# Patent

## Title
Method for preparing a non-self-aligned heterojunction bipolar transistor with a small emitter-to-base spacing

## Abstract

The present invention refers to a method for preparing a non-self-aligned heterojunction bipolar transistor comprising: preparing a patterned emitter metal on an emitter epi layer of a HBT epi structure on a substrate; preparing an emitter epitaxy below the emitter metal; applying a resist layer on the top surface covering the emitter metal and emitter epitaxy, and the base layer; applying lithography leaving the emitter epitaxy and the emitter metal covered by the resist vertically with a width pD and leaving a pattern according to the mask in the resist; depositing base metal on the entire surface; and removing the remaining resist and the base metal covering the resist defining a base metal, the base metal being spaced from the emitter epitaxy and the emitter metal by a distance xD from 0.05 μm to 0.7 μm. The present invention refers to a non-self-aligned heterojunction bipolar transistor as prepared by this method.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. application Ser. No. 10/966,219 filed Oct. 15, 2004, now U.S. Pat. No. 7,396,731.


STATEMENT OF GOVERNMENT INTEREST
This invention was made with government support under Contract No. F33615-02-C-1286 awarded by the Air Force Research Laboratory. The government has certain rights in this invention.


BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a non-self-aligned heterojunction bipolar transistor (HBT) and a method for preparing a non-self-aligned heterojunction bipolar transistor (HBT).
2. Description of Related Art
The use of self-aligned contacts in the HBT process is to minimize the base access resistance of the device. The use of this technique is known in that fabrication of early HBTs utilized contact lithography or early broadband wafer stepper systems for device fabrication. These systems are capable of limited layer-to-layer registration and therefore, the only technique available to minimize contact spacing was to employ the technique of self-aligned contacts. Large circuit demonstrating yield of known devices is described in T. P. Broekaert, W. Ng, J. F. Jensens, D, Yap, D. L. Persechini, S. Bourgholtaer, C. H. Fields, Y. K. Brown-Boegeman, B. Shi, and R. H. Walden, “InP-HBT Optoelectronic Integrated Circuits for Photonic Anaolog-to-digital Conversion”, IEEE Journal of Solid-State Circuits, Vol. 36, No. 9, September 2001, pp. 1335-1342, which is incorporated herein by reference.
While self-aligned HBT devices do demonstrate lower base resistance, this technique is prone to variations in device Beta due to lateral diffusion of minority carriers in the base and recombination at the base metal (BMET) contact which leads to variations in the base current of these devices. Since variations in base current lead to variations in Beta, the HBTs fabricated using self-aligned contacts demonstrate a large range of measured Beta.
A description of general HBT devices can be found in Jensen, J. F.; Stanchina, W. E.; Metzger, R. A.; Rensch, D. B.; Pierce, M. W.; Kargodorian, T. V.; Allen, Y. K., “AlInAs/GaInAs HBT IC technology”, Custom Integrated Circuits Conference, 1990, Proceedings of the IEEE 1990, 13-16 May 1990, Pages:18.2/1-18.2/4, which is incorporated herein by reference.
Another general description of a HBT device is disclosed by Hafizi, M.; Stanchina, W. E.; Sun, H.C., “Submicron fully self-aligned AlInAs/GaInAs HBTs for low-power applications”, Device Research Conference, 1995. Digest. 1995 53rd Annual, 19-21 Jun. 1995, Pages: 80-81, which is incorporated herein by reference.
Jensen, J. F.; Stanchina, W. E.; Metzger, R. A.; Rensch, D. B.; Allen, Y. K.; Pierce, M. W.; Kargodorian, T. V, “High speed dual modulus dividers using AlinAs-GainAs HBT IC technology”, Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, 1990. Technical Digest 1990, 12th Annual, 7-10 Oct. 1990, Pages 41-44 describe a general HBT device as well, which is incorporated herein by reference.
Base-emitter shorts are a leading limitation to HBT IC yield. Shorts due to metal spiting during the self-aligned base metal deposition occurs in about 1 in 600 transistors and limits current HBT IC yield to below 2000 transistors. The use of non-self-aligned emitter-base contacts eliminates the possibility of shorting of this type.
In HBTs designed for high-speed applications, current gain is not determined solely by recombination in the bulk of the base epitaxy layer. Both surface and bulk components of base current combine to decrease the overall device current gain. InP HBTs known in the state of the art employ self-aligned base-emitter structures to reduce the extrinsic base resistance and the base-collector junction capacitance. This alignment technique results in narrow contact spacing and potentially large values of surface recombination. Handbook of Heterojunction Bipolar Transistors, W. Liu, 1998 Wiley & Sons, FIGS. 3-54 & 3-55 shows that Beta is extremely sensitive to the spacing of the emitter and base contacts for values closer than about 1500 Å. In this range, minor variations in wet etch isotropy or in metal deposition source angle could lead to wildly varying values of Beta. In a self-aligned HBT, the emitter-base spacing can even approach zero.
The mechanism for reduction and variation in Beta is shown in FIG. 1. FIG. 1 shows that electrons, which are emitted by the emitter epi layer 2, migrate through the base layer 1 to the collector layer 5. A minority of electrons however laterally diffuses and does not reach the collector layer 5 and instead migrates to the base metal 4. Any electron that reaches the base metal 4 instead of collector layer 5 causes reduction and variation in device Beta.
