Abstract:
Methods are provided for laser patterning a partial depth surface portion of a glass body by controlling the amount of stress induced in the glass body. A laser beam is directed along an impinged path on the surface portion of the glass body to heat the glass body to form a swell. The glass body is then cooled and etched. The surface portion of the glass body is heated above the strain point at a heating rate HR to form a swell. The heating rate HR is a function of a target temperature T and an exposure time of the output laser beam. The exposure time is controlled to reach a target temperature above the softening point of the glass body and does not require a power density that would lead to laser ablation of the surface portion. The surface portion is cooled below the strain point to induce regions of localized stress. The unablated surface portion is etched while in a state of laser-induced localized stress to form a patterned glass body.

Description:
This application claims the benefit of U.S. Provisional Application No. 61/190,489 filed Aug. 29, 2008, entitled “Laser Patterning of Glass Bodies”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to method laser patterning of glasses and, more specifically of micromachining glasses using laser induced stress. 
     TECHNICAL BACKGROUND 
     It is known that glass bodies can be micromachined using particular pulsed lasers. In a laser micromachining process, laser energy is directed into a material and a portion of the irradiated material is removed by ablation. Laser micromachining can be used, for example, to drill, cut, and scribe glass so as to form structures including, for example, channels, grooves, or holes in the glass material. 
     In some micromachining processes, the laser energy comprises one or more laser pulses. However, when more than a single laser pulse is used, residual heat can accumulate in the bulk of the remaining material as successive pulses are incident upon the material. If the laser pulse repetition rate is sufficiently high, the accumulated heating can become severe enough to cause undesirable effects, such as melting, or other changes to the surrounding regions of the glass material. These regions are known as Heat Affected Zones (HAZ), and they lead to imprecision in the micromachining process. 
     In some pulsed laser micromachining processes, a higher laser pulse repetition rate is necessary to make the micromachining process economically feasible. One micromachining approach utilizes formation of microcracks created by the high repetition lasers, followed by the etching of the microcracked areas. However, the microcracks may propagate into the surrounding area, which would lead to imprecision in the micromachining process. Another approach is to use a high repetition femtosecond laser with water or other liquid in contact with the glass, to clean out the removed glass as damaged regions are created. 
     Yet another approach utilizes lithographical etching of the glass. In this approach the photoresist material is deposited on top of the treated material. The photoresist is exposed with UV radiation and cured. Uncured photoresist can be removed, and whole part is exposed to acid. Only material without photoresist on the top will be etched. After etching remaining photoresist can be removed. There are different modifications of this approach, but all of them use photoresist and UV exposure to create desired pattern. The laser is used to expose and/or cure the photo resist material, not to change the glass. 
     Accordingly, apparatus and methods are needed that enable a more efficient and simple, or more exact way of laser micromachining. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a method for laser patterning of a glass body, the method comprising the steps of:
         (i) providing a laser, said laser having an output beam at a laser wavelength λ;   (ii) providing a glass body having optical density at of at least 1.5/cm at said wavelength λ;   (iii) directing said laser output beam to (a) impinge on the glass body without ablating said glass, and (b) heat the glass body at a location proximate to said laser output beam so as to form a swell at this location; and   (iv) etching this location.       

