film precursor, an inert diluent gas such as helium (He), neon (Ne) or argon (Ar), a reactive gas such as hydrogen, nitrogen or oxygen, or diborane, trimethyl boron Or dopant gases such as phosphine can also be introduced. Typically, other gases are introduced into the enclosure as membrane precursor gases through the same port or ports, and the gases are mixed with each other, but can be drawn separately. The gas supply system should ensure proper flow of gases, typically in the range of 1 to 1000 sccm (standard cubic centimetre per minute) into the reactor.

The injection port for the gas generally consists of a single tube or a plurality of tubes 2 and is led into the deposition chamber. In order to ensure a more uniform gas distribution in the deposition chamber, the tubes, or in more than one case each tube, can be extended by a grid. Injection may be performed at any location within the reactor, but the film precursor gas is preferably directed towards the substrate surface.

As used herein, reference to the outlet arranged toward the gas towards the substrate surface refers not only to the case where the gas is directed directly to the substrate surface on which a film is deposited therefrom, but also from the outlet And all cases where the substrate surface is entirely located within an angle defined between a line extending from the outlet in the direction of gas flow therefrom and a line perpendicular to and passing through the outlet. Under these circumstances, the gas flow generated from the outlet will have a vector component directed to all parts of the surface.

One method of performing this injection, referred to as a "point" injection, is shown schematically in FIG. 1. In this arrangement, the film precursor is introduced through a pipe, or a plurality of pipes (two shown), each of which is located between a hot electron confinement envelope (shown in dotted line 4) and the substrate surface. It has an outlet 3, facing the surface.

The relative advantages of such implantation arrangements and other implantation arrangements are filed on the same day as the present application and pending applications together, "Method and apparatus for forming a film by deposition from a plasma" and "Device for forming a film by deposition from a plasma "(our references G28331EP and G28332 EP). Also, the properties of the films formed in the application "Method for forming a film of an amorphous material by deposition from a plasma" filed on the same date as the present application and pending application procedure (our reference G27557EP). Note that there is a discussion of the influence of the flow rate and residence tie of the membrane precursor gas on the bed.

The plasma chamber is equipped with a heated substrate support 5. As shown, at least one substrate 6, and optionally multiple substrates, is mounted on the support 6. The relationship between the substrate support and the substrate is further described below.

One of the functions of the substrate support is to heat the substrates to the required deposition temperature. It is usually between room temperature and 600 ° C., when depositing amorphous silicon, it is preferred to exceed 200 ° C., more preferably between 225 ° C. and 350 ° C. This can be obtained by circulating a high temperature fluid inside the substrate support, and can also be obtained by electrically heating a resistor inserted inside the substrate support. However, optionally, the substrate may be directly heated, for example using infrared lamps. The importance of heating to an appropriate temperature is discussed further below.

Another function of the substrate support is to allow polarization on the surface of the substrate to control the energy of ions towards the substrate. Polarization can be implemented using a source of RF voltage or using a DC voltage, requiring the substrate support to be electrically isolated from ground. In the case of RF polarization, polarization can be achieved by connecting the electrically insulated substrate support to an appropriate RF or DC generator 16 with a suitable matching circuit. In the case of depositing on an insulating substrate or on an insulating layer first deposited on a substrate (which may or may not be an insulator), the use of an RF generator is preferred, which is indicated by reference numeral 7 in FIG. It is shown on the marked RF generator. In the case of depositing on a conductive substrate or on a conductive layer first deposited on a conductive or non-conductive substrate, a bias is generated by an RF generator or a DC generator having an appropriate electrical connection to the substrate surface. ) May be applied. In a particular embodiment, the RF-bias may be applied using a 13.56 MHz dresser generator connected to the substrate support via an automatic tuning box. Even when using an RF generator, the resulting bias on the substrate surface includes a DC bias component as a result of the conditions in the plasma. A description of this is based on the description of a completely different plasma process, by Suzuki et al., "Radio-frequency biased microwave plasma etching technique: A method to increase SiO ₂ etch rate", (J. Vac. Sci Technol.B 3 (4), 1025-1033, Jul / Aug 1985). The importance of using an appropriate level of bias is discussed further below.