FIG. 11 is a pictorial of a state-of-the-art SA transistor that is currently being fabricated according to the prior art. The BMET layer 4 is a large square which completely overlays the emitter metal (EMET) 3 pattern. The process of manufacturing the SA transistor is shown in FIG. 6 to FIG. 11. First, a patterned emitter metal layer 2 is formed to yield an emitter metal 3 as shown in FIG. 6. This process can be carried out in two different ways. According to one method (1) a photoresist 4 is applied on the emitter layer and (2) exposed through a mask and (3) the exposed photoresist 4 is removed leaving a space pattern in the photoresist 4 for the emitter metal 3 and (4) emitter metal 3 is deposited on the top surface of the wafer and (5) the photoresist 4 and the emitter metal covering the photoresist 4 is removed leaving the emitter metal 3 and the base layer 1. According to a second method (1) an emitter metal layer 3 is deposited on the emitter epi layer 2 (2) photoresist is applied on the emitter metal layer 3 and (3) exposed through a mask and (4) the exposed photoresist 4 is removed leaving a photoresist 4 on the emitter metal layer 3 for the emitter metal 3 and (4) emitter metal 3 which is not covered by the photoresist 4 is removed and (5) the photoresist 4 covering the emitter metal 3 is removed leaving the emitter metal 3 and the base layer 1.
In a further process step the emitter epi layer 2 is etched off to yield an emitter epitaxy 2 below and in line with emitter metal 3. FIG. 7 shows an emitter epitaxy 2 below and in line with the emitter metal 3, wherein the emitter epitaxy has an undercut Δ. The undercut Δ is obtained by two etch process steps, a wet etch and a dry etch. The undercut Δ is necessary for devices according to the prior art between the base metal 4 and the emitter epitaxy 2 as shown in FIGS. 10 and 11. The emitter epitaxy 2 and the emitter metal 3 form a pedestal. In a next step a photoresist layer 8 is applied on the base layer 1 covering the base layer 1 and emitter epitaxy 2 and emitter metal 3 as shown in FIG. 8.
The following process is carried out according to image reversal photolithography as follows. The photoresist layer 8 is irradiated by light 10 through a mask 9. The mask 9 covers the area of the emitter metal 3 in excess, which is not irradiated. After the irradiation the surface of the photoresist layer 8, which was irradiated through the mask 9 becomes insensitive for removing. The area of the photoresist 8, which was covered and therefore not irradiated is sensitive and can be removed as shown in FIG. 8. After removing the sensitive area of the photoresist 8 part of the base layer 1, the emitter epitaxy 2 and the emitter metal 3 are laid free and are not covered by the photoresist layer 8 as shown in FIG. 9. In the next step base metal 4 is applied on the entire top surface as shown in FIG. 10. In the next step the photoresist 8 is removed together with the base metal 4 on top of the photoresist 8 as shown in FIG. 11. The pedestal comprising emitter epitaxy 2 below and in line with emitter metal 3 is covered by the base metal 4. The base metal 4 has a certain distance to the emitter epitaxy 2 in order to avoid a shorting between the base metal 4 and the emitter epitaxy 2. Since the emitter epitaxy 2 has the undercut A a shorting between the base metal 4 and the emitter epitaxy 2 is avoided. However, due to the undercut A of the emitter epitaxy 2, the contact area between the emitter epitaxy 2 and the emitter metal 3 is reduced.
It is desirable to prepare a device which reduces shorting between the base metal 4 and the emitter epitaxy 2. There is a need for a device that solves the recurring problem of low and varying values of current gain (Beta) in current state-of-the-art HBT devices. There is further a need to solve the problem of variations in HBT Beta from wafer to wafer, variations in HBT Beta within a single wafer and from wafer to wafer. Base emitter shorts, which occur in about 1/600 transistors, lead poor line yield of HBT wafers that needs to be improved.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for preparing a non-self-aligned heterojunction bipolar transistor. The method comprises in uninterrupted sequence: preparing a patterned emitter metal on an emitter epi layer of a HBT epi structure on a substrate; preparing an emitter epitaxy disposed exclusively below the emitter metal; depositing a base metal layer on an exposed top surface; applying a resist on the base metal layer; applying a lithography mask covering a peripheral edge of the emitter epitaxy and the emitter metal with a width PD and leaving a pattern according to the lithography mask in the resist; removing portions of the base metal layer not covered by resist; and removing remaining resist, the base metal being spaced from the emitter epitaxy and the emitter metal by a distance XD, wherein the distance XD ranges from 0.05 μm to 0.7 μm.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIG. 1 is a cross sectional view depicting pictorially the recombination mechanism that leads to reductions and variation in device Beta.
FIG. 2 depicts the measured 2-terminal forward Gummel data for three self-aligned devices (SA InP HBTs) measured on the same wafer (current [A] versus Vbe [V]).
FIG. 3 depicts the measured 2-terminal forward Gummel data for three non-self-aligned devices (NSA InP HBTs) on the same wafer and fields (current [A] versus Vbe [V]).