     In at least some of the embodiments, the laser treated area has preferential (i.e., faster) rate of etching than the surrounding areas. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a laser micromachining system according to one embodiment of the present invention; 
         FIG. 2  is a schematic illustration showing a glass body with a swell (i.e., a swollen area) formed along a path of the laser output beam produced by a laser micromachining system shown in  FIG. 1 ; 
         FIG. 3A  is an illustration of a first exemplary cross-sectional area of laser induced swollen area in a glass body, where the laser output beam was moved along the glass body at a rate of 15 mm/sec. 
         FIG. 3B  illustrates cross-sectional area of a glass body, after etching the swollen area shown in  FIG. 3A . 
         FIG. 3C  is an illustration of a second exemplary cross-sectional area of laser induced swells in a glass body, where the laser output beam was moved along the glass body at a rate of 10 mm/sec. 
         FIG. 3D  illustrates cross-sectional area of a glass body, after etching the swollen area shown in  FIG. 3C . 
         FIG. 3E  is an illustration of a third exemplary cross-sectional area of laser induced swollen area in a glass body, where the laser output beam was moved along the glass body at a rate of 5 mm/sec. 
         FIG. 3F  illustrates cross-sectional area of a glass body, after etching the swollen are shown in  FIG. 3E . 
         FIG. 3G  is an illustration of a fourth exemplary cross-sectional area of laser induced swollen area in a glass body, where the laser output beam was moved along the glass body at a rate of 5 mm/sec. 
         FIG. 3H  illustrates cross-sectional area of a glass body, after etching the swollen are shown in  FIG. 3G . 
         FIG. 4A  illustrates the stress areas (areas of birefringence) corresponding to the laser induced swollen areas of  FIGS. 3A ,  3 C,  3 E and  3 G, present in a cerium doped boro-silicate glass body situated between two crossed polarizers. 
         FIG. 4B  illustrates relationship between etching depth and average stress (psi) in laser exposed areas of Ce doped glass. 
         FIG. 4C  illustrates birefringent areas and stress patterns of a cross-sectional area of a glass body after laser exposure. 
         FIGS. 5A ,  5 C and  5 E illustration of cross-sectional area of iron doped glass after production of laser induced swells and subsequent etching, without an annealing step therebetween. 
         FIGS. 5B ,  5 D and  5 F illustrates cross-sectional area of a glass body, after production of laser induced swells and the subsequent etching step, but with an annealing step therebetween. These figures show that in these glass samples no pits were produced in glass samples that underwent annealing before etching. 
         FIG. 6  is a photograph of exemplary etched iron doped glass body.  FIG. 6  illustrates that both in this glass body both the front surface and the rear surface have been etched. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiment(s) of the invention, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One embodiment of the laser micromachining system of the present invention is shown in  FIG. 1 , and is designated generally throughout by the reference numeral  10 . The laser micromachining system includes a laser  20  for providing a laser output beam  22 , a bed  25  for the glass body  30 , and an optional translation stage  26  for the bed  25 . Alternatively, an optional scanning system (not shown) can be utilized to scan the laser output beam  22  across at least the required portion of the glass body  30 . The laser micromachining system includes a laser  20  may also include an optional focusing lens  35  for focusing laser output beam  22  on the glass body  30 . Preferably the glass body  30  may be doped with at least one of: iron, boron, or cerium, copper, vanadium, or other transition metals having absorption coefficient equal to or higher than 2 cm −1 . Preferably the focusing lens  35  has a numerical aperture NA of equal to or less than 0.4 (i.e., NA≦0.4). According to some embodiments, the laser  20  has laser wavelength λ in a wavelength range of 266 nm to 10 microns, and repetition rate of more than 1 kHz. For example, the laser  20  may be: a continuous wave laser, quasi CW laser with repetition rate greater than 1 kHz (e.g., &gt;10 kHz) a diode laser, an 810 nm laser, Nd:Yag laser, a CO 2  laser, or a single pulse laser. Preferably the laser output beam  22  is moving relative to the glass body  30  at a rate of at least 1 mm/sec, for example 1-20 mm/sec. 