The plasma fabrication apparatus comprises a series of individual plasma excitation devices 8, which are spaced apart from each other and work together to produce a uniform plasma with respect to the substrates. Each of the individual plasma excitation devices includes an extended microwave energy application 9. Each of the energy applying units 9 is connected with a microwave energy source 10, one of which ends is located outside of the enclosure 1. However, optionally a single microwave energy source may supply microwaves to all of the applicators, or there may be a small number of multiple energy sources relative to the number of applicators. For example, an array of 16 applicators can typically be supplied by two 2.45 GHz microwave generators, each of which has a maximum output of 2 kW and a power splitter and each slug Each of the eight grants is supplied through slug tuners. In order to ensure that the microwave energy is properly converted to plasma, each application is preferably installed with a matching device to minimize or at least reduce the reflected output.

Each microwave application has a free end connected with at least one permanent magnet 12. Each magnet has (preferably) a magnetic axis parallel to the long axis of the magnet itself. In a particular form of this arrangement, the plasma excitation devices all have magnets oriented in the same direction (unipolar configuration), ie all their north poles are on the top and all the south poles are on the bottom, or vice versa. In other cases, some of each pole is on the top and the other is on the bottom (multipole configuration). An example of the latter is an array, as seen at one end of FIG. 2, along any given column or row of devices, one intersecting successively with poles of alternating polarity. In other embodiments, all magnets in a given column (or row) have the same polarity and the rows (or columns) have alternating polarities. However, arrangements can be used such that the magnetic axes of the magnets are not parallel to the long axis of the magnets themselves, providing important areas where the lines of the magnetic field are parallel to the propagation vector of the microwaves. This is necessary to ensure the existence of critical areas where ECR damping can occur.

Discussion of the importance of ion energy and substrate temperature in providing certain desirable properties as described above and how to properly control ion energy and substrate temperature is provided.

Ion energy

Once the plasma is formed, the plasma is electrically neutral in bulk, meaning that the number of electrons is equal to the number of positive charges. However, at the boundaries, due to the mobility of the electrons being very high compared to the mobility of heavy ions, electrons are collected by the walls, so plasma electro-neutrality is the boundary. It does not exist in the immediate vicinity of the field. The region in which the electron-neutral is not present is called a sheath. This is illustrated in FIG. 3.

The plasma is thus connected to the wall through a sheath and the voltage drop balances the ion and electron currents by attracting cations and reflecting negative charges (electrons) (quasi neutrality is lost). The number of electrons needs to be equal to the number of ions lost). In order to limit fast electrons, the plasma potential is positive with respect to the walls. This means that cations are extracted and accelerated to the walls by the sheath, while slow electrons and anions are repelled in the plasma by the sheath.

Like the final loss energy or the final gain energy in the collision made in the sheath, the energy obtained by the ions accelerated in the sheath is a function of the potential drop (sheath potential) between the plasma and the substrate.

Except for RF plasmas, the thickness of the sheath is on the order of several Debye lengths. The latter is the characteristic length scale of the plasma associated with the electron movement. It can be calculated from the electron temperature and the plasma density, which is as follows.

Therefore, the sheath thickness is as follows.

a is in the range of 2 to 5.

_D는 90μm의 차수이고 따라서 시스 두께는, 180 내지 450μm 범위이고, 이는 매우 작다.Taking into account the characteristics of the DECR plasma used for amorphous silicon deposition, the electron temperature and charge density at the location of the substrate support are measured as kT _e = 1.5 eV and n _e = 10 ¹⁶ m ^-3 or 10 ¹⁰ cm ^-3 . . therefore,

_D is on the order of 90 μm and therefore the sheath thickness is in the range of 180 to 450 μm, which is very small.

The DECR plasma also operates at very low pressures; Typically in the range of 1 to 10 mTorr (about 0.133 to 1.33 Pa). Under these conditions the mean free path of the species (the average distance the particles can travel before colliding with other particles) is on the order of 50,000 to 5,000 μm, which is much larger than the sheath thickness. This means that the ions entering the sheath will be guided perpendicularly to the surface by the electric field, and most of the time will always reach the surface without colliding with other ions.