FIG. 4 depicts measurements of the frequency performance of six self-aligned and non-self-aligned InP HBTs devices from the same wafer and fields (F, [GHz] versus Ic [A]) for several devices as shown in FIG. 2 and FIG. 3.
FIG. 5 depicts a cross-sectional view of the starting epitaxial layer structure for an HBT.
FIG. 6 depicts a cross-sectional view of the layer structure after depositing and patterning emitter metal.
FIG. 7 depicts a cross-sectional view of the layer structure after removing emitter epitaxy layer under the emitter metal showing an undercut A of the emitter epitaxy according to the prior art.
FIG. 7a depicts a cross-sectional view of the layer structure after removing emitter epitaxy layer under the emitter metal without the undercut of FIG. 7.
FIG. 8 depicts a cross-sectional view of the layer structure after applying a photoresist layer on the layer structure as shown in FIG. 7 and irradiating the photoresist layer through a mask designed for the process of image reversal photolithography according to the prior art.
FIG. 9 depicts a cross-sectional view of the layer structure after removing photoresist layer in the desired areas according to the prior art.
FIG. 10 depicts a cross-sectional view of the layer structure after base metal deposition on the wafer according to the prior art.
FIG. 11 depicts a cross-sectional view of the layer structure after removing photoresist layer and base metal layer on top of the photoresist layer according to the prior art.
FIG. 12 depicts a cross-sectional view of the layer structure after applying photoresist layer and irradiating the photoresist layer through a mask on the layer structure as shown in FIG. 7a, designed for the process of image reversal photolithography according to the first embodiment of the present invention.
FIG. 13 depicts a cross-sectional view of the layer structure after applying photoresist layer and irradiating the photoresist layer through a mask, on a layer structure as shown in FIG. 7a, designed for the process of positive photolithography according to the first embodiment of the present invention.
FIG. 14 depicts a cross-sectional view of the layer structure after removing non-irradiated photoresist layer in the process of image reversal photolithography as shown in FIG. 12 or after removing of irradiated photoresist layer in the process of positive photolithography as shown in FIG. 13.
FIG. 15 depicts a cross-sectional view of the layer structure after base metal deposition on the wafer.
FIG. 16 depicts a cross-sectional view at line 23-23 of the non-self-aligned heterojunction bipolar transistor of FIG. 23 after removing photoresist layer and base metal layer on top of the photoresist layer.
FIG. 17 depicts a cross-sectional view of the layer structure after applying base metal layer according to a second embodiment of the present invention on a layer structure as shown in FIG. 7a. 
FIG. 18 depicts a cross-sectional view of the layer structure after applying a photoresist layer on the base metal layer and irradiating the photoresist layer through a mask designed for image reversal photolithography according to the second embodiment of the present invention.
FIG. 19 depicts a cross-sectional view of the layer structure after applying a photoresist layer on the base metal layer and irradiating the photoresist layer through a mask designed for positive photolithography according to the second embodiment of the present invention.
FIG. 20 depicts a cross-sectional view of the layer structure after removing photoresist layer in the desired areas.
FIG. 21 depicts a cross-sectional view of the layer structure after removing base metal layer from the base layer, which is not covered by the photoresist layer.
FIG. 22 depicts a cross-sectional view at line 23-23 of the non-self-aligned heterojunction Bipolar Transistor of FIG. 23 after removing the remaining photoresist layer. FIG. 22 shows a cross-sectional view of a device, which is shown in FIG. 16. FIGS. 12-16 depict the process according to the first embodiment of the present invention and FIGS. 17-22 depict the process according the second embodiment of the present invention. FIG. 16 and FIG. 22 show the cross section of a non-self-aligned heterojunction bipolar transistor as shown in FIG. 23 and in FIG. 24.
FIG. 23 depicts a top view of the emitter and base mask layers of a generic non-self-aligned heterojunction bipolar transistor.
FIG. 24 depicts a Scanning electron micrograph of the NSA HBT depicted in FIG. 23 above.
FIG. 25 depicts plots of measurements (DC Beta versus Ic [A]) for several non-self-aligned InP HBTs devices with different emitter-to-base spacing xD.
FIG. 26 depicts plots of measurements (Ft [GHz] versus Ic [A]) for several non-self-aligned InP HBTs devices with different emitter-to-base spacing xD.
FIG. 27 depicts plots of measurements (Fmax [GHz] versus Ic [A]) for several non-self-aligned InP HBTs devices with different emitter-to-base spacing xD.
FIG. 28 depicts plots of measurements (normalized DC Beta versus emitter-base spacing xD) for several non-self-aligned InP HBTs devices (SHBT and DHBT) with different emitter-to-base spacing xD.
FIG. 29 depicts plots of measurements (normalized Frequency Ft versus emitter-base spacing xD) for several non-self-aligned InP HBTs devices (SHBT and DHBT) with different emitter-to-base spacing xD.
FIG. 30 depicts plots of measurements (normalized Frequency Fmax versus emitter-base spacing xD) for several non-self-aligned InP HBTs devices (SHBT and DHBT) with different emitter-to-base spacing xD.