     In accordance with some embodiments of the present invention method of laser patterning of a glass body includes the steps of (i) providing a laser  20 , the laser  20  producing an output beam  22  at a laser wavelength λ that is within the absorption band of the glass body and (ii) providing a glass body  30  having optical density at of at least 1.5/cm, and preferably 2/cm at this wavelength λ. The laser power required to achieve the desired effect needs to be sufficiently high to bring the glass temperature above strain point (i.e., strain temperature) and preferably to or above softening temperature of the glass. (It is noted that overall higher absorption, or optical density of the glass is approximately inversely proportional to required laser power (given the same duration of the exposure and the same laser power density on the glass). The laser output beam  22  is directed to impinge on the glass body  30  without ablating glass, and to heat the glass body  30  at a location  32  proximate to the laser output beam  22  so as to form a “swell”  40  (i.e., swollen area  40 , for example as a bump or a ridge) with a height h of at least 1000 A at this location. Preferably, according to some embodiments, the height of the swell  40  is at least 1 μm and less than 350 μm, for example 2 μm to 200 μm, or 3 μm to 100 μm, which will vary based on the glass compositions. For example, a laser induced swell  40  may be 10 μm to 600 μm wide and 1 μm to 30 μm tall. The formation of the swell and/or height of the swelling is an indication that glass reached at least softening temperature. If the glass body  30  is translated relative to the impinging laser output beam  22 , for example by a translating bed  25 , or if the laser beam is scanned across at least a portion of the glass surface, the swell  40  forms along the path of the laser output beam  22  and is longer than it is wide (see  FIG. 2 , for example). 
     Preferably, after the laser exposure (and subsequent cooling), the swell  40  has laser-induced stress birefringence (for 0.7 mm thick glass) of at least 1.6 nm, preferably at least 3.2 nm, more preferably at least 4.8 nm, more preferably at least 6.4 nm, and even more preferably between 6.4 and 282 nm. Preferably, after the laser exposure (and subsequent cooling), the resulting laser induced stress is at least 200 psi, more preferably at least 300 psi, even more preferably at least 400 psi. For example, according to some embodiments, localized laser induced stress at or adjacent to location(s)  32  may be 200 psi to 30000 psi, or 400 psi to 25000 psi, or 1000 psi to 15000 psi. It is noted that the stress also penetrates the glass below the swells  40 , (i.e., the “stressed” areas have larger dimensions than the dimensions of the swells  40 .) 
     Preferably, when the laser output beam  22  impinges on the glass at location  32 , the glass at location  32  heats at a rate of 200° C./sec to 5000° C./sec (and preferably at the rate of 500° C./sec to 2500° C./), and then cools at up to 1,200,000° C./sec (and preferably at the rate of 10° C./sec to 1,000,000° C./sec). For example, in some exemplary embodiments, moving the laser output beam  22  relatively to the glass body  30 , at a velocity of 2 mm/sec to 30 mm/sec resulted in heating the localized area at a rate of 200° C./sec to 5000° C./sec and, after cooling created a pattern of localized laser-induced stress of at least 6.3 nm in the glass body that was 0.7 mm thick (this pattern corresponds to swells  40 ). 
     After the swell(s)  40  is formed, the glass body  30  is placed in an acid, base, or etchant gas, and the swollen area(s) etches out faster than the surrounding area(s), forming a depression at the location  32 . The depth of the etched out area is proportional to the amount of stress present in the swell(s)  40 . Preferably the swell  40  (or location  32 ) etches out at least 5 times faster than the surrounding glass. According to some embodiments the swell  40  (or location  32 ) etches out at least 10 times faster than the surrounding glass. Preferably the swell  40  (or location  32 ) etches out at least 20 times faster than the surrounding glass. Preferably the swell(s)  40 , or location(s)  32 , etch out at the rate of at least 5 μm/min, for example, more preferably at the rate of 5 to 50 μm/min. In some exemplary embodiments the etch rate is, for example, 6 to 30 μm/min. 