The combination of high density plasma and low operating pressure allows for efficient non-collision zones of the sheath. Thus, in addition to having an initial energy (which tends to be low due to the typical distance [to 10 cm] between the plasma source and the substrate), the ions are accelerated by the electric field in the sheath and are directly at the sheath potential. Reach the surface with an energy proportional to

If any polarization is applied to the substrate (corresponding to the "wall" in the context of the discussion above) it will be added to the plasma potential, thereby adjusting the ion energy by this bias potential.

E _ion = e (V _p -V _dc ) = eV _sheath

in this case,

V _p : potential difference between the plasma and ground given as reference

V _dc : DC bias according to the RF polarization

The situation is briefly summarized in FIG.

Sheath  electric potential

As mentioned above, the sheath potential that determines the acceleration of the ions before impacting the substrate is an important value to be determined.

In the absence of any external polarization, the substrate is automatically polarized by the asymmetrical collection of charges. This defines V _us , referred to as the unbiased sheath potential, in other words, the potential difference (at the substrate support) between the plasma bulk and the plasma boundary.

V _us = V _p -V _f

in this case,

V _f : potential difference between the “floating” substrate (ie not grounded) and ground given as a reference. The floating potential is defined as the bias voltage, and the probe (substrate) has no forward current and is determined by the balance of electrons and ion currents for the probe.

Since the power required to bias the substrate may be higher than expected due to the initial presence of excess charges, such excess charges should be taken into account when biasing the substrate externally.

As mentioned above, the biasing is typically implemented by applying a 13.56Mhz RF voltage on the substrate, which allows for polarization of both conductive and dielectric materials. Due to the I-V response of the diode type of the substrate in the plasma, a DC voltage is built up at the surface, as described in the above-mentioned Suzuki et al. This voltage is added to the plasma potential to represent the total potential difference across the sheath and the ions can accelerate.

To determine the floating potential, experiments were performed by applying a varying RF bias to the substrate support and measuring both the resulting DC polarization (V _dc ) and RF power required to reach equilibrium. This may also allow to determine the ion current at the substrate support.

Other experiments were conducted under similar conditions, which are described in the table below. In all cases, each antenna is supplied with the same MW power but only 4 or 16 antennas are used, with little effect on the ion current (see below).

The results are shown in FIG. 5, indicating that there is a linear relationship between the induced DC bias voltage and the injected RF power. For completeness, Table 1 above provides a substantial amount of information about the conditions under which the deposition was performed, and the meanings of the various columns of Table 1 are briefly summarized below. However, a problem in the sense of this part of the discussion is that the linear relationship exists under various conditions.

In Table 1:

"Temp" represents the nominal temperature of the substrate, for reasons described below, except for sample d040206, this is not the actual temperature.

The carrier plate indicates whether the substrate is located on the carrier plate, which is located on the heated substrate support or directly on the substrate support.

"Yes" of the "silver" glue means that the sample is adhered to the substrate support, which is further discussed with respect to the temperature control.

The "antenna" column indicates the number of antennas present, and in the case of a four antenna arrangement, the antennas are located in the central region of the reactor as viewed from above.

"Distance" means the vertical distance between the substrate and the bottom of the magnets of the antennas.

"Injection" means the type of infusion. The term "point injection" is described above. In "tube" injection, the precursor gas is injected in a direction from the position just above or within the hot electron confinement envelope towards the substrate.

The second column after Table 1 shows the rate at which the precursor gas is introduced (sccm, standard cubic cm per minute).

The last column of Table 1 shows the power of the pulsed microwave source in kW.

The linear relationship described above indicates that the ion current does not change significantly even when the negative RF bias increases, and the DC bias is directly proportional to the applied RF-power. The current is due to charge collection on the substrate support as well as on the substrate. As is already known, the more negative the bias is on flat substrates, the faster the ion current reaches a constant value very quickly. The change in sheath thickness, known that the charge collection is constant and changes as the square root of the applied voltage, affects charge collection on a flat substrate (except for substrates where edge effects are important, ie small surface area). Does not have

Floating  electric potential

Some RF power needs to keep the substrate at OV relative to ground. This is due to the fact that the charge potential is apparently positive with respect to ground, and charge compensation is needed as soon as possible when the bias potential becomes more negative than the floating potential.

It has been found that the power delivered by the RF generator can be approximated as shown below as soon as it is connected to the substrate support.