REFERENCE NUMERALS


    
    
        1 base
        2 emitter layer
        3 emitter metal
        4 base metal
        5 collector
        6 subcollector
        7 substrate
        8 photoresist
        9 mask
        10 light
    
    


DETAILED DESCRIPTION OF THE INVENTION
The base metal 4 is placed according to the invention at least some multiple of a diffusion length (LD) away from the emitter 2 in order to avoid reduction and variation in device Beta. This is accomplished in self-aligned (SA) HBTs by a method of wet etching of the emitter pedestal (emitter metal 3 and emitter epitaxy 2) to move the emitter epi layer further under the EMET pattern 4. The present invention provides a method of lithography to control this emitter-to-base spacing and to minimize the diffusion of electrons.
Two-terminal forward Gummel characteristics for three self-aligned HBT devices were measured at three different sites across a three-inch InP wafer as shown in FIG. 2. The top three curves (I, II, III) plot the collector current and show the variation in device turn-on voltage across the wafer. The bottom three curves (IV, V, VI) are the measured base current of the devices, which shows the large variation in leakage current across the wafer for these self-aligned devices.
Two-terminal forward Gummel characteristics for three non-self-aligned (NSA) HBT devices were measured at three different sites across a three-inch InP wafer as shown in FIG. 3. The device measurements plotted here were taken from devices adjacent to the three from FIG. 2. The top three curves (I, II, III) plot the collector current and show that these devices demonstrate a great improvement in minimizing the variation in device turn-on voltage across the wafer. The bottom three curves (IV, V, VI) are the measured base current of the devices which shows significantly lower leakage current as well as reduced variation across the wafer for these non-self-aligned devices. The reduced leakage current demonstrated by these NSA devices leads to higher gain and usable gain at extremely low power levels.
The unity gain cutoff frequency (Ft) for the six devices from FIG. 2 and FIG. 3 were measured and are shown in FIG. 4. Extrapolation of ft was done from a single frequency point s-parameter measurement assuming an h21 versus frequency slope of −20 dB/decade. This plot shows that the NSA devices operated at higher frequencies, compared to the self-aligned devices, across the entire range of current or power levels.
The sole difference between SA and NSA transistors is a simple change to a single layer in the device fabrication process. The starting material is preferably a layer structure, which comprises a base 1. An emitter epi layer 2 is deposited or grown on the base 1. An emitter metal layer (EMET) 3 is deposited on the emitter epi layer 2. Underneath the base layer 1 a collector layer 5 and below the collector layer 5 a subcollector layer 6 is deposited on the substrate 7. The described layer structure beginning with substrate 7, and on the substrate preferably the following layers are deposited: subcollector 6, collector 5, base layer 1, emitter epi layer 2 and emitter metal layer 3 is a preferred starting material for manufacturing SA and NSA transistors. This structure is called the HBT epi layer structure.
Preparing of Self-Aligned HBT by the Additive Method
The starting layer structure as shown in FIG. 7a is used for the method according to this preferred embodiment. FIG. 7a does not show an undercut A of the emitter epitaxy 2. The process for yielding emitter epitaxy 2 involves only dry etch. The step of wet etch is omitted. The wet etch process leads to the undercut Δ of the emitter epitaxy 2 which is necessary according to the prior art to avoid shorting between the base metal 4 and the emitter epitaxy 2. The process of the present invention makes it possible to deposit the base metal 4 on the base 1 wherein a controlled distance between the base metal 4 and the emitter epitaxy 2 is reached to avoid shorting as shown in FIGS. 16 and 22.
A photoresist layer 8 is applied on the top surface layer. Two different methods can be preferably applied to irradiate and remove photoresist layer 8, image reversal photography and positive photolithography. Image reversal photography is shown in FIG. 12 and already explained in the above in regard to FIG. 8. The photoresist layer 8 is irradiated by light 10 through a mask 9. The mask 9 leaves the area of photoresist 8 covering the emitter epitaxy 2 and the emitter metal 3 plus the later emitter-to base spacing xD on the left and right sight open to be irradiated. After the irradiation the surface of the photoresist layer 8, which was irradiated through the mask 9 becomes insensitive for removing. The area of the photoresist 8, which was covered and therefore not irradiated is sensitive and can be removed as shown in FIG. 12 and FIG. 14.