     The heating rate of the glass is at least partially determined by laser power and duration of laser exposure. The typical laser exposure is 1-2 seconds. The shorter exposure time requires higher laser power and higher power densities on the glass, which potentially could lead to the glass ablation. So, at the shorter exposure limit, the heating rate is determined by the high power density leading to laser ablation. We define the heating rate HR to be: HR=T(target)/exposure time, where T(target) is target temperature. The target temperature, T(target) (i.e., temperature T when the swell  40  starts to form) needs to be above softening point of the glass, typically above 800° C.-1200° C. Preferable laser exposure duration time is greater than 50 milliseconds. Typical exemplary shortest exposure time is 0.25 sec, preferably 0.5 sec, so preferable maximum average heating rate of the glass at location(s)  32  is 5000° C./sec, more preferably 2400° C./sec. It is preferable for the heating rate to be in the range of 1000° C./sec to 5000° C./sec, more preferably, 1500° C./sec to 2400° C.°/sec. When the heating rate is too low, heat diffusion makes heat spread to a very large area, making the treated area very large. For example the heat may spread over an area wider than about 3-5 mm, leading to loss of ability to create small features on glass. In addition, a larger heating zone may lead to glass cracking, and a slower cooling rate. 
     The cooling rate of the glass in the previously heated area (location(s)  32 ) is determined by the heat diffusivity rate in the glass, and heating spot size. Typically cooling rates that were measured experimentally were found significantly higher than the heating rates. The typical cooling time is about 1-10 ms from the same peak temperature, so maximum average cooling rate is about 1200° C./1 ms, or about 120000° C./sec. This cooling rate creates a significant stress in area(s)  40  of the glass body  30  at the location(s)  32 . It is preferable for the cooling rate to be in the range of 100° C./see to 120000 C/sec ° C./sec, more preferably, 15000° C./sec to 100000° C./sec. Slow cooling rates allow glass to completely relax, leading to absence of stress in the glass, which is often undesirable. (Slow cooling rates for glass are cooling rates that in the range of a few degrees per minute.) Faster cooling rates (100° C./sec to 120000 C/sec ° C./sec), especially around the glass strain point (also referred to as strain temperature herein) lead to higher stress, which is more desirable. 
     The cooling rate depends on temperature of the glass, because heat radiation loss is proportional to T 4 , where T is temperature of the glass in the treated region. At high temperatures in the treated region cooling is fast, at lower temperatures the cooling is slower. The most important temperature for stress formation is strain point. (At temperatures above strain point no additional significant stress is formed.) 
     The parameters for absorption and laser power and power density could be determined as follows: The maximum absorption is in the range of 30 cm −1 , while minimum absorption is 1 cm −1 . The maximum absorption determines the depth of “laser treated” region, while minimum absorption assumes that all thickness of the glass (1-3 mm piece) is treated, but requires very high laser power. The treated areas of the glass body  30  (location(s)  32 ) need to reach temperature when glass can flow, or at least be above strain point, in order to generate adequate degree of stress in this location(s). These temperatures are glass specific, but typically strain point is about 450° C. to 800° C., while glass softening points are in the 800° C. to 1200° C. range. Therefore, laser power density, glass absorption, and exposure time needs to be such that glass at location(s)  32  reaches this temperature during laser treatment (either line or spot exposure). In a first approximation the relationship between target temperature T and other parameters may look like this: T≈P(λ)τ 1/2 α(λ)/(a 3/2 D 1/2 ), where P is laser power, α is glass absorption, a is laser spot size, D is glass heat diffusivity (almost the same for all glasses), τ is duration of laser pulse, λ is laser wavelength. The typical size of laser spot is 0.1 to 2 mm wide. According to some embodiments, T&gt;900-1500° C., depending on glass composition. 
     The desired depth of the etched region can thus be provided by choosing a glass with absorption that is appropriate to laser wavelength of the available laser(s), or by ether choosing a laser with a wavelength λ, or by adjusting laser wavelength λ to match absorption properties of the glass. A wide range of etched depths from a few um up to all thickness of glass body may be achieved. 
     The invention will be further clarified by the following examples. 
     EXAMPLE 1 
     In this example, the glass body  30  was irradiated with a laser  20 , at four different locations, each corresponding to one of four different beam velocities. More specifically, the laser output beam  22  was moved along the top surface of the glass body at a velocity V, where V was 15 mm/sec, 10 mm/sec, 5 mm/sec and 2 mm/sec. The composition of the cerium doped glass in this example was as follows: 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 (Mol %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SiO2 
                 80.85 
               