P _RF = I * V _sheath = I * (V _dc -V _f )

P _RF = applied RF power

I = ion current (when V _dc is more negative than V _us )

V _dc = induced DC bias voltage to ground

V _f = Floating Potential to Ground

This is rather an oversimplification in that it does not take into account the power lost in the decoupling capacitor, so that the actual RF-power applied to the substrate support is not known exactly. Furthermore, despite the linear relationship, in particular close to the floating potential, the RF-current may not always be directly directly proportional to the DC-bias.

Thus the floating potential is calculated from the slope and the intercept of the P _RF / V _dc line, while the ion current is determined from the slope of the line. The results are shown in the table below.

The floating potential is likely to change significantly even with similar deposition conditions. Looking at the available data, V _f is in the range of + 8V to + 29V when depositions are performed with 16 antennas, but is in the range of 0V to + 6V when generated with four antennas. In both cases, the change is very large and even the difference between the two measurements d240605 and d240605a made on the same day amounts to 6V. Attempts have been made to find correlations to the process parameters, but no trends were found from the data, except that using 16 antennas showed a larger V _f value.

The value of the plasma potential was determined for the floating potential in the hydrogen plasma using a device similar to a Lanmuir probe. Since the silicon film is deposited onto the probe, it is difficult to perform this measurement in the silane plasma. From the table below, the plasma potential is about 5V higher than the floating potential, which allows recalculation of the sheath potential under biased conditions and silane deposition.

When comparing the deposition conditions, a careful consideration is the actual potential difference experienced by the ions when entering the sheath. When an external bias is applied to the substrate support, the ion acceleration potential will be the sum of the external bias voltage and the plasma potential. The plasma potential is determined from the floating potential plus 5V as defined above. Given the values found in the other deposition conditions and the plasma potential must be taken into account to determine the sheath potential, a DC bias of -30V will not necessarily provide an acceleration voltage of 30V (see Table 1). . For example, for the d070705 sample (line 2 of Table 2), the acceleration voltage would be 30 + 29 + 5 = 64 V. Thus the ion acceleration can be much larger than expected based on the value of the bias voltage. This is an important effect to be taken into account when controlling the deposition on thin films.

Influence of ion energy

The effect of ion energy on the quality of amorphous silicon layers deposited by DECR PECVD has been studied under various process conditions, in particular silane (silane or disilane) precursor gas and (in continuous or high frequency or low frequency mode). The method of supplying the MW (microwave) to the reactor was studied.

Each dataset will be described in detail.

Silane  - CW ( continuous wave , Continuous wave) DECR mode

In order to determine the effect of the ion energy on the quality of the deposited amorphous silicon layer, the results of depositing ten series of films have been calculated taking into account the plasma potential as previously defined, the sheath. Correlated with potential. The ten types are different in view of the diversity of the other elements shown in Table 1, but within each type these elements remain constant and the bias potential is varied.

E _ion = e (V _p -V _dc ) = eV _sheath

The optical properties of the films are determined from spectroscopic ellipsometry data by the Tauc-Lorentz model, and the charge transfer properties are SSPG (steady state photocarrier grating). Whereas the results are shown in FIGS. 6A, 6B and 6C for surface roughness, disorder parameter C and ε _i (max), respectively. A description of the methods for determining them is described, for example, in "Structure and hydrogen content of polymorphous silicon thin" by A. Fontcuberta i Morral, P. Roca i Cabarrocas, C. Clerc. films studied by spectroscopic ellipsometry and nuclear measurements "(PHYSICAL REVIEW B 69, 125307 / 1-10, 2004).

As can be seen from the graphs, as the sheath potential becomes more negative, both the surface roughness and the disordered parameter are improved, reaching an asymptotic value of acceleration voltage greater than 50 / 60V. The fact that the cis potential must be negative in order to implement an appropriate improvement indicates that the species involved in this densification process are cations.

The material density indicated by the maximum value of the imaginary part of the refractive index, ε _i (max) reaches around its maximum value of −35 V, and then slowly decreases as ion collisions become more active. This shows that damage can occur if the ion energy becomes too large and the operating range of the substrate polarization should be defined.

As shown in FIG. 6D, as the collision ion energy is increased, the charge transfer characteristics have also improved, and a sheath potential of at least −40 V is required to obtain a hole diffusion length L _d exceeding 100 nm.