The other method, positive photolithography is shown in FIG. 13. The photoresist layer 8 is irradiated by light 10 through a mask 9. The mask 9 covers the area of the emitter epitaxy 2 and the emitter metal 3 plus the later emitter-to base spacing xD on the left and right sight. After the irradiation, the surface of the photoresist layer 8, which was irradiated through the mask 9, becomes sensitive for removing. The area of the photoresist 8, which was covered and therefore not irradiated, stays insensitive and cannot be removed.
According to both methods the mask 9 is carefully designed so that after removing the non-irradiated photoresist 8 in the image reversal photography and after removing the irradiated photoresist 8 in the positive photolithography, parts of the base layer 1 are still covered by photoresist layer 8. Both methods, image reversal photolithography and positive photolithography yield a structure as shown in FIG. 14. The two methods use a positive or negative mask and require different photoresist material. Both methods, image reversal photolithography and positive photolithography are well known in the art. Both methods are described in Microlithography, Micromachining, and Microfabrication, Vol. 1: Microlithography, Editor P. Rai-Choudhury, 1997 SPIE Optical Engineering Press, Bellingham, Wash. and Semiconductor Lithography Principles, Practices, and Materials, Wayne M. Moreau, 1988 Plenum Press, New York, which are both incorporated herein by reference.
It is very important for the present invention that the emitter epitaxy 2 and the emitter metal 3 are not laid free and vertically and horizontally covered by the photoresist layer 8 as shown in FIG. 14. The width pD of the vertical coverage of the emitter metal 3 with photoresist 8 predetermines the later emitter-to-base spacing xD. The thickness of the vertical coverage of the emitter metal 3 predetermines the emitter-to-base spacing xD as shown in FIG. 16. In the next step the entire top surface becomes covered by base metal 4 as shown in FIG. 15. In the next step the photoresist 8 is removed together with the base metal 4 on top of the photoresist 8 as shown in FIG. 16.
A comparison between the SA design of the state of the art in FIG. 11 to the NSA design prepared by the method invention in FIG. 16 shows that an emitter-to-base spacing xd is created by the method according to the present invention. The pedestal comprising emitter epitaxy 2 below and in line with emitter metal 3 is not covered by the base metal 4 according to this embodiment of the invention.
Preparing of Self-Aligned HBT by the Subtractive Method
The same starting layer structure as shown in FIG. 7a is used for the method according to this preferred embodiment, wherein a base metal 4 layer is applied on the top surface as shown in FIG. 17. Then a photoresist layer 8 is applied on the base metal 4 layer as shown in FIG. 18. Two different methods can be applied to irradiate and remove photoresist layer 8: the image reversal photography as shown in FIG. 18 and the positive photolithography as shown FIG. 19. Both methods are described in the preceding sections in regard to FIG. 12 and FIG. 13. According to both methods the mask 9 is carefully designed so that after removing the non-irradiated photoresist 8 in image reversal photolithography and after removing the irradiated photoresist 8 in the positive photolithography, parts of the base layer 1 are still covered by photoresist layer 8. The distance pD between the edge of the photoresist 8 covering the base metal 4 and emitter epitaxy 2 and the emitter metal 3 as shown in FIG. 20 predetermines the emitter-to-base spacing xD as shown in FIG. 22. In the next step the base metal layer 4, which is not covered by photoresist 8 is removed, wherein base metal 4 covered by photoresist 8 stays on the base 1 as shown in FIG. 21. In the next step the photoresist 8 is removed as shown in FIG. 22. The pedestal comprising emitter epitaxy 2 below and in line with emitter metal 3 is not covered by the base metal 4 according to this embodiment of the invention.
The Non-Self-Aligned Heterojunction Bipolar Transistor according to the present invention as shown in FIGS. 16 and 22, and 23 and 24 can be prepared by both methods of the present invention shown in FIGS. 12 to 16 and FIGS. 17 to 22.