               
                   
                 Al2O3 
                 1.21 
               
               
                   
                 B2O3 
                 12.44 
               
               
                   
                 Na2O 
                 3.5 
               
               
                   
                 CeO2 
                 1 
               
               
                   
                 TiO2 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     The following Table 1 summarizes the achieved results, which are illustrated in  FIGS. 3A ,  3 C,  3 E and  3 G. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Ce-doped borosilicate glass 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 velocity V (mm/s) 
                 15 
                 10 
                 5 
                 2 
               
               
                 Resulting stress (psi) 
                 407 
                 1019 
                 1630 
                 4076 
               
               
                 Resulting Birefringence 
                 6.3 
                 15.7 
                 25.1 
                 62.77 
               
               
                 (nm) for 0.7 thick glass 
               
               
                 swell peak h (μm) 
                 1.4 
                 1.78 
                 3 
                 5.5 
               
               
                   
               
             
          
         
       
     
     The glass body  30  was then dipped in 50% HF acid for 5 minutes. The results are summarized in Table 2 and are shown in  FIGS. 3B ,  3 D,  3 F and  3 H. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 FIG. 3B 
                 FIG. 3D 
                 FIG. 3F 
                 FIG. 3H 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 etched depth (μm) 
                 25 
                 25 
                 140 
                 164 
               
               
                   
                   
               
             
          
         
       
     
     More specifically,  FIGS. 3A ,  3 C,  3 E and  3 G are illustration of exemplary cross-sectional areas of laser induced swells  40  in a Ce doped borosilicate glass body  30 , where the laser output beam  22  was moved along the glass body at a rate V, where V was 15 mm/sec, 10 mm/sec, 5 mm/sec and 2 mm/sec, respectively. The laser  20  is a NG:Yag laser, with an output laser wavelength λ of 355 nm, average laser power of 2 W, and repetition rate of 140 kHz. The laser beam spot size on the glass surface was about 200-300 μm in diameter. Because of the presence of Ce ions, this glass body has an absorption band in UV region, thus enabling the glass body to absorb energy corresponding to the 355 nm wavelength. As shown in  FIGS. 3A ,  3 C,  3 E and  3 G the faster the relative motion of the laser beam across the glass surface, the lower is the height h of the swells. This difference in height h can be explained by higher power density on the glass produced by a slower moving beam (more dwell time at any given particular location), and lower power density produced by a faster moving beam (less dwell time at any given particular location). For example, the height h of the swell  40  corresponding to  FIG. 3E  (V=5 mm/sec) is 8 μm, while the height h of the swell  40  corresponding to  FIG. 3C  (V=10 mm/sec) is 3.25 μm. 
       FIGS. 3B ,  3 D,  3 F,  3 H illustrate cross-sectional areas of respective glass bodies, after etching in a 50% hydrofluoric acid for a period of 5 min the swells  40  that correspond, respectively, to cross-sections illustrated in  FIGS. 3A ,  3 C,  3 E and  3 G. As we can see, the deeper areas correspond to lower velocities and/or the areas with higher stress (higher induced birefringence). 
       FIG. 4A  illustrates the stress areas (areas of birefringence) corresponding to the laser induced swollen areas of  FIGS. 3A ,  3 C,  3 E and  3 G, present in a cerium doped boro-silicate glass body situated between two crossed polarizers. 
       FIG. 4B  illustrates relationship between etching depth and measured average stress for Ce doped glass (after laser expose and cooling). As we can see, the relationship is almost linear. The stress in swells  40  was measured to be in 50 to 4000 psi range and the resulted etching depth for 5 min processing with acid was 20 to 160 μm. 
       FIG. 4C  shows stress birefringence pattern for laser exposed and quenched (i.e., cooled) glass with the view taken perpendicular to the laser exposure (laser exposure comes from the top). The stress pattern shows depth of the laser affected area, which area will be preferentially etched in the etching step. The front surface FS is the surface irradiated by the laser. 
     The experiment (using translation velocities V of 5 mm/sec and 10 mm/sec) was then repeated, but with the Ce doped borosilocate glass body  30  partially annealed. That is, the glass body  30  of this example was heated above strain point and slowly cooled (10° C./min). This reduces stress, but not completely erases it, before the etching step. We have discovered that the annealing step resulted in a significant reduction in etch depth. The data (for velocities V mm/sec of 5 and 10 mm/sec) is summarized in Table 3. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Etch depth 
               