The data association with the sheath potential is quite noteworthy, indicating that the ion acceleration voltage plays an important role in the DECR deposition process.

Based on the roughness, the disorder parameter, and the data of ε _i (max), a useful “full” bias (cis potential) ranges from -30 to -105V. Based on the L _d data for the 16 antennas mode, this “full” bias range produces a material of L _d greater than 100 nm. At least under the specific conditions used above, L _d in four antenna modes is known to be less. However, even in this mode, the same range of cis potentials is generally considered to be desirable.

The above-described correlations for the optical and charge transfer properties of amorphous silicon layers deposited by DECR with the cis potential are also obtained with the RF power. This is expected as discussed above, which shows that it takes into account the auto-polarization of the surface under conditions where the applied RF power is not biased to induce DC polarization of the substrate surface. The RF power is proportional to the bias voltage measured with respect to ground and the potential drop between the plasma and the substrate.

A useful sheath potential range of -30 to -105V corresponds to an RF power range of 25 to 120W. As explained below, this relationship is maintained only at a given area of the substrate support. In this case it was 484 cm ² . In the "Conclusion" section, a description is provided on how to standardize RF power requirements to be applied to which substrate support area. This correlation of sheath potential and RF power was also obtained using a microwave power of 2 kW, a silane flow rate of 100 sccm, and a 10 cm gap between the substrate and the bottom of the magnet. The RF power used will have to be adjusted in response to other process conditions.

Disilane  - CW DECR mode

When using disilane as the precursor gas, the effect of the sheath potential on the optical and electrical properties of the amorphous silicon layer was determined. The use of this gas allowed deposition of the a-Si: H layer at about twice the rate possible with silane, and rates of greater than 50 dl / s were measured.

The observed surface roughness, disordered parameters, and the tendency of ε _i (max) when using disilane are shown in FIGS. 7A-7C, and are the same as the tendencies seen when using silane as the film precursor gas.

The optimal cis potential range (-30 to -105 V) of the silanes defined above seems to be suitable even for disilane. Thus, when using silane or disilane, no significant difference is seen, except that the latter deposition rate is twice that of the former.

Silane  High frequency pulse DECR mode

When using silane as precursor gas in combination with pulsed MW conditions, the effect of the sheath potential on the optical and electrical properties of the amorphous silicon layer was also determined. The reason for wanting to pulse the MW energy is the applicant, "Method for forming a film with a graded bandgap by deposition of an amorphous material from a plasma using distributed electron cyclotron resonance" filed on the same date as the present application. Reference number G28555EP), which is discussed in the section on preparing a gradient bandgap structure.

This MW application mode is evaluated at high and low frequencies. In addition to the evaluation of the frequency range, this allowed for an assessment of the collision when synchronizing the RF bias with the MW pulse.

Pulsing the MW at high frequencies (10 kHz in these experiments) is because the response time of the substrate support setup is too slow and the polarization between the two MW pulses is not fully relaxed. It does not allow the effect of RF-bias synchronization. This is not the case for low frequencies, which will be explained in the next section.

The results for the high frequency pulse mode are shown in Figs. 8A, 8B and 8B. From these, the trends seen in the CW mode are still observed even when pulsing the MW at high frequencies, despite the fact that the plasma composition is somewhat different. It also underlines the importance of ion bombardment to deposit high quality layers.

The sheath potential range (-30 to -105 V) determined to be optimized for silane in CW mode to achieve low surface roughness, low disorder parameters and high density seems to be suitable in the pulsed mode as well. Interestingly, it was found that for silane in pulsed MW mode, the bandgap can also be improved (ie, reduced), which is an effect not observed in CW mode.

Silane  Low frequency pulse DECR mode

When using silane as the precursor gas in combination with low frequency pulsed MW conditions, the effect of the sheath potential on the optical and electrical properties of the amorphous silicon layer was also determined.

Contrary to the high frequency case, the pulsed MW signal and the RF-bias signal can be synchronized so that conditions for the case where no bias is applied when the MW pulsed signals are in the OFF mode can be obtained. have. The data obtained is shown in FIGS. 9A-9D.