Non-Self-Aligned HBT Devices with Different Emitter Metal to Base Metal Spacing (XD)
FIG. 25 shows a plot of seven curves 0 (xD=0 μm), 3 (xD=0.3 μm), 4 (xD=0.4 μm), 5 (xD=0.5 μm), 6 (xD=0.6 μm), 8 (xD=0.8 μm), 10 (xD=1.0 μm) of DC Beta versus Ic [A] (Collector Current). The spacing is between the emitter metal and the base metal, the Emitter pedestal and the closest edge of the base metal (XD). 0_and 3 result in a much lower DC Beta versus collector current than 4-10. The differences in the measured values for DC Beta versus Ic [A] are small between 4, 5, 6, 8 and 10. For distances greater than 0.4 microns diminishing returns are observed.
FIG. 26 shows a plot (0, 3, 4, 5, 6, 8, 10) of the same devices as shown in FIG. 25 of Ft [GHz] (unity gain transfer frequency) versus IC [A]. The 0 curb represents the self-aligned emitter base heterojunction device, which is the prior art (emitter base spacing of 0 μm is prior art.). The Fts of 3, 4, 5, 6, 8 and 10 are higher than 0 but there is not a big difference between the Fts of 3, 4, 5, 6, 8 and 10. Generally the distance makes the effect and it doesn't make a big difference between 0.3 and 1.0 microns. In terms of Ft, which is the unity gain transfer frequency, there is no difference in the emitter base spacing of the criteria, just greater than 0.
FIG. 27 shows a plot (Fmax [GHz] versus IC [A]) frequency max versus collector current. Fmax is the maximum oscillation frequency. Fmax has the highest value for the 0.3 μm and 0.4 μm spacing and peaks at about 140 GHz. This shows that the smaller emitter base spacing gives a better device. The smallest emitter base spacing 0.3 μm that is in this measured data gave the best Fmax. It's very probable that if data were available for devices with emitter-base spacing equal to 0.2 or 0.1 that they would be even higher. The tendency shows that the smaller the emitter-base spacing, the better the device gets. At 0.3, 0.4, 0.5, 0.6 then it goes down. The 0 device is a self-aligned device just by nature of the way the device is fabricated creates leakage currents, which adversely affect device performance. But comparing the self-aligned device according to the prior art to the invention here for emitter-base spacing of 0.8 and 1.0 microns prepared according to the invention show worse results than according to the prior art with a spacing of 0 μm. With respect to this performance metric, Fmax, in this invention should specify emitter-base spacing ≦0.7 μm, preferably ≦0.6 μm and more preferably ≦0.5 μm.
FIG. 28 shows normalized DC Beta versus emitter-base spacing xD. The values of DC Beta are normalized to the peak value measured on the wafer. The units on the x-axis are microns. The two curves are for differing types of HBT devices (SHBT=Single Heterojunction Bipolar Transistor and DHBT=Double Heterojunction Bipolar Transistor). The two curves used the invention non-self-aligned layouts. The two types of devices differ only by their base-collector junction. The SHBT has a homojunction at the base-collector while the DHBT has a heterojunction. The DHBT devices used InP as the collector material as opposed to InGaAs for the SHBT devices. However, both the SHBT and DHBT devices have in common the emitter-base junction, according to this invention. This figure shows that the same phenomenon happens for both types of HBT devices (SHBT and DHBT). If the base metal is moved closer and closer to the emitter pedestal, the normalized DC Beta is stabile until about 0.2 μm to 0.3 μm, then there is a cliff and the device gain falls off precipitously. This figure shows that in terms of device gain (DC Beta) values of emitter-base spacing less than  ̃50 nm should be avoided. The figure shows that the same phenomenon of lateral diffusion of minority in the base happens for both types of devices.
FIG. 29 shows a plot of normalized Frequency Ft versus emitter-base spacing xD. The values of Ft are normalized to the peak value measured on the wafer. The units are microns. This figure shows that in terms of Ft that device performance does not suffer significantly for increasing emitter-base spacing until 0.4 μm or 0.6 μm. The values are fairly constant for values of emitter-base spacing between 0.3 μm and 0.6 μm. It can be as far away as 0.6 μm and not suffer in the device performance of Ft. This is very surprising.