               
                   
                 Peak 
                 Birefringence 
                   
                 Etch dept  
                 (μm), 
               
               
                   
                 swell, 
                 nm, for 0.7 mm 
                 Stress, 
                 (μm), sample 
                 sample not 
               
               
                 V 
                 μm 
                 thick glass 
                 psi 
                 annealed 
                 annealed 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  5 mm/s 
                 8.0 
                 43.9 
                 2853 
                 −40 
                 −58 
               
               
                 10 mm/s 
                 3.25 
                 22 
                 1426 
                 −20 
                 −30 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 2 
     In this example, a glass body  30  was irradiated with a 3 W CO 2  laser  20 , at two different locations. The composition of the glass body in this example was as follows: 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 (Mol %) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SiO2 
                 81 
               
               
                   
                 B2O3 
                 13 
               
               
                   
                 Al2O3 
                 2 
               
               
                   
                   
               
             
          
         
       
     
     At both locations the laser output beam  22  was moved along the top surface of the glass body at a velocity V=5 mm/sec. The laser output beam  22  spot size was about 150 μm. The glass body was then cut in half, resulting in two identical glass samples, each containing one laser beam irradiated area. One of the glass samples was annealed prior to etching and another was not annealed prior to etching. As in the previous example, the etching step was performed with a HF acid. The resulting depression after etch was −8.7 μm deep for a glass sample that was not annealed, and for partially annealed glass the depression was −3 μm deep. Again, we observed that annealing reduced the depth of the etched depression. Thus, in order to achieve maximum depression depth (without additional reduction in velocity V, or increase in laser power) one should preferably not anneal the glass body between the steps of laser beam irradiation step and etching. 
     EXAMPLE 3 
     In this example, an iron-doped glass body  30  was irradiated with an 810 nm, CW laser  20 , at two different locations. The composition of the glass body in this example was as follows: 
                                                       (Mol %)                                        SiO2   82.28           B2O3   8.13           Al2O3   1.21           Na2O   5.38           Fe2O3   1           TiO2   2                        
The laser power and resulting birefringence of swells  40  are tabulated below in Table 4.
 
                                                                                         TABLE 4                           Corresponding figure                FIG. 5a   FIG. 5b   FIG. 5c   FIG. 5d   FIG. 5e   FIG. 5f                        power W   14   14   12   12   8   8       stress, psi   18,140   0   9783   0   5300   0       Birefringence,   279.4   0   150.7   0   81.6   0       nm for 0.7       mm glass       Sample   no   yes   no   yes   no   yes       annealed       50% HF/5 min   yes   yes   yes   yes   yes   yes       etched depth   −35   23   −33   12   −27   2.3       um       center bump in   26       15       etch cavity                    
In Table 4 the negative number corresponding to the etch depth means that the etched hole or groove is below the surface of the glass. Positive etch depth corresponds to the (at least partially) annealed examples, which indicate that no depression was formed by the etching process. In some glass bodies, the grove or hole contained a central “bump”—i.e., a raised area in the center and its height h′ (shown in the last line of table 4) was measured relative to the deepest etched area of the glass body. For example (see  FIG. 5   a ), an etched depth of −35 μm and a central bump height of 23 μm indicate that the etched area has a “w shaped” cross-section, with the deepest measured portions being 35 μm below the top surface of the glass body, and that the top of the “central bump” situated within the etched area is 26 μm above the lowest point, or 9 μm below the top surface of the glass body. For example, positive etch depth of 23 depth μm (see  FIG. 5   b ) indicates that no depression was formed by the etching process because of annealing, and that the laser exposed area retained a swell that is 23 μm high.
 