From the data, correlation with the RF-power was established, and experiments to determine the sheath potential were not performed. However, because of the relationship between the RF-power and the sheath potential, the correlation with the RF-power is equal to the correlation with the sheath potential. For the low frequency pulse mode, the range of optimal RF-power may be shifted below the RF-power range 35 to 120W determined to be optimal for the CW and high frequency modes.

Another important point should be emphasized. 9A, 9B, and 9C, the RF-bias is determined for the maximum value of the imaginary part of the surface roughness, the disorder parameter and the refractive index, ε _i (max), _i. It is highly desirable to synchronize with low frequency MW pulses. As discussed the importance of ion bombardment on the growing film to improve the quality of the growing film, synchronizing the RF-bias with the MW pulse will optimize the use of the ions. If the ions are not actually produced there is no need to polarize the substrate. On the other hand, the generation of ions will increase further at the beginning of the MW pulse, and synchronization will ensure optimal use of the ions.

Conclusions Regarding Ion Energy

Based on the data, like most of the charge transfer properties, between most of the optical properties of the material (determined by ellipsometry) and the sheath potential (or the applied RF power) in the substrate There are very good relationships. This indicates the importance of the ion bombardment for the deposition of high quality layers.

From all data sets (silane, disilane, CW-MW mode and pulsed MW mode), and combinations thereof, the optimal ranges are as follows:

RF power: 25-120 W

Sheath potential: -30 to -105 V

This should be corrected to the size of the substrate support having ions collected and having an area of 484 cm ² in the experiment. This correction assumes that the substrate support is greater than or equal in size to the substrate. This would be a common case, but if the substrate support is smaller than the substrate, the relative area for correction will be that of the substrate. In either case, what is to be done is standardization of the entire ion collection surface. The power density for optimal deposition conditions is therefore:

RF power: 50 to 250 mW / cm ²

Sheath potential: -30 to -105 V

Influence of temperature

Although it is known from the prior art that a substrate on which an amorphous film is deposited on top by plasma deposition must be heated, the present inventors are more important than previously known, and the methods used to heat the substrate so far are In fact it was found that the substrate was not reliably heated to a given temperature.

The results of the study on the substrate directly bonded onto the substrate support using a thermally conductive adhesive member (Ag-filled adhesive member) without using an intermediate carrier plate are described. The adhesive member may comprise silicone resin (instead, other resins such as, for example, epoxy) may be used, solvents, and silver flakes. Pieces of sufficiently high concentration are used to ensure that the pieces are in contact with each other when the solvent evaporates, so that the resulting layers of the adhesive member are thermally and electrically conductive. Other adhesive members that can be used include resin filled with carbon black, and aluminum oxide paste. The latter, although not equipped with electrical conductivity, can be thermally conductive and are used in electronic applications. The use of Ag-filled adhesive members has been described as allowing for good control of the temperature of the substrate surface and good resulting substrate properties, as described in the table below. An optional possibility, although seen to be inferior, includes the carrier plate and adheres the substrate to the carrier plate using an adhesive member, as described above, between the substrate support and the carrier plate, for example To locate a thermally conductive layer, such as a carbon foil. Another possibility is to use what is referred to as "backside gas heating". This typically involves injecting helium gas between the substrate support and the carrier plate to have a high enough pressure inside this space to ensure good heat transfer.

As shown in the table above, most data points are the average of two measurements.

From the graphs shown in FIGS. 10A-10E, the effect of the temperature can be seen more clearly.

Thus, the influence of the maximum value of the imaginary part of the dielectric constant (see FIG. 10C) and the temperature of the material bandgap (see FIG. 10D) is very clear and essentially the same no matter what RF-power is applied. In both cases, increasing the temperature significantly improves the optical properties of the material.

The disorder parameter is also influenced by the temperature (see FIG. 10B), but in the graph, the very large influence of the bias can also be clearly seen, so that the medium range order of the atoms in the film matrix This characteristic, associated with a range order, indicates that it is mostly affected by the energy of the ions impinging on the membrane surface, rather than by temperature. This may be due to the fact that most of the ions in the SiH ₄ / DECR plasma are H _x ⁺ and do not supply much kinetic energy for rearrangement to the growing film. The ion contribution may be further associated with hydrogen recombination, diffusion or implantation into the membrane. The nature of the disorder parameter with respect to temperature and bias is very interesting in that the bias is the most important factor and the temperature loses its effect as the bias increases.