FIG. 30 shows a plot of normalized Frequency Fmax versus emitter-base spacing xD. The values of Fmax are normalized to the peak value measured on the wafer. When the separation between the emitter and the base is greater than 0 the highest F are obtained. This is obtained for xD between 0.1 μm and 0.4 μm, preferably between 0.2 μm and 0.3 μm. Increasing the emitter base spacing of more than 0.6 μm decreases the device performance in terms of Fmax. The measured Fmax for devices with emitter-base spacing between 0.1 and 0.3 were comparable.
The preferred spacing between the emitter metal and the base metal (XD), which means between the Emitter pedestal and the closest edge of the base metal (XD), is from 0.05 μm to 0.6 μm, preferably from 0.05 μm to 0.5 μm, more preferably from 0.05 μm to 0.3 μm and most preferably from 0.05 μm to 0.15 μm.
The following preferred materials for metals and epitaxy are preferably used within the present invention. Any of the materials from each list can be used for each. That is, any metal on the list will work for any of the three contacts and any of the epi materials listed will work for any of the three epitaxy layers.
One or more of the following metals or metal alloys: InGaAs, InAlAs, InAs, InSb, AlInGaAs, InGaSb, GaAs, InP, InGaP, Si or SiGe can preferably be used as Emitter Epitaxy or Base Epitaxy or Collector Epitaxy.
One or more of the following metals or metal alloys: Titanium, Molybdenum, Aluminum, Gold, Platinum, Copper, Gold-Germanium, Nickel, Tantalum or Tungsten can preferably be used as Emitter Metal or Base Metal or Collector Metal.
One or more of the following metals or metal alloys: Silicon, SiGe, InP, GaAs, SiC, Quartz or Sapphire can preferably be used as Substrates.
Both light (photolithography) and even electron beam (e-beam) lithography are applicable to the invention here. With photolithography there is a standard chrome-on-quartz mask used, but any general mask works. The lithography could be either positive photolithography or image reversal photolithography. Both are applicable.
The following are examples for the process according to the present invention.
EXAMPLES
Example 1
Image Reversal Lithography Process
A photoresist layer of 1 μm was applied on the top surface of a layer structure (wafer) with emitter epitaxy and emitter metal as shown in FIG. 7. A photoresist SPR 955-1.1 of Shipley Company was applied as a liquid by spin-on processing to the wafer. The wafer was then heated on a hot plate by 90° C. for 60 seconds. The wafer was then exposed to a light through a mask as shown in FIG. 12. The light was i-line 365 nm. After the irradiation the wafer was baked in an image reversal oven using NH3 gas at a temperature of 100° C. for 90 minutes. Then the entire wafer was exposed to a broadband illumination without a mask to complete the cross-linking process in the photoresist. Then the wafer was exposed to a photoresist developer MF 321 (Metal Ion Free) of Shipley Company. A wafer was obtained as shown in FIG. 14. Then base metal was deposited on the entire top surface of the wafer as shown in FIG. 15 using the process of thermal evaporation. A base metal was heated until the temperature of evaporation. First a thin layer of titanium as adhesion layer, then platinum as diffusion barrier and finally gold was deposited on the top surface of the wafer. Then the entire wafer was exposed to an acetone bath, wherein the photoresist was removed and a HBT precursor was obtained as shown in FIG. 16.
Example 2
Positive Lithography Process
A photoresist layer of 1 μm was applied on the top surface of a layer structure (wafer) with emitter epitaxy and emitter metal as shown in FIG. 7. A photoresist SPR 955-1.1 of Shipley Company was applied as a liquid by spin-on processing to the wafer. The wafer was then heated on a hot plate by 100° C. for 60 seconds. The wafer was then exposed to a light through a mask as shown in FIG. 13. The light was i-line 365 nm. Then the wafer was exposed to a photoresist developer MF 26A (Metal Ion Free) of Shipley Company. A wafer was obtained as shown in FIG. 14. Then base metal was deposited on the entire top surface of the wafer as shown in FIG. 15 using the process of thermal evaporation. A base metal was heated until the temperature of evaporation. First a thin layer of titanium as adhesion layer, then platinum as diffusion barrier and finally gold was deposited on the top surface of the wafer. Then the entire wafer was exposed to an acetone bath, wherein the photoresist was removed and a HBT precursor was obtained as shown in FIG. 16.
Although the description above contains much specificity, this should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. For example the process described above can involve additional or fewer steps and different priorities of steps.
Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.