     The laser output beam  22  was moved along the top surface of the glass body at a velocity V=10 mm/sec. The laser output beam  22  spot size was about 600 μm. The glass body was then cut in half, resulting in two identical glass samples, each containing three laser beam irradiated areas corresponding to three swells  40 , with each swell produced by a different beam intensity. One of the glass samples was annealed prior to etching and another was not annealed prior to etching. As in the previous example, the etching step was performed with a HF acid. The depths of resulting depression after the etch are tabulated in Table 4, both for the glass sample that was not annealed, and for glass sample that was partially annealed (i,e., stress was not completely removed). We observed that annealing interfered with etching and the annealed sample formed no depressions. We also observed that higher laser power corresponded to deeper pit formations. This suggests that stress plays an important role in formation of these features. 
     In the case with the iron doped glasses, corresponding to  FIGS. 5A-5F , we observed an additional effect. The stress is not only relegated to the surface that directly sees the laser irradiation, but can be transferred to the rear surface, if the power density is sufficient (i.e., the power density is such that the laser energy can radiate to the rear surface). The amount of power required to heat the glass (e.g., at the rear surface) is related to reduction of the optical density of the glass during radiation, in this example due to changing of oxidation states of the Fe ions. This is due to the fact that that during cooling, Fe ions convert from an absorbing oxidation state to a non-absorbing oxidation state for 810 nm laser wavelength. 
     As seen in the cross section ( FIG. 6 ), the etch rate in Fe doped glass is much more pronounced than in other glasses. This is due to deeper penetration of the heating zone into the glass. Since the glass becomes transparent during exposure, the stress is formed all the way through to the back surface. As with other glasses, we discovered that the etch rate was greatly reduced if the samples received an annealing step before HF acid etch.  FIG. 6  is a photograph of exemplary etched iron doped glass body. Only the front surface of this glass body was irradiated, but both the front and the rear surfaces were exposed to acid (50% HF/5 min) simultaneously, i.e., both surfaces were etched simultaneously.  FIG. 6  illustrates that in this glass body both the front surface FS and the rear surface RS have been etched and that the etched area is deeper on the rear surface RS. This is because laser induced stress was transferred from the laser exposed area(s) on the front surface to the rear surface of the glass body, and the rear surface experienced greater stress than the front surface. Table 5 shows the relationship between laser power during exposure, formed stress, and depth of etching by HF. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Iron doped glass 
               
             
          
           
               
                   
                 power W 
               
             
          
           
               
                   
                 2 
                 6 
                 12 
                 15 
                 22 
                 18 
                 15 
                 22 
                 18 
               
               
                   
                   
               
             
          
           
               
                 stress psi 
                 1630 
                 7540 
                 12,640 
                 19,974 
                 23,640 
                 23,640 
                 19,974 
                 23,643 
                 23,643 
               
               
                 Birefringence, nm for 
                 25.1 
                 116.1 
                 194.6 
                 307.6 
                 364 
                 364 
                 307.6 
                 364.1 
                 364.1 
               
               
                 0.7 mm glass 
               
               
                 Sample annealed 
                 no 
                 no 
                 no 
                 No 
                 no 
                 no 
                 no 
                 no 
                 no 
               
               
                 50% HF/5 min 
                 yes 
                 yes 
                 yes 
                 Yes 
                 yes 
                 yes 
                 yes 
                 yes 
                 yes 
               
               
                 etched depth um 
                 27 
                 31 
                 31 
                 30 
                 30 
                 30 
                 162 
                 162 
                 162 
               
               
                 center bump in etch 
                   
                 12 
                 28 
                 36 
                 55 
                 45 
               
               
                 cavity 
               
               
                 surface measured 
                 front 
                 front 
                 front 
                 Front 
                 front 
                 front 
                 rear 
                 rear 
                 rear 
               
               
                   
               
             
          
         
       
     
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.