The surface roughness (see FIG. 10A) is also more affected by the ion energy than the temperature.

In the case of diffusion distance (see FIG. 10E), the contribution of temperature is very clear, although the ion bombardment energy also has a major influence. Referring to FIG. 10, it can be seen that the diffusion distance improves as the temperature increases, but the influence of the substrate bias should not be overlooked.

Conclusions Regarding Temperature

Based on the above data and considering the bias range in which these films have grown, it is clear that deposition of amorphous silicon should be performed at a temperature of at least 200 ° C. However, it is known that amorphous silicon can be thermally crystallized at temperatures in excess of 600 ° C. Therefore, this value should be considered as an absolute upper limit. However, in the manufacture of devices such as thin film solar cells, when the amorphous silicon is used as the intrinsic layer, a smaller upper limit must be set. This is because the devices also have a p-doped layer and an n-doped layer, where the n-doped layer is generally resistant, but the p-doped layer is particularly very temperature resistant. This is because it is sensitive and generally cannot withstand temperatures above 350 ° C. If a temperature higher than 350 ° C. is used in the manufacture of the devices, post-diffusing boron into the bottom layer, ie during or during the high temperature deposition of the intrinsic layer Thereafter, modifications of the process, such as to diffuse boron from the substrate to the bottom of the intrinsic layer, will be necessary (in this case using a material such as phosphorus instead of boron, after high temperature deposition of the intrinsic layer, Similar processes can be used to produce n-type layers).

Overall conclusions

From the above discussion, it can be seen that ion bombardment energy and substrate temperature have a major influence on the above properties of amorphous silicon hydride (a-Si: H) films, and that accurate selection of both is essential to grow high quality materials. there was.

For surface roughness and the disorder parameter, the effect of the ion energy is absolutely essential for the growth of high quality membranes. For other properties, especially for the material bandgap, the most important is temperature, and a good quality material can be grown without large bias. However, there are other properties, such as the maximum value of the imaginary part of the dielectric constant and the charge diffusion distance, which require that the material and the ion collision be included in a certain range in order to have a good quality. This means that these two deposition parameters cannot be considered separately from each other, both of which are essential for the growth of high quality materials.

Based on the generated data, DECR plasma deposition should be operated in the following range.

RF power: 50 to 250 mW / cm ²

Sheath potential: -30 to -105 V

Temperature: 200-600 ° C.

Under these conditions, high quality amorphous silicon hydride (a-Si: H) can be grown from Si-based film precursor gases at a very high rate (> 20 kW / s). This also applies to all modes of supplying MW to the reactor. In addition, in low frequency pulse mode, it has been found that synchronizing the MW pulse and the RF-bias is beneficial for the quality of the material.

Claims (8)

Positioning the substrate in an enclosure; Introducing a film precursor gas into the enclosure at a flow rate; Extracting unreacted and decomposed gas from the enclosure to provide a low pressure within the enclosure; Generating a plasma therein using a distributed electron cyclotron resonance (DECR) and applying microwave energy to the gas in the enclosure such that material is deposited from the plasma onto the substrate; Including, The substrate is maintained at a temperature in the range of 200 to 600 ° C. during deposition, A method of forming an amorphous silicon film on a substrate by plasma deposition, comprising applying a bias voltage to the substrate at a level that increases a sheath potential in the range of -30 to -150V. The method of claim 1, Wherein the sheath potential is in the range of -35 to -85V. 10. A method of forming an amorphous silicon (a-Si: H) film on a substrate by plasma deposition. The method according to claim 1 or 2, The substrate is attached to the substrate support using a conductive adhesive member, And supplying heat to the substrate support, wherein the amorphous silicon film is formed on the substrate by plasma deposition. The method of claim 3, wherein The substrate is non-conductive, And wherein said bias voltage is applied to said substrate by a source of RF power. The method of claim 4, wherein Wherein the RF power is in the range of 50 to 250 mW / cm ² in the region of the ion collection surface. The method according to any preceding claim, And said temperature is not as high as 350 [deg.] C. The method according to any preceding claim, Said temperature is at least 225 ° C. A method for forming an amorphous silicon film on a substrate by plasma deposition. The method according to any preceding claim, And wherein the film precursor gas is introduced into the enclosure in a direction towards the substrate.

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