## Claims

1. A method for preparing a non-self-aligned heterojunction bipolar transistor comprising in uninterrupted sequence:
a) preparing a patterned emitter metal on an emitter epi layer of a HBT epi structure on a substrate;
b) preparing an emitter epitaxy disposed exclusively below the emitter metal;
c) depositing a base metal layer on an exposed top surface;
d) applying a resist on the base metal layer;
e) applying a lithography mask covering a peripheral edge of the emitter epitaxy and the emitter metal with a width PD and leaving a pattern according to the lithography mask in the resist;
f) removing portions of the base metal layer not covered by resist; and
g) removing remaining resist,
h) the base metal being spaced from the emitter epitaxy and the emitter metal by a distance XD, wherein the distance XD ranges from 0.05 μm to 0.7 μm.

2. The method according to claim 1, wherein the emitter-to-base distance XD is from 0.05 μm to 0.6 μm.

3. The method according to claim 1, wherein the emitter-to-base distance XD is from 0.05 μm to 0.5 μm.

4. The method according to claim 1, wherein the emitter-to-base distance XD is from 0.05 μm to 0.3 μm.

5. The method according to claim 1, wherein the emitter-to-base distance XD is from 0.05 μm to 0.15 μm.

6. The method according to claim 1, wherein the emitter epitaxy and the emitter metal are prepared by etching off the emitter metal layer and the emitter layer.

7. The method according to claim 1, wherein the photolithography is image reversal photolithography or positive photolithography.

8. The method according to claim 1, wherein the photolithography is light photolithography or electron beam lithography.

9. The method according to claim 1, wherein the photolithography is carried out with a chrome-on-quartz mask.

10. The method according to claim 1, wherein a subcollector layer is applied on a substrate and a collector layer is applied on the subcollector layer and the base layer is applied on the subcollector layer.

11. The method according to claim 1, wherein one or more of the following metals or metal alloys are used to prepare emitter epitaxy, base epitaxy or collector epitaxy: InGaAs, InAlAs, InAs, InSb, AlInGaAs, InGaSb, GaAs, InP, InGaP, Si or SiGe.

12. The method according to claim 1, wherein one or more of the following metals or metal alloys are used to prepare emitter metal, base metal or collector metal: Titanium, Molybdenum, Aluminum, Gold, Platinum, Copper, Gold-Germanium, Nickel, Tantalum or Tungsten.

13. The method according to claim 1, wherein one or more of the following metals or metal alloys are used to prepare substrates: Silicon, SiGe, InP, GaAs, SiC, Quartz or Sapphire.

14. The method according to claim 1, wherein the epi layer structure comprises subcollector, collector, base and epi layer.

15. The method according to claim 1, wherein the emitter layer is an emitter epi layer.

16. The method according to claim 1, wherein the HBT structure is a HBT epi structure.

17. The method according to claim 1, wherein the epi layer structure comprises subcollector, collector, base and epi layer.

18. The method according to claim 1, wherein the heterojunction bipolar transistor is non-self-aligned.

19. A heterojunction bipolar transistor as prepared according to a method of claim 1.

20. A heterojunction bipolar transistor as prepared according to a method of claim 18.